cardiovascular disease

cardiovascular disease


      any of the diseases, whether congenital or acquired, of the heart and blood vessels (blood vessel). Among the most important are atherosclerosis, rheumatic heart disease, and vascular inflammation. Cardiovascular diseases are a major cause of health problems and death in developed countries.

      Life depends on the functioning of the heart; thus, the heart is involved in all death, but this does not account for its prominence in causing death. To some degree, as medical science advances, more people are saved from other illnesses only to die from one of the unsolved and uncontrolled disorders of the cardiovascular system (human cardiovascular system). Some forms of cardiovascular diseases are becoming less frequent causes of death, and continued research and preventive measures may provide even greater benefits.

      Heart disease as such was not recognized in nontechnological cultures, but the beating heart (heart failure) and its relationship to death have always been appreciated. Sudden death, now usually attributed to heart disease, was recognized as early as the 5th century BC by the Greek physician Hippocrates and was noted to be more common in the obese (obesity). The role of disease in affecting the heart itself did not become apparent until the 17th century, when examination of the body after death became acceptable.

      Gradually, the involvement of the heart valves, the blood vessels, and the heart muscle was observed and categorized in an orderly fashion. The circulation of the blood through the heart was described in 1628 by the British physician William Harvey (Harvey, William). The recognition of the manifestations of heart failure came later, as did the ability to diagnose heart ailments by physical examination through the techniques of percussion (thumping), auscultation (listening) with the stethoscope, and other means. It was not until early in the 20th century that the determination of arterial blood pressure and the use of X-rays (X-ray) for diagnosis became widespread.

      In 1912 James Bryan Herrick (Herrick, James Bryan), a Chicago physician, first described what he called coronary thrombosis (he was describing symptoms actually caused by myocardial infarction; see below Myocardial infarction (cardiovascular disease)). angina pectoris (see below Angina pectoris (cardiovascular disease)) had been recorded centuries earlier. Cardiovascular surgery in the modern sense began in the 1930s, and open-heart surgery began in the 1950s.

      The exact incidence of heart disease in the world population is difficult to ascertain, because complete and adequate public health figures for either prevalence or related deaths are not available. In the more technologically developed countries of the world—such as the United States, the United Kingdom, and most other European countries—arteriosclerotic (arteriosclerosis) heart disease (heart disease resulting from thickening and hardening of the artery walls) constitutes by far the most predominant form. In other areas, such as the countries of Central Africa, other forms of heart disease, often nutritional in nature, are a common cause of death. In Asia and the islands of the Pacific, hypertensive (hypertension) cardiovascular disease, disease involving high blood pressure, constitutes a major health hazard.

James V. Warren Ed.

      The heart's complicated evolution during embryological development presents the opportunity for many different types of congenital defects (congenital disorder) to occur. congenital heart disease is one of the important types of diseases affecting the cardiovascular system, with an incidence of about 8 per 1,000 live births. In most patients the causes appear to fit in the middle of a continuum from primarily genetic (genetics, human) to primarily environmental.

      Of the few cases that have a genetic nature, the defect may be the result of a single mutant gene (mutation), while in other cases it may be associated with a chromosomal abnormality (chromosomal disorder), the most common of which is Down syndrome, in which about 50 percent of afflicted children have a congenital cardiac abnormality. In the even smaller number of cases of an obvious environmental cause, a variety of specific factors are evident. The occurrence of rubella (German measles) in a woman during the first three months of pregnancy is caused by a virus and is associated in the child with patent ductus arteriosus (nonclosure of the opening between the aorta and the pulmonary artery). Other viruses may be responsible for specific heart lesions, and a number of drugs, including antiepileptic agents, are associated with an increased incidence of congenital heart disease.

      In most cases, congenital heart disease is probably caused by a variety of factors, and any genetic factor is usually unmasked only if it occurs together with the appropriate environmental hazard. The risk of a sibling of a child with congenital heart disease being similarly affected is between 2 and 4 percent. The precise recurrence can vary for individual congenital cardiovascular lesions.

      Prenatal diagnosis of congenital cardiovascular abnormalities is still at an early stage. The most promising technique is ultrasonography (ultrasound), used for many years to examine the fetus in utero. The increasing sophistication of equipment has made it possible to examine the heart and the great vessels (blood vessel) from 16 to 18 weeks of gestation onward and to determine whether defects are present. amniocentesis (removal and examination of a small quantity of fluid from around the developing fetus) provides a method by which the fetal chromosomes can be examined for chromosomal abnormalities associated with congenital heart disease. In many children and adults the presence of congenital heart disease is detected for the first time when a cardiac murmur is heard. A congenital cardiovascular lesion is rarely signaled by a disturbance of the heart rate or the heart rhythm.

      Congenital cardiac disturbances are varied and may involve almost all components of the heart and great arteries. Some may cause death at the time of birth, others may not have an effect until early adulthood, and some may be associated with an essentially normal life span. Nonetheless, about 40 percent of all untreated infants born with congenital heart disease die before the end of their first year.

      Congenital heart defects can be classified into cyanotic and noncyanotic varieties. In the cyanotic varieties, a shunt bypasses the lungs (lung) and delivers venous (deoxygenated) blood from the right side of the heart into the arterial circulation. The infant's nail beds and lips have a blue colour due to the excess deoxygenated blood in the system. Some infants with severe noncyanotic varieties of congenital heart disease may fail to thrive and may have breathing difficulties.

Abnormalities of individual heart chambers
      Abnormalities of the heart chambers may be serious and even life-threatening. In hypoplastic left heart syndrome, the left-sided heart chambers, including the aorta, are underdeveloped. Infants born with this condition rarely survive more than two or three days. In other cases, only one chamber develops adequately. Survival often depends on the presence of associated compensatory abnormalities, such as continued patency of the ductus arteriosus or the presence of a septal defect, which may allow either decompression of a chamber under elevated pressure or beneficial compensatory intracardiac shunting either from right to left or from left to right.

Abnormalities of the atrial (atrium) septum
      The presence of a septal defect (atrial septal defect) allows blood to be shunted from the left side of the heart to the right, with an increase in blood flow and volume within the pulmonary circulation. Over many years the added burden on the right side of the heart and the elevation of the blood pressure in the lungs may cause the right side of the heart to fail.

      Defects in the atrial septum (atrial septal defect) may be small or large and occur most commonly in the midportion in the area prenatally occupied by the aperture called the foramen ovale. Defects lower on the atrial septum may involve the atrioventricular valves and may be associated with incompetence of these valves. In its most extreme form, there may be virtually no septum between the two atrial chambers. Atrial septal defect is a noncyanotic type of congenital heart disease and usually is not associated with serious disability during childhood. A small defect may be associated with problems in young adults, although deterioration can occur in later life. Atrial septal defects, unless small, must usually be closed in childhood.

Abnormalities of the ventricular (ventricular septal defect) septum
      Defects in the interventricular septum (ventricular septal defect), the partition that separates the lower chambers of the heart, may be small or large, single or multiple, and may exist within any part of the ventricular septum. Small defects are among the most common congenital cardiovascular abnormalities and may be less life-threatening, since many such defects close spontaneously. Small defects often create loud murmurs but, because there is limited flow of blood from left to right, no significant change in the circulation occurs. On the other hand, when a defect is large, a significant amount of blood is shunted from the left ventricle to the right, with a high flow and volume of blood into the pulmonary circulation.

      The pulmonary circulation may be damaged by the stresses imposed by a high blood flow over a long period of time. If unchecked, this damage can become irreversible. A further hazard in both small and large ventricular septal defects is the increased risk of bacterial endocarditis (inflammation of the heart lining as a result of bacterial infection). This risk is likely to be high during procedures such as dental extractions, when infection may enter the bloodstream.

      Ventricular septal defects are often combined with other congenital cardiac defects. The best-known of these is tetralogy of Fallot (Fallot, tetralogy of), named for the French physician Étienne-Louis-Arthur Fallot, who first described it. In this condition there is a ventricular septal defect, pulmonary stenosis (narrowing of the opening to the pulmonary artery), deviation of the aorta to override the ventricular septum above the ventricular septal defect, and right ventricular hypertrophy (thickening of the muscle of the right ventricle). As a result of the obstruction imposed by the pulmonary stenosis, deoxygenated venous blood is shunted from the right to the left side of the heart into the arterial circulation. Significant amounts of deoxygenated blood in the systemic circulation impart a blue-gray cast to the skin (called cyanosis). A child with this cyanotic form of congenital heart disease can survive beyond infancy, but few survive to adulthood without surgery.

Abnormal origins of the great arteries
      In many complex forms of congenital heart disease, the aorta and pulmonary artery do not originate from their normal areas of the ventricles. In one of the most common of such cases—transposition of the great arteries—the aorta originates from the right ventricle and receives deoxygenated blood from the superior and inferior venae cavae (vena cava), and the pulmonary artery arises from the left ventricle and receives fully oxygenated pulmonary venous blood. Survival in such cases depends on a naturally occurring communication between the two sides of the heart that allows oxygenated blood to enter the aorta; if such a communication is not present naturally, it may be created medically or surgically. Both the aorta and the pulmonary artery may originate from the right ventricle; this form of abnormal origin of the arteries usually is associated with a ventricular septal defect and, on occasion, pulmonary stenosis. This combination of defects is a severe form of cyanotic heart disease.

Abnormalities of the valves
      The most common congenital abnormality of the cardiac valves affects the aortic valve. The normal aortic valve usually has three cusps, or leaflets, but the valve is bicuspid in 1 to 2 percent of the population. A bicuspid aortic valve is not necessarily life-threatening, but in some persons it becomes thickened and obstructed (stenotic). With age the valve may also become incompetent or act as a nidus (focus of infection) for bacterial endocarditis. Congenital aortic valve stenosis, if severe, results in hypertrophy of the left ventricular myocardium and may rarely be responsible for sudden death in asymptomatic individuals. Even minor forms of aortic valve stenosis may grow progressively severe and are likely, with the passage of time, to require surgical treatment.

      In contrast to aortic valve stenosis, pulmonary valve stenosis, if mild, is usually well tolerated and does not require surgical treatment. More severe forms of the disease may require surgery or balloon dilation (see below Surgical treatment of the heart (cardiovascular disease)).

Abnormalities of the myocardium and endocardium
      Congenital abnormalities in the myocardium—for example, tumours (tumour)—may be present at birth, but they are rare. Abnormalities of the endocardium may be present at birth, but they are also rare. They include fibroelastosis, a disease in which the endocardium develops a thick fibrous coat that interferes with the normal contraction and relaxation of the heart. This condition cannot be treated surgically and is usually life-threatening.

Abnormalities of the coronary arteries (coronary artery)
      The coronary arteries (coronary artery) may arise abnormally from a pulmonary artery rather than from the aorta, with the result that deoxygenated blood instead of oxygenated blood flows through the heart muscle. Abnormal openings, called coronary arterial venous fistulas, may be present between the coronary arteries and the chambers of the heart. One or more of the three main coronary arteries may be absent. While these abnormalities are frequently asymptomatic, they may be associated with early, often sudden, death. If necessary, most coronary arterial abnormalities can be corrected surgically.

Abnormalities of the aorta
      One of the most common congenital cardiovascular abnormalities involves the aorta. In coarctation of the aorta (aorta, coarctation of the) there is a narrowing of the aortic wall, usually at that portion of the aorta just beyond the site at which the main blood vessel to the left arm (the subclavian artery) originates. As a result of the narrowing or obstruction at this point, blood flow to the lower half of the body is diminished, and hypertension develops in the upper half of the body. This defect may give rise to heart failure in early infancy or complications in later childhood and adulthood.

      During fetal life and immediately after birth, the ductus arteriosus connects the pulmonary artery and the first segment of the descending thoracic aorta. The function of this duct in utero is to shunt blood away from the lungs (lung). If the ductus remains open after birth, excessive blood may flow into the lungs, resulting in pulmonary congestion and heart failure. Spontaneous closure of the ductus arteriosus may be delayed in premature newborn infants, exacerbating the respiratory problems common to them. If necessary, the ductus arteriosus can be induced to close with drugs (drug) in premature infants, and it can be closed in older infants and children by surgery or insertion of a prosthetic occluder by cardiac catheterization. Finally, there may be direct communication between the aorta and pulmonary arteries because the truncus arteriosus has either partially or completely failed to partition.

Anomalous pulmonary venous return
      The pulmonary veins (vein) from the right and left lungs may connect either directly or indirectly to the right, instead of the left, atrium. In this condition the abnormal venous channel draining to the right side of the heart may become obstructed. Infants born with total anomalous (abnormal) pulmonary venous drainage usually develop problems within the first few weeks or months and thus require cardiac surgery. Partial forms of anomalous pulmonary venous return, in which only one or two pulmonary veins are connected abnormally, may have few symptoms, although surgical correction may be done if required.

Anomalies of the venae cavae (vena cava)
      The most common abnormalities of the venae cavae (vena cava), the major veins returning venous blood to the right side of the heart, are a persistent left superior vena cava (normally there is only one superior vena cava opening to the right side of the heart) and an abnormal connection of the inferior vena cava to the heart. These abnormalities are frequently associated with intracardiac structural faults.

Michael Francis Oliver Michael James Godman

Acquired heart disease
      Acquired heart diseases are conditions affecting the heart and its associated blood vessels (blood vessel) that develop during a person's lifetime, in contrast to congenital heart diseases (congenital heart disease), which are present at birth. Acquired heart diseases include coronary artery disease, coronary heart disease, rheumatic heart disease, diseases of the pulmonary (pulmonary circulation) vessels and the aorta, diseases of the tissues of the heart, and diseases of the heart valves. For more information about diseases of the major arteries, including atherosclerosis, see the section Diseases of the arteries (cardiovascular disease). For more information about surgical procedures used to treat diseases of the heart, see the section Surgical treatment of the heart (cardiovascular disease).

 The term coronary artery disease describes the diseases that lead to obstruction of the flow of blood in the vessels that supply the heart. These diseases can occur in other arteries as well. Coronary artery disease is commonly used as a synonym for the more specific condition of atheromatous intrusion into the artery lumen (cavity). Coronary heart disease is a term used to describe the symptoms and features that can result from advanced coronary artery disease (see below). The same symptoms are also diagnosed as ischemic heart disease, because the symptoms result from the development of myocardial ischemia (reduced blood flow to the heart muscle). There is no one-to-one relationship between coronary atherosclerosis and the clinical symptoms of coronary artery disease or between coronary artery disease and coronary heart disease.

      Coronary artery disease due to atherosclerosis is present to varying degrees in all adults in industrialized nations. The symptoms of the disease, however, will occur only when the extent of the lesions or the development of acute thrombosis (the formation of a blood clot which blocks a coronary artery) reduces the flow of blood to the heart muscle below a critical level. One or more major coronary arteries may progressively narrow without leading to any symptoms of coronary heart disease, provided the area of the heart muscle supplied by that artery is adequately supplied with blood from another coronary artery circuit. The small coronary arteries anastomose (interconnect) and are not, as previously thought, end arteries. Thus, they can open up and provide a collateral, or supportive, circulation that protects against progressive occlusion (obstruction). Exercise improves coronary collateral flow and for this reason may protect against coronary heart disease.

      Although coronary artery disease is most frequently caused by atherosclerosis, inflammation of the blood vessels may, in rare cases, cause obstructive lesions of the coronary vessels. In persons with familial hypercholesterolemia (genetically inherited high cholesterol), the disease may involve the mouth of the coronary vessels as they leave the aorta and cause an obstruction to blood flow. On rare occasions, clots arising from the left atrium or left ventricle may enter the coronary vessels and cause acute obstruction and symptoms of disease.

      There are influences, or “triggers,” that convert coronary artery disease into coronary heart disease; these include coronary thrombosis (formation of blood clots), coronary spasm, and the hemodynamic (blood-flow) needs of the heart muscle. Influences within the heart muscle itself also may increase the demand for blood flow above the level available, making the myocardium vulnerable to alterations in function, contractility, and the maintenance of normal rhythm.

      Coronary heart disease is a general term for a number of syndromes. Ischemic heart disease, an alternative term, is actually more correct because the syndromes described are all to some degree manifestations of myocardial ischemia (a lack of blood supply to the myocardium, or heart muscle).

      Coronary heart disease includes a number of interdependent syndromes: angina pectoris, acute myocardial infarction (death of some tissue of the heart muscle because of reduced blood supply), and sudden cardiac death (due to lethal arrhythmia—that is, irregular heart rhythm). There are also features of coronary occlusion (blockage of a coronary artery) that indicate the presence of myocardial ischemia. Knowledge of the mechanisms that lead to a particular syndrome is inexact. Thus, a coronary thrombosis may lead to myocardial infarction in one person, sudden death in another, a minor episode of angina in a third, or no symptoms at all in a fourth. There is, however, no alternative to using the orthodox syndromes as the means of recognizing and recording the incidence of coronary heart disease.

      Coronary heart disease is the leading cause of death worldwide, although its occurrence is unevenly distributed. It is the most common single cause of death in North America and Europe; the only region in which another cause of death leads heart disease is Africa. The disease was once relatively uncommon in Asia (including China, Japan, India, and the Middle East), central Africa, and Central and South America. As Western diets become more prevalent in these countries, however, the incidence of heart disease rises accordingly. Thus, although rates for heart disease were once low all over Asia and are still low in Japan, the incidence of heart disease in China is increasing steadily. Studies link the geographic differences in coronary heart disease with diet and with various aspects of lifestyle, such as cigarette smoking, physical inactivity, and obesity.

Risk factors
      Three main risk factors have been identified: cigarette smoking, a high level of cholesterol in the blood (hypercholesterolemia), and high blood pressure ( hypertension). Important as these risk factors are, they are found only in about one-half of those who experience heart attacks. The proportion of persons with any or all of these three risk factors is greater in young and middle-aged adults than in older adults. It is impossible to incriminate any one of these risk factors over another, since the manifestations of coronary heart disease are undoubtedly due to many independent and interdependent influences, but the coexistence of the three greatly increases the risk of developing the disease.

      The familial predisposition to the disease is not well understood, although it is stronger in families with hypercholesterolemia and hypertension. It is most likely to develop prematurely in the presence of familial (genetic) hypercholesterolemia. There is a progressive relationship between serum cholesterol concentrations and the incidence of coronary heart disease. This is also true for hypertension. Of the three major risk factors, however, excessive cigarette smoking is probably the most important. Other influences—such as a predisposition to develop thrombosis, diabetes mellitus, physical inactivity, obesity, and, rarely, oral contraceptives—may induce premature coronary heart disease in susceptible persons.

      The term angina pectoris was first used in 1772 by the British physician William Heberden when he wrote:

There is a disorder of the breast.…The seat of it, and sense of strangling and anxiety, with which it is attended, may make it not improperly be called angina pectoris. Those, who are afflicted with it, are ceased [sic] while they are walking and most particularly when they walk soon after eating, with a painful (pain) and most disagreeable sensation in the breast, which seems as it would take their breath away, if it were to increase or to continue; the moment they stand still, all this uneasiness vanishes.

      Heberden's initial description is still accurate; however, there are no truly characteristic symptoms of angina pectoris. Whereas the chest discomfort may be variously described as “constricting,” “suffocating,” “crushing,” “heavy,” or “squeezing,” there are many patients in whom the quality of the sensation is imprecise. The discomfort is usually, but not always, behind the breastbone, but pain radiating to the throat or jaw or down the inner sides of either arm is common. There may be no physical abnormalities, and an electrocardiogram may be normal or show only transient changes with exercise.

      Coronary arteriography assesses the extent of coronary artery occlusion (blockage), which may vary from a small increase in coronary artery muscle tone at a partly blocked site in a branch of one of the three main coronary arteries to a 90 percent or greater blockage of the left main coronary artery with involvement of other major coronary arteries. But the extent of coronary artery disease revealed by coronary arteriography does not predicate action or treatment.

      The myocardial ischemia (reduced blood supply to the heart muscle) that causes angina is due to a disturbance of the balance between heart muscle demands and supply. If demands are reduced sufficiently, the temporarily endangered supply may be adequate. The disturbance of the equilibrium may be short lived and may correct itself. Unstable angina has an appreciably worse prognosis than stable angina because of a higher risk of myocardial infarction (tissue death of a piece of the heart muscle) and sudden cardiac death, and it requires daily observation and active intervention.

      When coronary arteriography reveals relatively isolated, incompletely obstructive lesions, there are two alternative treatments—medication or coronary angioplasty (balloon dilation of the localized obstruction by a special catheter). When coronary arteriography reveals a severe blockage of the left main coronary artery or proximally in one or more of the major arteries, coronary artery bypass graft surgery may be necessary.

      In unstable angina pectoris, coronary arteriography may help determine whether coronary angioplasty or coronary artery bypass surgery is needed. Drugs that cause coronary dilation and peripheral arterial vasodilation (dilation of blood vessels) and that reduce the load on the heart are usually necessary. Drugs that reduce the work of the heart by blocking adrenoreceptors (receptors in the heart that respond to epinephrine (epinephrine and norepinephrine)) and drugs that reduce a patient's tendency to form blood clots are given at this stage. For patients with stable angina, drugs that reduce the heart's work are administered.

      A syndrome of prolonged, severe chest pain was first described in medical literature in 1912 by James Bryan Herrick (Herrick, James Bryan), who attributed the syndrome to coronary thrombosis, the development of a clot in a major blood vessel serving the heart. As a result, the disorder was termed coronary thrombosis or coronary occlusion (blockage of a coronary artery). Later evidence indicated, however, that, though thrombotic occlusion of an atheromatous lesion in a coronary artery is the most common cause of the disorder, the manifestations are the result of the death of an area of heart muscle (infarction). The term myocardial infarction, therefore, is more appropriate. The less specific term heart attack may be more desirable because of these difficulties in describing the causation of the disease entity.

      Myocardial infarction is characterized by cellular death ( necrosis) of a segment of the heart muscle. Generally, it involves an area in the forward wall of the heart related to the blood distribution of the anterior descending coronary artery, though in other instances the inferior wall or the septum (partition) of the ventricle is involved. A blocked coronary artery is present in a majority of the hearts examined at autopsy and undoubtedly plays an important role. In some instances, changes in metabolic demands of the heart muscle in the presence of a restricted blood flow may be enough to cause the death of blood-deprived cells.

      The outstanding clinical feature of myocardial infarction is pain, similar in many respects to that of angina pectoris. The important difference is that the pain lasts for a much longer period—at least half an hour and usually for several hours and perhaps for days. The pain is described as “crushing,” “compressing,” and “like a vise” and is often associated with some difficulty in breathing. As with angina pectoris, the pain may radiate to the left arm or up the neck into the jaw. There is often nausea, vomiting, and weakness. Fainting ( syncope) may occur. The affected person is frequently pale and may perspire profusely. Infrequently, these symptoms may be absent, and the occurrence of infarction can then be detected only by laboratory tests. Laboratory studies may show an elevation of the number of white blood cells in the blood or a rise in the enzyme content of the blood, indicating leakage from damaged heart muscle cells. The electrocardiogram in most instances shows distinct and characteristic abnormalities at the onset, but the electrocardiographic abnormalities may be less characteristic or totally absent.

      In most persons who experience an acute myocardial infarction, the circulation remains adequate, and only by subtle evidence such as rales (abnormal respiratory sounds) in the lungs or a gallop rhythm of the heartbeat may the evidence of some minor degree of heart failure be detected. In a small percentage of cases, the state of shock occurs, with pallor, coolness of the hands and feet, low blood pressure, and rapid heart action. In these cases myocardial infarction is deadly, with low survival rates. Mortality is also related to age, for the process is more lethal in the elderly. In a small number of persons there may be thromboembolism (obstruction caused by a clot that has broken loose from its site of formation) into an artery elsewhere in the body.

      In some individuals the damage caused by the infarction may interfere with the functioning of the mitral valve, the valve between the left upper and lower chambers, and result in a form of valvular heart disease. It may cause a rupture of the interventricular septum, the partition between the left and right ventricles, with the development of a ventricular septal defect, such as is seen in some forms of congenital heart disease. Rupture of the ventricle also may occur.

      Drugs are used to control arrhythmias (arrhythmia) and to strengthen the heart muscle. Convalescence from an acute myocardial infarction may last several weeks, allowing time for scar tissue to form in the area of an infarction and for a gradual return to activity. Although some persons may have residual evidence of heart failure or other cardiac malfunction, most individuals may return to an active lifestyle after a period of weeks and are not in any way invalided by the process. These individuals do, however, have an increased potential for subsequent myocardial infarction.

      The term sudden death is used imprecisely and includes death that is almost instantaneous as well as death in which rapidly deteriorating disease processes may occupy as much as two or three days. In heart disease both may occur, but the term characteristically refers to instantaneous death, which is frequent in coronary heart disease. Sudden death from coronary heart disease occurs so frequently that less than half of the persons who die from heart attacks each year in the United States survive long enough to reach the hospital.

      Instantaneous cardiac (arrhythmia) death is usually due to ventricular fibrillation (an uncontrolled and uncoordinated twitching of the ventricle muscle), with total mechanical inadequacy of the heart and erratic and ineffective electrical activity. Sudden death may occur without any previous manifestations of coronary heart disease. It may occur in the course of angina pectoris and causes about one-half of the deaths due to acute myocardial infarction in hospitalized patients, though this number is decreasing with the more widespread use of coronary care units. Although a reduced supply of blood to the heart undoubtedly is the precipitating factor, acute myocardial infarction does not always occur. In most persons who have died almost instantaneously, no infarction was present, but there was widespread coronary artery disease. In rare instances sudden death occurs without a major degree of coronary artery disease.

      The use of cardiopulmonary resuscitation (CPR) coupled with electrical defibrillation (the use of electrical shocks), if applied within a few minutes of the sudden death episode, may successfully resuscitate the majority of patients. In coronary care units, where the facilities and trained personnel are immediately available, the percentage of successful resuscitations is high. In general hospitals where resuscitation teams have been established, the percentage is less satisfactory. Sudden death usually occurs outside the hospital, of course, and thus presents a more difficult problem. Mobile coronary care units responding as emergency ambulances improve a patient's chance of survival considerably, but effective resuscitation depends upon the prompt arrival of the unit. The use of drugs and other means to prevent the onset of sudden death has been relatively successful in the coronary care unit, except in situations in which the disease has been present for a long period of time.

Survival during and after a heart attack
      The risk of death from an arrhythmia is greatest within the first few minutes of the onset of a blockage in a coronary artery or of acute ischemia (reduction of blood flow) occurring in the region of the heart muscle. Thus, of those likely to die during the first two weeks after a major heart attack, nearly half will die within one hour of the onset of symptoms.

      During the first few hours most persons have some disturbances of rhythm and conduction. Ventricular fibrillation is particularly common in the first two hours, and its incidence decreases rapidly during the next 10 to 12 hours. If undetected, ventricular fibrillation is lethal. Once the patient has reached the hospital, fibrillation can be reversed in 80 to 90 percent of patients with the use of appropriate electronic devices for monitoring heart rhythm, for giving a direct-current shock to stop it, and for resuscitation. Given that the vast majority of heart attacks occur at home, treatment that allows for intervention in the first critical minutes holds the greatest hope of increasing survival rates. CPR training targeted to people that are most likely to witness a heart attack and the availability of automatic external defibrillators for use in a home setting by people with no medical training are common means of saving heart attack sufferers.

      Both the immediate and the long-term outlook of persons after myocardial infarction depends on the extent of myocardial damage and the influence of this damage on cardiac function. Efforts to limit or reduce the size of the infarct have been unsuccessful in improving the short- or long-term outlook. Procedures that cause thrombi (clots) to dissolve (thrombolysis), however, have led to the dramatic and immediate opening of apparently blocked coronary arteries. When such measures are implemented within four hours (and preferably within one hour) of the onset of a heart attack, the chances of survival are greater and the long-term prognosis is improved. Naturally occurring lytic enzymes (such as streptokinase) and genetically engineered (genetic engineering) products are used, as is aspirin.

  coronary artery bypass surgery is used to restore adequate blood flow to the heart muscle beyond severe atheromatous obstruction in the main coronary arteries (coronary artery). The most common operation is one in which lengths of superficial veins (vein) are taken from the legs and inserted between the aorta and joined to a part of a coronary artery below the obstructive atheromatous lesion. Multiple grafts are often used for multiple atheromatous occlusions. The internal mammary arteries are also used to provide a new blood supply beyond the point of arterial obstruction; however, since there are only two internal mammary arteries, their use is limited.

      There are two principal uses for coronary artery bypass surgery. One is to relieve chest angina that is resistant to medication. The other is to prolong a person's life; however, this is only achieved when all three main coronary arteries are severely obstructed and when the contractility of the left ventricle has been impaired somewhat. Coronary artery bypass surgery does not prolong life when it is used to overcome an obstruction in only one or even two arteries. As a nonsurgical option, coronary angioplasty is also used to unblock arteries.

Prevention of coronary heart disease
      To prevent heart disease, physicians recommend that patients quit smoking; eat a diet in which about 30 percent of the calories (calorie) come from fat, choosing polyunsaturated fats and avoiding saturated fat and trans fat; reduce high blood pressure; increase physical activity; and maintain a weight within normal limits. Although the circumstantial evidence from many kinds of studies supporting these measures is impressive, not all these measures have been shown to be as effective as expected or predicted. Quitting smoking does lower the risk of cardiovascular disease; within a few years of quitting, patients show a risk factor for heart disease nearly equal to that of people who have never smoked. People with familial hypercholesterolemia (high cholesterol) benefit greatly from reduction of high levels of serum cholesterol. Rather surprisingly, studies suggest that even people who have borderline high cholesterol benefit from drugs that lower cholesterol. Results from studies in which participants modify their diet have had unexpected results, however, in that a low-fat diet does not seem to lead to reduced coronary risk. More studies on the effect of diet on heart disease are needed. And, although lowering blood cholesterol does have a great impact on heart disease, reducing high blood pressure has not been shown to lower coronary mortality as significantly.

Rheumatic heart disease
      Rheumatic heart disease results from inflammation of the endocardium (heart lining), myocardium (heart muscle), and pericardium (the sac that surrounds the heart) that occurs during acute rheumatic fever, an infection with Streptococcus pyogenes organisms. The disease includes those later developments that persist after the acute process has subsided and that may result in damage to a valve, which may in turn lead to heart failure.

      Rheumatic fever is poorly understood. The disease process occurs days or weeks following the initial streptococcal (Streptococcus) infection. Later infections may bring about recurrences of rheumatic fever that damage the heart. Immunologic processes (reactions to a foreign protein) are thought to be responsible for the response that damages the heart and particularly the heart valves. Rapid and effective treatment or prevention of streptococcal infections stops the acute process.

      Many other factors of a geographic, economic, and climatic nature influence the incidence of rheumatic fever but are not the primary causes. Rheumatic fever became less common in the second half of the 20th century, and, with better control of streptococcal infections, there is an indication of a sharp decline in rheumatic heart disease.

      It is thought that the basic pathologic lesion involves inflammatory (inflammation) changes in the collagen, the main supportive protein of the connective tissue. There is also inflammation of the endocardium (endocarditis) and the pericardium (pericarditis). Only a relatively small percentage of deaths occur in the acute phase, with evidence of overwhelming inflammation associated with acute heart failure. There may be a disturbance of the conduction system of the heart and involvement of other tissues of the body, particularly the joints (joint). About one-half of the persons found to have late rheumatic valvular disease give some indication that they have had acute rheumatic fever.

      The major toll of rheumatic fever is in the deformity of the heart valves created by the initial attack or by frequently repeated attacks of the acute illness. Although there may be valve involvement in the acute stages, it usually requires several years before valve defects become manifest as the cause of heart malfunction. The valve most frequently affected is the mitral valve, less commonly the aortic valve, and least common of all, the tricuspid valve. The lesion may cause either insufficiency of the valve, preventing it from operating in a normal fashion and leading to regurgitation, or stenosis (narrowing) of the valve, preventing a normal flow of blood and adding to the burden of the heart.

      Mitral valve involvement is usually symptomless initially but may lead to left ventricular failure with shortness of breath. Heart murmurs are reasonably accurate signposts for specific valvular diagnoses. A murmur during the diastolic (diastole), or resting, phase of the heart, when blood normally flows through the mitral valve to fill the ventricle, generally indicates the presence of mitral stenosis. On the other hand, a murmur during systole, or contraction, of the left ventricle, indicates an abnormal flow of blood back through the mitral valve and into the left atrium (mitral regurgitation). When this latter condition is present, each beat of the heart must pump enough blood to supply the body as well as the wasted reflux into the pulmonary vascular system (pulmonary circulation). This additional workload causes dilation and enlargement of the ventricle and leads to the development of congestive heart failure.

      Involvement of the aortic valve is common, and again there may be evidence of stenosis or insufficiency. The presence of aortic stenosis may lead to a marked hypertrophy (enlargement) of the left ventricle of the heart. Involvement of either the tricuspid or pulmonic valve occurs in a similar fashion. In many persons with rheumatic valvular disease, more than one valve is involved. The specific type of valve involved influences the clinical picture of congestive failure.

The heart, the pulmonary artery, and the aorta
      In various lung diseases an obstruction to blood flow through the network of vessels in the lungs (lung) develops. This places a burden on the right side of the heart, which normally pumps against a low-pressure load with little resistance to blood flow. Pulmonary-artery pressures are normally low compared with those in the aorta.

      Pulmonary heart disease may be divided into acute and chronic forms. The classic form of acute pulmonary heart disease (acute cor pulmonale) occurs when there is a sudden obstruction to the pulmonary blood-flow pattern, as occurs with a massive embolus (embolism)—a blood clot that has broken loose from its point of formation. This impairs blood flow through the lungs, causes additional reflex changes that add to the heart's burden, and creates an acute form of high blood pressure in the pulmonary artery, with dilation and failure of the right ventricle. The right ventricle's pumping ability is acutely reduced, and, therefore, the amount of blood available for the left side of the heart is also restricted, so that systemic circulatory failure occurs.

      Respiratory symptoms are not prominent, and the disorder in its early stages is not accompanied by edema (the accumulation of excess fluid) in the lung. The clinical picture in the more severe form is one of shock, with cold, pale, and clammy skin, low arterial pressure, and a high pulse rate. oxygen transfer in the lungs is severely impaired, and the heart may be acutely dilated. Treatment is with anticoagulant drugs (anticoagulant) (such as streptokinase) and oxygen, which relieve the hypoxia (low serum oxygen levels), or, in some instances, surgical removal of the obstruction.

      Chronic cor pulmonale may be caused by a form of pulmonary disease—such as chronic bronchitis or emphysema—in which lung tissue is destroyed and replaced with air spaces, causing a loss of pulmonary blood vessels, or it may be caused by multiple blood clots in the vessels of the lung or by a primary disorder of the pulmonary blood vessels. The result is a form of heart failure partly based on an obstruction to blood flow through the pulmonary vessels, producing high blood pressure in the pulmonary artery. Cyanosis (bluish discoloration of the skin) may be evident, indicating that the arterial blood is not saturated with oxygen. In patients with chronic bronchitis and emphysema, the lack of oxygen contributes to pulmonary hypertension. The manifestations of heart failure are present—particularly where there is edema—except that shortness of breath is often due to the underlying lung disease. The right side of the heart is enlarged, the valve sounds from the pulmonic valve may be loud, and there may be electrocardiographic evidence of chronic strain on the right side of the heart. Drugs that dilate the pulmonary blood vessels or relieve the edema and drugs with anticoagulant effects can be useful in the treatment of chronic pulmonary heart disease. However, the course that affords the best chance of improvement in patients with cor pulmonale due to chronic bronchitis and emphysema includes prompt treatment of infection, termination of smoking, and correction of the lack of oxygen.

Hypertensive heart disease
      Arterial hypertension is a disease in which the regulation of blood pressure is abnormal, resulting in arterial pressure that is chronically higher than normal. Hypertension results from several causes, but the cause of the most common form (essential hypertension) is not understood. A family tendency to hypertension has been found in persons with the disease, and there may be a basic genetic (genetics, human) abnormality involving the permeability of cell membrane in the blood vessels. This defect might make such persons less able to tolerate salt and in turn more responsive to hormonal (hormone) or nervous (nervous system, human) stimulation.

      Excessive dietary intake of salt has long been held to be responsible for hypertension in certain people. stress has also been shown to cause hypertension, and fear and anxiety can induce a rise in blood pressure owing to increased activity in the sympathetic nervous system. Hormones and other vasoactive substances (substances that relax or contract the blood vessels) have a direct effect on blood pressure, but the interaction of these factors remains unclear. Hypertension also results from a number of types of chronic renal (kidney) diseases (renal system disease) and from some tumours of the adrenal gland. In certain structural abnormalities of the aorta, such as coarctation (aorta, coarctation of the), in which the artery's middle coat is deformed with resultant narrowing of the channel, arterial pressure in the upper half of the body is abnormally high.

      Regardless of the cause but in some ways coloured by it, the effects on the cardiovascular system (human cardiovascular system) are similar. The impact on the vascular system varies from person to person. In some persons, for unknown reasons, the body withstands the abnormal elevation of blood pressure with minimum change in the heart and blood vessels. In other persons, blood vessel damage is early and severe, coupled with serious deterioration of heart function. In general, the rule is that the higher the blood pressure, the higher the degree of cardiovascular damage, though there are many exceptions. Rarely, a vicious and damaging form of hypertension occurs, often called malignant hypertension, that results in damage to small blood vessels throughout the body but particularly affecting the heart, brain, and kidneys (kidney).

      People with hypertensive disease have an increased susceptibility to atherosclerosis of the coronary arteries, thus making it difficult to separate the cardiac manifestations from those actually caused by hypertension. Hypertensive people, therefore, may eventually have congestive heart failure following enlargement of the heart caused by the chronic increase in arterial pressure. In addition they may suffer the effects of a decline in blood supply to the heart because of coronary artery disease and the classic manifestations of coronary arteriosclerosis, such as angina pectoris or myocardial infarction. Hypertensive cardiovascular disease may also become manifest through defects in the vessels supplying the brain, leading to stroke. Furthermore, hypertensive cardiovascular manifestations may be complicated by the development of kidney failure and the resultant abnormal retention of fluid in the tissues, adding to the problems of congestive heart failure.

      Before the use of antihypertensive drugs, high blood pressure was associated with a greatly increased mortality, with survival measured in months in the most severe cases. Antihypertensive drugs have dramatically increased the life expectancy of patients with severe hypertension; stroke and kidney failure are now relatively uncommon in treated hypertensive patients. The reduction in coronary heart disease among this group of patients, however, has not been as substantial. Other factors, such as smoking and diet, are important in this aspect of therapy.

Other diseases of the aorta and the pulmonary artery
       arteriosclerosis may involve the aorta and its major branches. Indeed, it seems to be an almost inevitable process with increasing age, but the rate of development and the extent of involvement vary greatly. The process may merely limit the elasticity of the aorta and allow for some dilation and increased complexity of the path of the blood flow as age advances. In more severe instances, there may be a major degree of dilation or localized formation of aneurysms (aneurysm) (bulging of the vessel wall at a point of weakness), generally in the abdominal portion of the aorta. These aneurysms may result in pain and may occasionally rupture, causing sudden death. The arteriosclerotic process may impair the flow of blood to the tributaries of the aorta and lead to a variety of ischemic states—i.e., result in various types of damage that come from an insufficient supply of blood. This condition is particularly notable when the renal (renal artery) vessels are involved, creating a state of renal ischemia, occasionally creating hypertension, and possibly terminating in renal failure.

      Medial necrosis is a lesion of the aorta in which the media (the middle coat of the artery) deteriorates, and, in association with arteriosclerosis and often hypertension, it may lead to a dissecting aneurysm. In a dissecting aneurysm a rupture in the intima, the innermost coat of the artery, permits blood to enter the wall of the aorta, causing separation of the layers of the wall. Obstruction to tributaries may occur, which is usually associated with severe chest pain. In many instances there is a secondary rupture of the exterior wall, which may lead to fatal internal bleeding. The aortic wall may become inflamed as an isolated process.

       calcium salts that deposit in the aorta wall may occur as a part of the arteriosclerotic process or of other disease involvement. In certain conditions, such as congenital heart disease, blood clots (thrombi) may form in the pulmonary (pulmonary embolism) artery, and these may break loose. Blood clots in the lungs (pulmonary emboli (pulmonary embolism)) may arise from this and other sources in the systemic venous circulation. These fragments of clot may be small, causing no detectable manifestations, or large, causing obstruction of either the total pulmonary arterial flow or of flow to an area of lung.

syphilis of the heart and aorta
       syphilis, a disease caused by infection with the microorganism Treponema pallidum, is widespread in its early stages, affecting the entire body. During this initial phase there may be transient inflammation of the heart muscle, but usually with little or no impairment of the circulation. In the late stages of the disease, there may be syphilitic involvement of the heart, confined almost purely to the aorta and aortic valve. A particularly severe form of aortic insufficiency may develop, with subsequent dilation and enlargement of the heart and, eventually, heart failure. The disease process often involves the base of the aorta and the blood flow through the openings into the coronary vessels from the aorta, causing impairment of the coronary circulation, with resultant angina pectoris and, on rare occasions, myocardial infarction, the death of portions of heart muscle.

      The syphilitic process may also involve the wall of the aorta; the result is the loss of the aorta's elastic properties, the dilation of the aorta, and, at times, the formation of aneurysms of the aorta. The aneurysms may become large and interfere with blood flow through the aortic tributaries in the involved area. They may be the source of pain and eventually may rupture, causing sudden death from loss of blood into the heart cavity. Syphilis of the aorta was common in the past, but, with the advent of more-modern control mechanisms, plus effective early treatment with the use of penicillin, the disorder has become much less common. Late complications can be effectively avoided with early antisyphilitic treatment.

Diseases of the endocardium and valves
      Bacterial (bacteria) endocarditis—a disease in which bacterial (bacteria) or fungal (fungus) infection becomes established on the surface of a heart valve or, less commonly, in a blood vessel wall or in the endocardium (inner lining) of the heart—usually occurs where there has been some previous lesion, either congenital or acquired. Most frequently the location is at the line of closure of the valve. The disease may be acute and severe, or it may be chronic, often referred to as subacute bacterial endocarditis. It may erode the valve structure, or it may be of an inflammatory nature, producing nodules with the ulcerative surface of active infection. Because the bacteria are embedded in the lesion, the blood's normal immune (immune system) defenses have difficulty entering into play; for this reason, certain types of bacterial endocarditis become more chronic and more slowly progressive. The effects of the lesion are complex, being related to the presence of a bacterial infection in the body, local damage to the valve, and systemic damage caused by fragments of a blood clot that breaks off and travels through the bloodstream to distant organs. These clots cause infarctions or abscesses (abscess), a type of kidney disease, and other small areas of bleeding and necrosis in the skin, eyes (eye, human), and other parts of the body.

      Before the advent of antibiotic therapy, bacterial endocarditis was almost always a fatal disease. Many affected persons can now be successfully treated, given the best conditions, though the mortality rate still remains relatively high. Inflammation of the heart lining, such as endocarditis that is not caused by infection, may occur in some illnesses, but it does not result in the formation and breaking loose of blood clots.

      In the course of rheumatoid arthritis, a chronic inflammation of the joints of unknown cause, a type of valvular damage has been recognized. It is different from that caused by rheumatic fever but leads to valvular insufficiency and stenosis (narrowing) in much the same fashion and is particularly likely to attack the aortic valve. The tendencies toward heart failure and toward impairment of heart function are the same as in rheumatic valvular disease.

Diseases of the myocardium
      There has been increasing recognition of a type of heart disease characterized as primary myocardial disease. The cardiomyopathies (cardiomyopathy) are diseases involving the myocardium (heart muscle) itself. They are unique in that they are not the result of hypertensive, congenital, valvular, or pericardial diseases and are rarely the result of ischemic heart disease. This form of heart disease is often sufficiently distinctive, both in general symptoms and in patterns of blood flow, to allow a diagnosis to be made. Increasing awareness of the condition, along with improved diagnostic techniques, has shown that cardiomyopathy is a major cause of morbidity and mortality. In some areas of the world, it may account for as many as 30 percent of all deaths due to heart disease.

      Some cardiomyopathies are primary; i.e., the basic disease involves the myocardium rather than other heart structures, and the cause of the disease is not known and not part of a disorder of other organs. In other cardiomyopathies the cause of the myocardial abnormality is known, and the cardiomyopathy is a manifestation of a systemic disease process. Clinically, the cardiomyopathies fall into three categories: dilated cardiomyopathy, characterized by ventricular dilation and often by symptoms of congestive heart failure; hypertrophic cardiomyopathy, characterized by hypertrophy of the ventricle, particularly the left ventricle; and restrictive cardiomyopathy, marked by scarring of the ventricle and impairment of filling in diastole.

      A large number of cardiomyopathies are apparently not related to an infectious process but are not well understood. A number of these are congenital and many cause enlargement of the heart. About one-third of these diseases are familial, and some of these are transmitted as a non-sex-linked autosomal dominant trait (dominance) (i.e., a person may be affected if he inherits the tendency from one parent). They are particularly common among African Americans. A number of metabolic diseases (metabolic disease) associated with endocrine (endocrine system, human) disorders may also cause cardiomyopathies. Other metabolic disorders that may contribute to cardiomyopathy include beriberi, caused by a nutritional deficiency, and a form of cardiomyopathy seen in chronic alcoholics (alcoholism). Cardiomyopathies can also be caused by cobalt poisoning, which is sometimes seen in workers exposed to pigments. There are also rare cardiomyopathies caused by drugs. Infections, such as acute rheumatic fever and several viral (virus) infections, may cause any of a number of types of myocarditis. Myocarditis may also occur as a manifestation of a generalized hypersensitivity (allergic (allergy) or immunologic) reaction throughout the body.

      The cardiomyopathies may cause no symptoms and may be detected only by evidence of an enlarged heart and disturbances in cardiac conduction mechanisms detected with an electrocardiography. In other instances, extensive involvement may lead to heart failure. Some cases may be chronic, with exacerbations and remissions over a period of years.

      The heart may be affected by any of a considerable number of collagen diseases (connective tissue disease). collagen is the principal connective-tissue protein, and collagen diseases are diseases of the connective tissues (connective tissue disease). They include diseases primarily of the joints (e.g., rheumatoid arthritis) and the skin (e.g., scleroderma), as well as systemic diseases (e.g., systemic lupus erythematosus).

Diseases of the pericardium
      Pericardial disease (pericarditis) may occur as an isolated process or as a subordinate and unsuspected manifestation of a disease elsewhere in the body. Acute pericarditis—inflammation of the pericardium (the sac that surrounds the heart)—may result from invasion of the pericardium by one of a number of agents (viral, fungal, protozoal (protozoan)), as a manifestation of certain connective-tissue and allergic diseases, or as a result of chemical or metabolic disturbances. cancer and specific injury to the pericardium are also potential causes of pericardial disease.

       pain is the most common symptom in acute pericarditis, though pericarditis may occur without pain. A characteristic sound, called friction rub, and characteristic electrocardiographic findings are factors in diagnosis. Acute pericarditis may be accompanied by an outpouring of fluid into the pericardial sac. The presence of pericardial fluid in excessive amounts may enlarge the silhouette of the heart in X-rays (X-ray) but not impair its function. If the pericardial fluid accumulates rapidly or in great amounts, if there is a hemorrhage into the sac, or if the pericardium is diseased so that it does not expand, the heart is compressed, a state called cardiac tamponade. There is interference with the heart's ability to fill with blood and reduction of cardiac output. In its more severe form, cardiac tamponade causes a shocklike state that may be lethal. Removal of the fluid is lifesaving in an emergency and aids in the identification of the cause.

      Chronic constrictive pericarditis, caused by scar tissue in the pericardium, restricts the activity of the ventricles (ventricle). In many instances the cause is not known, but in some it is the result of tuberculosis or other specific infections. It is treated most effectively by surgery. Tumours (tumour) that either arise directly from the pericardium or are secondary growths from other sources may impair cardiac function and cause pericardial effusion (escape of fluid into the pericardium).

James V. Warren Michael Francis Oliver

Disturbances in rhythm and conduction

Determinants of cardiac rhythm
 The cardiac muscle cell is a type of “excitable” cell, meaning that it is capable of conducting electrical impulses that stimulate the heart muscle to contract. Excitable cells, which also include neurons (neuron) and muscle cells, possess a unique ability to sense differences in voltage across their cell membrane. This transmembrane voltage gradient arises from the presence of ion-specific voltage-sensitive channels that are made up of proteins (protein) and are embedded in the lipid layers of the cell membrane. As their name implies, voltage-sensitive channels respond to changes in voltage (excitation) that lead to depolarization of the cell. When a cell is excited, each channel opens and transports specific ions (i.e., potassium [K], sodium [Na], calcium [Ca], and chloride [Cl]) from one side of the membrane to the other, often exchanging one ion species for a different ion species (i.e., the Na+/K+ ATPase channel transports three sodium ions out in exchange for two potassium ions pumped into the cell). Ion exchange is required for depolarization, reestablishing intracellular homeostasis, and cell repolarization.

      Once the cell returns to its resting state (periods of time between electrical impulses when the cell is repolarized), voltage-sensitive channels close, and the cell is ready to receive another impulse. Cardiac cells at rest are fully repolarized when the intracellular environment reaches a specific negative charge (approximately –90 millivolts) relative to the extracellular environment (approximately 0 millivolt). The cycle of depolarization and repolarization in the heart is known as the cardiac action potential and occurs approximately 60 times every minute. In addition, cardiac muscle cells are unique from other types of excitable cells in that they remain permeable to potassium in the resting state. This facilitates the intracellular response to depolarization and, in combination with other potassium channels, ensures proper duration between and during action potentials.

 Normal cardiac muscle cells do not spontaneously depolarize. For this reason, cardiac rhythm is dependent upon specialized conduction cells, called pacemaker cells, to generate the initiating impulse for depolarization. These cells contain a complement of channels that aid in the generation of a rhythmic, spontaneous depolarization that initiates excitation. In healthy individuals, heart rate (impulse generation) is controlled by the pacemaker cells of the sinoatrial node. Under pathological conditions, and with some pharmacological interventions, other pacemakers elsewhere in the heart may become dominant. The rate at which the sinoatrial node produces electrical impulses is determined by the autonomic nervous system. As a result, heart rate increases in response to increased sympathetic nervous system activity, which is also associated with conditions that require increased cardiac output (i.e., exercise or fear). In contrast, the parasympathetic nervous system slows heart rate.

 Once the electrical impulse is generated in the sinoatrial node, it is propagated rapidly throughout the heart. Specialized connections between conduction cells in the heart allow the electrical impulse to travel rapidly from the atria (atrium) to the atrioventricular node and bundle of His (known as the atrioventricular junctional tissue), through the bundle branches and Purkinje fibres (known as the ventricular (ventricle) conduction system), and into the ventricular muscle cells that ultimately generate cardiac output. The conduction system in the atria is poorly defined but clearly designed to initiate atrial depolarization, as well as to propagate the impulse toward the ventricle. The atrioventricular node and bundle of His represent important supraventricular control points in the heart that distribute impulses to the ventricles via the right and left bundle branches. The impulse proceeds through the ventricular conduction system and into specialized conduction tissue in the subendocardial (innermost) layer of the ventricle. This tissue propagates impulses that travel from the inner wall to the outer wall of the heart. The atrioventricular node is also under autonomic control, through which sympathetic stimulation facilitates conduction and parasympathetic stimulation slows conduction. Abnormalities in this conduction system often create cardiac rhythm disturbances.

Premature contractions
      While vulnerable to pathological, physiological, and pharmacological stressors, cardiac rhythm control is remarkably constant and robust. Many people develop abnormalities in this system that have little pathological consequence. While the sinoatrial node pacemaker is dominant, occasional spontaneous premature beats may arise anywhere in the conduction system. Depending on their origin, they are described as premature atrial contractions, premature nodal contractions, or premature ventricular contractions. They typically do not interfere with normal cardiovascular (human cardiovascular system) function and are seen more frequently under circumstances of increased excitability and impulse generation, such as that occurring with physiological stress, stimulants (stimulant) (e.g., caffeine), and certain drugs. While they may be benign and of no physiological consequence, they may also be harbingers of more-serious cardiac abnormalities.

Supraventricular arrhythmia
      Rhythm disturbances in the atrium can occur as a result of increased or decreased conduction rate, both of which may potentially compromise cardiac function. The electrophysiologic mechanisms for these changes are important with respect to prognosis and treatment.

      Supraventricular tachycardia (increased heart rate) is initiated in the atria and arises from a number of conditions, including an increase in sinoatrial node impulse rate that normally occurs during conditions of high excitation, such as hyperthyroidism or exercise. Referred to as physiologically appropriate sinus tachycardia, this response stems from a demand for increased cardiac output. In contrast, pathological tachycardia is defined by its presence under circumstances where it is physiologically inappropriate. In some cases, symptoms may go unnoticed or cause slight increases in heart rate, such as in paroxysmal (sudden) supraventricular tachycardia, which occurs in many people as a relatively benign increase in heart rate, ranging anywhere from 160 to 240 beats per minute. This condition is easily controlled by a variety of physical or pharmacological approaches and can be prevented or reduced with beta-adrenergic blocking agents (beta-blocker) (beta blockers; drugs that diminish excitatory response) or calcium channel blocking agents. Some conditions, however, require more aggressive pharmacological intervention or pacemaker implantation.

      Atrial flutter (rapid atrial beat) may occur suddenly and unpredictably or may be a chronic sustained arrhythmia. The heart rate in atrial flutter approximates 300 beats per minute and is difficult to treat pharmacologically. In general, only a fraction of the atrial beats (one-third to one-fourth) are transmitted to the ventricle, which is done in a systematic manner so that the ventricular rate appears constant. Atrial flutter can also occur as a variant of paroxysmal supraventricular tachycardia in overdose of digitalis, which causes the atria to beat faster than the ventricles because atrial transmission to the ventricles is blocked. Patients with atrial flutter are susceptible to marked increases in heart rate with relatively little stimulation unless treated pharmacologically with beta-adrenergic blocking agents, digitalis, or calcium channel blocking agents. The sustained condition of atrial flutter is treated with electric countershock followed by antiarrhythmic therapy to maintain normal rhythm. In many patients with chronic atrial flutter, the rhythm ultimately changes to atrial fibrillation. Atrial fibrillation is a chaotic disorganization of the atrial muscle in which multiple and organized electrical impulses arise. A small fraction of impulses are transmitted to the ventricle in an unpredictable manner, and the heart rate is described as irregularly irregular. As in atrial flutter, patients are treated pharmacologically to control ventricular heart rate. Atrial fibrillation may have severe consequences that require various approaches to treatment.

      Tachycardias that are sometimes resistant to treatment may arise from a series of abnormalities called Wolff-Parkinson-White syndrome. This syndrome is characterized by the presence of an alternative conduction source from atrium to ventricle that bypasses the atrioventricular node, causing impulses to reach the ventricle too soon. A variety of tachycardias can occur under these circumstances that may be very rapid and life-threatening. Catheter ablation, in which the electrical conduction pathway is destroyed in the problematic cells, has been used to treat severe cases of this syndrome.

Bradycardia and heart block
       bradycardia (low heart rate) can arise from two general mechanisms. The sinoatrial node may not function properly either as a result of slow generation of impulses or of blocking of the propagation of impulses. As a result, other pacemakers in the heart become responsible for impulse generation, and these have intrinsically slower rates. The condition, while not harmful in and of itself, is usually an indication of problems with the atrial conduction system and frequently results in the development of atrial fibrillation. In some circumstances, paroxysmal supraventricular tachycardia will abruptly terminate, and the sinoatrial node will not take up normal sinus rhythm. This results in a profound bradycardia that may cause fainting ( syncope), a condition known as tachycardia-bradycardia syndrome.

      Another mechanism for slow ventricular rates is heart block. Under these circumstances the sinoatrial node generates an appropriate impulse rate, but the impulses are not transmitted properly through the atrioventricular node and the His bundle. The block is classified as first-degree (normal heart rate but delayed transmission of atrial impulse to ventricle), second-degree (only some atrial beats are transmitted to the ventricle), or third-degree (no transmission from the atrium to the ventricle occurs). In some cases, first-degree heart block may be a side effect of medication (i.e., digitalis). Treatment is not required for first-degree heart block, as it is a benign condition with a generally good prognosis. If heart block progresses into severe second-degree or third-degree stages, a pacemaker is implanted for proper function. Heart block may occur as a result of severe injury, such as myocardial infarction, in which an emergency pacemaker may be implanted; however, it frequently occurs as a function of normal aging because of fibrosis of the His bundle. Third-degree heart block initiated in the His bundle results in a very slow heart rate and almost always requires a pacemaker. Third-degree heart block can also occur from blocks of the atrioventricular node in patients with congenital heart block. These patients are generally asymptomatic and capable of maintaining cardiac output under most circumstances. This is because the presence of other, more rapid, pacemaker cells below the level of the block is sensitive to metabolic demand, allowing some increase in heart rate. The use of pacemakers in patients with congenital heart block is not usually required.

Ventricular arrhythmia
      Ventricular arrhythmias represent the major mechanism of cardiac sudden death, which is the leading cause of death in the United States, where each year more than 325,000 people die suddenly. Almost all of these deaths are related to ventricular fibrillation. While this rhythm disturbance may be associated with heart attack (myocardial infarction), evidence suggests that more than half are not related to heart attack.

      The mechanism by which ventricular arrhythmias occur is not completely understood. One basic mechanism appears to result from spontaneous generation of cardiac impulses within the ventricle. It is not clear whether this condition results from pathologically altered ventricular cells or from cells in the specialized conduction system. A second mechanism of ventricular arrhythmia is associated with reentry of an impulse. In this situation, slowed impulse conduction in the ventricle leads to the generation of ectopic impulses (electrical impulses derived from an area of the heart other than the sinus node) that are primarily the result of temporal dispersion of the impulse between adjacent areas of the ventricle. This sets up an electrical impulse circuit within the ventricle that may progress into an arrhythmia. Reentry mechanisms are important components of ventricular arrhythmias and may be as simple as a premature ventricular beat coupled to a normal beat or as serious as a dangerous ventricular tachycardia. Under any circumstance where cardiac injury has occurred, a ventricular arrhythmia may potentially become a lethal ventricular event. In contrast, premature ventricular contractions can occur spontaneously in healthy people without any consequence.

      The chaotic nature of excitation and inefficient ventricular contraction in pathological ventricular arrhythmias frequently compromises circulation. Even ventricular tachycardia can potentially cause shock and be lethal in its own right. However, the primary danger of ventricular tachycardia is that it will decay into ventricular fibrillation, which is incapable of sustaining life and represents the majority of sudden cardiac death cases. Thus, the indication that ventricular tachycardia or ventricular fibrillation might occur demands prompt therapeutic intervention.

      There has been considerable investigation into methods of evaluating premonitory signs that might predict susceptibility to serious ventricular arrhythmias. One approach involves monitoring the heartbeat continuously for long periods of time (24 to 72 hours), with patients recording their activity in diaries during the monitoring process (called Holter monitoring). In addition to evaluating ventricular rhythm disturbances associated with serious cardiac arrhythmias, this method also allows for the identification of potential causative conditions. Patients with coronary artery disease often undergo an exercise test that examines ventricular rhythm under circumstances in which part of the heart is receiving insufficient blood. This is a useful way of predicting potential problems associated with ventricular arrhythmias in these patients.

Treatment of ventricular arrhythmias
      Since coronary artery disease is the most common cause of ventricular arrhythmias, correction of coronary occlusion either by angioplasty or coronary artery bypass is quite common and successful. However, if the ventricle has already been significantly damaged, ventricular arrhythmias may persist. In addition, a significant group of people who have no evidence of coronary artery disease develop a propensity for ventricular arrhythmias. Treatment of ventricular arrhythmias in patients without coexisting cardiac disease is variable and, in some cases, is not required.

      In patients with moderate to severe congestive heart failure, cardiac arrhythmias are the most common cause of death. For many years the principle therapeutic approach was to treat patients with drugs that altered the electrophysiology of the heart. The efficacy of these drugs was assessed based on their ability to control the frequency of premature ventricular contractions and other transient ventricular arrhythmias. However, even though these drugs may reduce premature contractions, they are not effective in reducing sudden cardiac death. An example of a highly effective therapeutic agent used for arrhythmias is amiodarone, a structural analog of thyroid hormone. This drug is unique because it has multiple mechanisms of action, including blood vessel dilation and a calcium channel blockade. However, it takes weeks for the drug to reach therapeutic levels in the body and can produce serious side effects, such as “halo” vision, discoloration and increased sensitivity of the skin to sunlight, and thyroid disorders. In addition, if proper dosage levels are not maintained, amiodarone can become arrhythmogenic. Because of these adverse effects, amiodarone is not used in patients whose heart function is otherwise compromised, such as in patients who have experienced myocardial infarction.

      Improvements in the technology and implantation procedures of internal ventricular defibrillation devices has provided an alternative way to reduce risk of sudden death from ventricular arrhythmias in high-risk patients. An internal defibrillatory device works very similar to an external electrical defibrillator used to treat cardiac emergencies and is wholly contained within the chest (similar to a pacemaker); it stops ventricular arrhythmias with internal shocks. In some patients these defibrillators also contain a pacemaking mechanism.

      Progress in the treatment of coronary artery disease, as well as predicting the propensity for ventricular arrhythmias (with the initiation of proper treatment), has reduced the rate of cardiac sudden death. In addition, cardiopulmonary resuscitation (CPR), which can keep patients undergoing sudden cardiac arrhythmias alive until proper therapy is available, and a growing trend to make external cardiac defibrillators available in public areas have improved survival rates in cardiac emergencies. Improvement in the prevention and treatment of coronary artery disease and cardiac arrhythmias has also contributed to the reduced incidence of ventricular arrhythmias in sudden cardiac death.

Mark L. Entman

      Congestive heart failure (also called heart failure) is a condition resulting from a variety of cardiac diseases associated with an inadequate pumping function of the heart. The inability of the heart to pump effectively leads to accumulation of blood in the lungs (lung) and veins (vein), reduced blood flow to tissues, and accumulation of fluid in tissues ( edema), causing circulatory congestion. Congestive heart failure results in part from the consequences of mechanisms that compensate for cardiac dysfunction and in part from direct effects of decreased blood flow to the heart. These problems are often related to salt and water retention in tissues and can vary from minimal symptoms to pulmonary edema (abnormal accumulation of fluid in the lungs) to sudden cardiac death.

      In healthy individuals, cardiac output is adjusted by a rapid increase in the strength of contraction that occurs almost immediately upon an increase in activity. After this increased contractility, additional changes in cardiac output arise from adjustment of the heart rate. For this reason, maximum cardiac output is closely linked to the maximum achievable heart rate. While improved strength and efficiency of contraction can be demonstrated in athletes, maximum achievable heart rate appears to be almost entirely a function of age. Maximum achievable heart rate begins to decline at approximately 30 years of age and gradually decreases throughout the remainder of life. The percentage maximum of cardiac work an individual patient achieves under certain workloads (i.e., during exercise testing) is a measure of how well the patient's heart is functioning. Disturbances in cardiac output may be a sign of cardiac dysfunction that can lead to congestive heart failure.

      Causes of congestive heart failure include coronary artery disease, myocardial infarction, cardiomyopathy, untreated hypertension, congenital heart defects (congenital heart disease), heart valve disease, and chronic kidney disease (renal system disease). However, a large group of people develop ventricular dysfunction and congestive heart failure with no obvious cause. While the incidence of myocardial infarction, and the resulting severity of cardiac injury, has fallen, it remains one of the most common etiologies of congestive heart failure. This occurs in part because of the marked increase in survival of myocardial infarction patients who have severely compromised hearts. Heart failure due to cardiac valve disease has decreased in the developed world because of the marked reduction in rheumatic heart disease and the improvement of cardiovascular surgical approaches. Similarly, surgical approaches to congenital heart abnormalities have reduced the incidence of congestive heart failure related to congenital syndromes.

      Studies using molecular genetics techniques have demonstrated the presence of specific genetic mutations (mutation) in cardiac proteins (protein) associated with cardiomyopathy clustering in families. It is not clear whether spontaneous cardiomyopathies are associated with random genetic mutations of these proteins. The etiology of congestive heart failure affects both preventative and therapeutic approaches, which are discussed later under Therapy (cardiovascular disease).

Ventricular dysfunction in heart failure
      The major role of the ventricles (ventricle) in pumping blood to the lungs and body means that even a slight decrease in ventricular efficiency can have a significant impact on heart function. If the left ventricle encounters either absolute or relative functional insufficiency (called left ventricular heart failure, or left-sided heart failure), a series of compensatory reactions are initiated that may temporarily provide a return to sufficient ventricular function. One mechanism of compensation associated with left ventricular failure is left ventricular enlargement, which can increase the volume of blood that is ejected from the ventricle, temporarily improving cardiac output. This increase in size of the ventricular cavity (called ventricular dilation), however, also results in a reduction in the percentage of the left ventricular volume of blood that is ejected (called ejection fraction) and has significant functional consequences. Ejection fraction, therefore, is a benchmark for assessing ventricular function and failure on a chronic basis.

      The result of a fallen ejection fraction is an enlargement of the ventricular volume during diastole that occurs by ventricular dilation, which serves as a first-line compensatory mechanism. When this happens, the ventricle recruits additional contractile units in myocardial cells that cause the cells to stretch further than they would normally, so they can generate a stronger contraction for ejection. Dilation is necessary for the dysfunctional ventricle to maintain normal cardiac output and stroke volume (the volume of blood ejected with each contraction). This acute compensatory mechanism, called the Frank-Starling mechanism (named for German physiologist Otto Frank and British physiologist Ernest Henry Starling (Starling, Ernest Henry)), may be sufficient in patients with mild heart failure who only require ventricular compensation during exercise, when demand for cardiac output is high. Increased ventricular volume, however, results in an increase in internal load. Over time the ventricle responds by increasing the size of individual muscle cells and thickening the ventricular wall (ventricular hypertrophy). Ventricular hypertrophy causes increased stiffness of the left ventricle, thereby placing a limitation on the amount of compensatory increase in ventricular volume that can be generated.

      The need for increased ventricular filling in a stiff ventricle results in an increase in left ventricular filling pressure during the period of time that blood is flowing from the left atrium to the left ventricle (diastole). Atrial pressure must be increased in order to fill the ventricle, resulting in increased pulmonary venous pressure. Increased pulmonary venous pressure results in congestion (due primarily to a distended pulmonary venous population), which stiffens the lung and increases the work of breathing (dyspnea). Thus, compensation for ventricular dysfunction results in shortness of breath, particularly on exertion, which is the cardinal feature of congestive heart failure.

      Other features of congestive heart failure result from a compensatory mechanism in the body to maintain stroke volume. Receptors located in the large arteries (artery) and the kidneys are sensitive to alterations in cardiac function. The latter respond by secreting an enzyme called renin that promotes sodium retention, which leads to fluid retention. Thus, a compensatory mechanism for inadequate blood circulation is expansion of the blood volume. Increased blood volume is an indication that fluid is being lost from the circulation into the extracellular fluid. Fluid accumulation in tissues (edema) accounts for several of the clinical signs of congestive heart failure. Edema is frequently seen as swelling, particularly of the lower extremities, where there is accumulation of subcutaneous fluid. When severe enough, pressure on this swelling results in a temporary crater or pit (pitting edema). Similarly, edema may occur in the pulmonary circulation (pulmonary edema). The symptoms may vary from shortness of breath on very little exertion to a medical emergency in which the patients feel as though they are suffocating (suffocation). Congestive symptoms may also result in enlargement of the liver and spleen and loss of fluid into the abdominal cavity ( ascites) or the pleural cavity (pleural effusion), profoundly affecting organ function and respiratory (respiration, human) function.

      In patients with less severe disease, congestive symptoms at rest are minimal because of decreased cardiac load associated with inactivity. However, if fluid overload persists, when the patient lies down and elevates dependent extremities (e.g., the legs), large amounts of fluid become mobilized, resulting in rapid expansion of the blood volume and in shortness of breath. Shortness of breath on lying down is called orthopnea and is a major symptom of heart failure. In addition, the patient may experience acute shortness of breath while sleeping (paroxysmal nocturnal dyspnea) that is related to circulatory inadequacy and fluid overload. When this occurs, the patient is awakened suddenly and suffers severe anxiety and breathlessness that may require half an hour, or longer, from which to recover.

      A limited amount of heart failure is initiated in the right ventricle, though it may also be caused by cor pulmonale or disease of the tricuspid valve. Right ventricular heart failure (sometimes called right-sided heart failure) results in right-sided alterations in the pulmonary circulation. These alterations may be associated with severe lung diseases, such as chronic obstructive lung disease, and poorly understood primary diseases, such as primary pulmonary hypertension. Since the right side of the heart is the direct recipient of venous blood, the primary signs of this illness are venous congestion and enlargement of the liver. Compensatory mechanisms also cause expansion of fluid volume and edema in the feet and legs. Pulmonary congestion does not occur in right ventricular heart failure because back pressure into the lungs is required for this condition, and the normal function of the right ventricle is to pump blood forward into the pulmonary circulation. In severe (terminal) right ventricular heart failure, cardiac output becomes significantly reduced, leading to metabolic acidosis. Historically, right ventricular heart failure was also associated with mitral valve disease and congenital heart disease, but the incidence of these two conditions has been greatly reduced as a result of surgical advancements.

Therapy (therapeutics)
      Therapy for heart failure is generally aimed at treating the underlying causes of the condition. For example, surgical (surgery) intervention may be used to repair congenital or valvular heart defects. The primary goal of this approach is to avoid potential heart failure associated with complications of congenital or valvular defects, such as ventricular overload. Despite improved therapies for coronary artery disease and efforts to educate people about the importance of reducing risk factors for atherosclerosis, coronary artery disease remains one of the most common causes of heart failure.

      Treatment of myocardial infarction has important consequences with respect to long-term mechanical function of the ventricle. Therapy is often designed to reduce the amount of damage caused by rapid revascularization immediately following myocardial infarction. The process of revascularization plays an important role in stimulating ventricular remodeling that leads to ventricular dysfunction. Improved emergency response and prevention of complications that may arise during myocardial infarction, such as arrhythmias (arrhythmia), have resulted in a significant reduction of cardiac deaths from heart attack. Therapies designed to promote efficient repair and scar formation in the ventricle also reduce sudden death and the incidence of heart failure. Congestive heart failure is the major cause of cardiac death after myocardial infarction, often appearing within one to two years after the initial heart attack. Drugs used to treat these conditions include beta-adrenergic blocking agents (beta-blocker) (beta-blockers), which reduce excitatory reaction in response to sympathetic nervous system stimulation, and vasodilators. Administration of both of these classes of agents have been shown to have considerable benefits directly related to their ability to control blood pressure. Treatment of cardiomyopathies has generally been aimed at symptom relief, such as lowering blood pressure and controlling arrhythmias.

      Therapy of progressive heart failure is generally targeted toward decreasing blood volume by increasing salt and water excretion. In patients who have no symptoms at rest and only mild symptoms while exercising (sometimes called incipient heart failure), salt restriction and diuretics (diuretic) may be sufficient. In patients with marked restriction of exercise capacity or with symptoms at rest (mild to moderate heart failure), there is significant benefit from low doses of beta-blockers, renin-angiotensin system inhibitors, and inhibitors of aldosterone (a steroid hormone that regulates the balance of salt and water in the body). Patients with symptoms at rest or with minimal activity (moderate to severe heart failure) have a particularly poor long-term prognosis, with approximately half of these patients dying within two years from cardiac dysfunction or rhythm disturbances. Thus, more aggressive strategies have arisen to maintain these patients and to improve their prognosis.

      Heart transplants (heart transplant) have been performed since 1967 but are much more successful today because of effective treatments that reduce immune (immune system) rejection of the donor heart. However, cardiac transplant is still limited by the availability of donor hearts, and, while antirejection strategies have been generally effective, they may cause complications, such as accelerated atherosclerosis and changes in cardiac cells, that ultimately result in transplant failure. While life expectancy following a heart transplant is difficult to predict, the average recipient will live 8 to 10 years. This has fostered ongoing investigation into better strategies to manage immune rejection.

 Because of the unpredictable nature of obtaining a donor heart, left ventricular assist devices have been developed to increase patient survival while awaiting a transplant. These devices work by taking part of the blood from the left ventricle and mechanically pumping it into the arterial circulation. This mechanical assistance reduces the amount of work required of the left ventricle. Some patients who have received left ventricular assist devices as “bridges” to transplant have actually demonstrated significant recovery of their native ventricular function. A dramatic improvement in health and quality of life in some of these patients has eliminated the need for a transplant. Long-term ventricular assist devices, for use in patients who are not candidates for heart transplant, have been approved as well.

Mark L. Entman

Surgical treatment (surgery) of the heart

Cardiopulmonary bypass
      Cardiopulmonary bypass serves as a temporary substitute for a patient's heart and lungs (lung) during the course of open-heart surgery. The patient's blood is pumped through a heart-lung machine (heart–lung machine) for artificial introduction of oxygen and removal of carbon dioxide. Before its first successful application to operations on the human heart in the early 1950s, all heart operations had to be done either by the sense of touch or with the heart open to view but with the patient's whole body held to a subnormal temperature ( hypothermia). The latter procedure was feasible only for very brief periods (less than five minutes).

      The first heart-lung machine (pump oxygenator) resembled only slightly the complicated apparatus currently used for correction of cardiac defects. With this machine the blood bypasses the heart and lungs so that the surgeon has an unobstructed view of the operative field. Cardiopulmonary bypass is accomplished by use of large drainage tubes (catheters) inserted in the superior and inferior venae cavae (vena cava), the large veins (vein) that return the blood from the systemic circulation to the right upper chamber of the heart. The deoxygenated blood returning to the heart from the upper and lower portions of the body enters these tubes and by gravity drainage flows into a collecting reservoir on the heart-lung machine. Blood then flows into an oxygenator, the lung component of the machine, where it is exposed to an oxygen-containing gas mixture or oxygen alone. In this manner, oxygen is introduced into the blood, and carbon dioxide is removed in sufficient quantities to make the blood leaving the oxygenator similar to that normally returning to the heart from the lungs.

      From the oxygenator, blood is pumped back to the body and returned to the arterial tree through a cannula (small tube) introduced in a major systemic artery, such as the femoral (groin) artery. Oxygenated blood then flows to the vital organs, such as the brain, kidneys (kidney), and liver. Meanwhile, the heart may be opened and the corrective operation performed. This procedure permits a surgeon to operate on the heart for many hours, if necessary.

      The assemblage and sterilization of the components of the heart-lung machine are essential considerations, because the blood comes in contact with the apparatus outside of the body. Heart-lung machines have totally disposable tubing and plastic bubble oxygenators. Cardiopulmonary bypass is now more often carried out by using cardioplegic solutions designed to provide the heart with the necessary minimal nutrient and electrolyte requirements. Blood is also needed, and administration of an anticoagulant ( heparin) prevents clotting of the blood while it is circulating in the heart-lung machine.

Repair of congenital (congenital heart disease) cardiac defects
      Most congenital cardiac defects can be repaired surgically. Operations are of two general types: those that can be performed without a heart-lung machine, such as surgeries for patent ductus arteriosus and coarctation of the aorta (aorta, coarctation of the), and those, such as intracardiac abnormalities, that require a heart-lung machine.

Persistent (patent) ductus arteriosus
      The ductus arteriosus is the channel in utero between the pulmonary (pulmonary circulation) artery and the first segment of the descending thoracic aorta. Before birth, blood flows from the right ventricle into the pulmonary artery and across the ductus arteriosus to the descending aorta. The ductus shunts blood away from the lungs because oxygen–carbon dioxide exchange begins only at the time of birth. Normally, functional closure of the ductus arteriosus is completed within the first few days after birth, although complete anatomic closure may not occur for several months. If it remains open, excessive levels of blood may flow through the lungs. Ligation of the ductus arteriosus performed by Robert E. Gross in Boston in 1938 was the first successful operation for congenital heart disease and initiated the modern era of cardiac surgery for congenital cardiovascular lesions. Today the ductus arteriosus can be closed with a nonsurgical prosthesis inserted by catheter.

Coarctation of the aorta (aorta, coarctation of the)
      Coarctation of the aorta, which is a constriction of the aorta, usually in the same region as the ductus arteriosus, is one of the most common congenital cardiac defects. It was first successfully repaired by Clarence Crafoord in Sweden in 1944. In older children and adolescents the narrowed area is repaired by cutting out the constriction and stitching the two normal ends together. In infants a modified operation is used in which the left subclavian artery (the artery that supplies the left arm) is tied, divided, and used as a flap to repair the narrowed aortic area. With this procedure the stricture has less of a tendency to redevelop at that site. In adults it often may be necessary to bridge the narrowed area with a graft tube, which is attached to the aorta above and below the narrowed segment; the blood is thus able to bypass the constricted area to reach the organs below the defect.

Pulmonary (pulmonary stenosis) valve stenosis
      The most common congenital defect of the valves in children is a narrowing of the pulmonary valve (the valve opening to the pulmonary artery), called pulmonary stenosis. The valve cusps in this condition are not well formed, and, as a result, the valve cannot open normally. The valve cusps are thickened, and the size of the orifice varies in diameter from 1 mm to about two-thirds of the circumference of the pulmonary artery. As a result, blood flow from the right ventricle into the lungs is obstructed. Mild stenosis is usually compatible with normal activities and normal life, but moderate and severe stenosis may result in clear symptoms.

      The surgical procedure used to correct this condition is usually performed on cardiopulmonary bypass, with the valve approached through the pulmonary artery and cut in three places to create a valve with three cusps. An alternative approach to surgery is the use of a special balloon catheter, which is passed from the femoral vein (the vein in the groin) into the right side of the heart and positioned across the pulmonary valve. A balloon at the tip of the catheter is then inflated to enlarge the valve orifice.

Aortic (aortic stenosis) valve stenosis
      Although mild aortic valve stenosis is manageable in children, deterioration may occur with growth. Severe aortic stenosis in infancy and childhood may be associated with either sudden death or heart failure. The usual basis for the stenosis is fusion of the valve, which is usually bicuspid rather than tricuspid. The valve is often both obstructed and incompetent (allowing blood to leak back from the aorta into the left ventricle). Patients with more than a trivial degree of aortic stenosis usually should not take part in competitive sports, such as swimming or football. In moderate to severe degrees of aortic stenosis, surgery usually is necessary and is performed using cardiopulmonary bypass. The aorta is opened just above the valve, and the surgeon incises the valve sufficiently to convert severe stenosis to a mild or moderate degree of obstruction. In older patients the valve is often thickened and calcified, and it may need to be replaced.

Atrial and ventricular septal defects
      If atrial (atrium) and ventricular septal defects require surgical closure, the patient's circulation must be supported by the heart-lung machine. Atrial septal defects (atrial septal defect) are usually repaired by sewing the tissue on either side of the defect together, although very large defects may require a patch of material to close the opening. Because of the frequency of spontaneous natural closure, small ventricular septal defects (ventricular septal defect) are observed for a period of time before the decision is made to perform surgery. Large ventricular septal defects are usually closed by a patch.

Cyanotic cardiovascular abnormalities
      The first attempt to treat “blue babies (Fallot, tetralogy of)” affected with cyanotic abnormalities was performed by the American physicians Alfred Blalock (Blalock, Alfred) and Helen B. Taussig (Taussig, Helen Brooke) in 1944. This procedure transformed the outlook for cyanotic children and for the first time made survival possible. In the early 1950s, heart-lung cardiac surgery and procedures for repair were developed. Surgical treatment of the tetralogy of Fallot (Fallot, tetralogy of) has been an important model for developments in more complex forms of cardiac surgery, and long-term results have been excellent. Most, but not all, forms of cyanotic congenital heart disease can now be repaired, and palliative surgery may produce considerable benefits for those in whom definitive treatment is not possible.

Repair of acquired cardiac defects
Valvular disease
 Destroyed heart valves can be replaced with artificial valves (prostheses (prosthesis)) made of stainless steel, Dacron™, or other special materials. The heart-lung machine is used during these operations, in which one, two, or even three cardiac valves may be removed and replaced with the appropriate artificial valve. The use of both homograft valves (obtained from human beings after death) and heterograft valves (secured from animals) is widespread. One of the advantages of both types is the absence of clotting, which occurs occasionally with the use of artificial valves. Most homograft and heterograft valves have a durability of 10–15 years. There is a risk of endocarditis with all types of valves.

Chronic constrictive pericarditis
      Chronic constrictive pericarditis can affect the surface of the heart and the sac (pericardium) surrounding it. The pericardium becomes thickened and fibrotic, and over a period of time it constricts the heart so that the normal filling of the ventricles during the resting phase of the cardiac cycle is limited. This condition in turn reduces the output (cardiac output) of the heart and eventually affects all the organ systems, including the brain, liver, and kidneys. Treatment is the surgical removal of the thickened pericardium around the heart, which permits normal filling and expansion of the ventricles and restores adequate cardiac output to the vital organs.

Cardiac pacemakers
      The normal rhythm of the heart is generated by spontaneous electrical activity in cells in an area of the heart called the sinoatrial node. The electrical activity is usually at a rate of about 70 beats per minute at rest and is transmitted to the pumping chambers of the heart, the atria, and the ventricles through a specialized conducting system. The electrical activity causes contraction of the heart muscle, which results in a detectable pulse at the wrist and elsewhere. Disease of the sinus node (sick sinus syndrome) or the conducting system ( heart block) can cause an abnormally slow rhythm of the heart; because blood supply to the brain is inadequate, severe disease can cause loss of consciousness. This occurs if there is no heartbeat for about six seconds.

      A pacemaker is a device that artificially stimulates the heart when the abnormal electrical activity is absent. A pacemaker comprises a pulse generator connected to the heart by wire or electrode. The pulse generator has a battery power source and electronic circuitry that can generate an artificial stimulus at a predetermined rate. It can also detect normal activity of the heart so that the artificial stimulus is only discharged when the natural activity is absent. In this way the pacemaker functions on demand, inserting an artificial beat as required.

      The pulse generator is usually placed under the skin over the right or left chest and has enough power to last several years. The electrode is passed from the pulse generator along a vein and is connected to either the atrium or ventricle, depending on whether the underlying problem is sick sinus syndrome or heart block. In many models the performance of the pacemaker can be altered by using radio-frequency signals to alter its programmed settings. Some pacemakers may last up to 15 years and can be reused; the most common lifetime is seven years.

Heart wounds (wound)
      Heart wounds are caused by blunt or penetrating instruments. The rapid deceleration often experienced in automobile accidents is a common cause of injury to the heart muscle, resulting in bruising (bruise) and even disruption of a valve or the ventricular septum. Both bullet and stab wounds account for many patients treated in the emergency clinics of major hospitals. Prompt diagnosis and effective surgical treatment, usually consisting of control of bleeding by sewing the heart muscle at the point of entry of the foreign object, have resulted in a high rate of successful treatment.

Coronary arterial disease (coronary heart disease)
 Operations have been devised to bring a new blood supply into the heart when the coronary arteries (coronary artery) become narrowed by atherosclerosis. A commonly used technique is to use a vein removed from the leg as a bypass (coronary artery bypass) around the diseased portion. The vein is attached to the aorta above as it leaves the left ventricle. The other end of the vein is then sutured directly to any one of the coronary arteries. Large quantities of blood can be delivered to the heart muscle by this direct form of myocardial revascularization. Implantation of an artery below the breastbone (internal mammary artery) into a coronary artery beyond the block is increasingly used and is associated with longer graft survival.

 The development of catheters with strong inflatable balloons constructed toward their end and along the line of the catheter has greatly changed cardiac surgery. The balloons can be inflated by compressed air at different controlled pressures. They are used for dilation of a partly obstructed coronary artery (percutaneous transluminal coronary angioplasty, or PTCA), with restoration of blood flow to the heart muscle, and of a severely obstructed heart valve, particularly the aortic valve, relieving the pressure on the left ventricle.

      The procedure generally requires no anesthetic and, using specialized radiological imaging techniques, is sometimes done on an outpatient basis. Several coronary arteries may be dilated in this way, with flattening of the atheromatous material against and into the arterial wall. Although there are operative risks, such as emboli (embolism) and tearing, the results are excellent, and the technique may be repeated if necessary. However, the use of drug-coated stents has decreased the need for repeat angioplasty.

Heart transplantation (heart transplant)
      If the heart muscle has been damaged beyond surgical repair, heart transplantation (heart transplant) may be performed. The diseased heart is removed, and the donor's heart is sewn in position. This procedure is particularly useful in advanced cardiomyopathy. About 65 to 70 percent of all heart transplant patients are still alive five years after the surgery. Heart-lung transplants are used for some intractable cardiopulmonary diseases, such as cystic fibrosis.

Michael E. DeBakey Edward Bronson Diethrich Michael Francis Oliver

Diseases of the arteries
      There are many types of arterial diseases. Some are generalized and affect arteries (artery) throughout the body, though often there is variation in the degree they are affected. Others are localized. These diseases are frequently divided into those that result in arterial occlusion (blockage) and those that are nonocclusive in their manifestations.

Occlusive disease
  atherosclerosis, the most common form of arteriosclerosis, is a disease found in large and medium-sized arteries. It is characterized by the deposition of fatty substances, such as cholesterol, in the innermost layer of the artery (the intima). As the fat deposits become larger, inflammatory white blood cells (leukocyte) called macrophages try to remove the lipid deposition from the wall of the artery. However, lipid-filled macrophages, called “foam cells,” grow increasingly inefficient at lipid removal and undergo cell death, accumulating at the site of lipid deposition. As these focal lipid deposits grow larger, they become known as atherosclerotic plaques and may be of variable distribution and thickness. Under most conditions the incorporation of cholesterol-rich lipoproteins (lipoprotein) is the predominant factor in determining whether or not plaques progressively develop. The endothelial injury that results (or that may occur independently) leads to the involvement of two cell types that circulate in the blood—platelets (platelet) and monocytes (a type of white blood cell). Platelets adhere to areas of endothelial injury and to themselves. They trap fibrinogen, a plasma protein, leading to the development of platelet-fibrinogen thrombi. Platelets deposit pro-inflammatory factors, called chemokines, on the vessel walls. Observations of infants and young children suggest that atherosclerosis can begin at an early age as streaks of fat deposition (fatty streaks).

 Atherosclerotic lesions are frequently found in the aorta and in large aortic branches. They are also prevalent in the coronary arteries (coronary artery), where they cause coronary artery disease. The distribution of lesions is concentrated in points where arterial flow gives rise to abnormal shear stress or turbulence, such as at branch points in vessels. In general the distribution in most arteries tends to be closer to the origin of the vessel, with lesions found less frequently in more distal sites. Hemodynamic forces are particularly important in the system of coronary arteries, where there are unique pressure relationships. The flow of blood through the coronary system into the heart muscle takes place during the phase of ventricular relaxation ( diastole) and virtually not at all during the phase of ventricular contraction ( systole). During systole the external pressure on coronary arterioles is such that blood cannot flow forward. The external pressure exerted by the contracting myocardium on coronary arteries also influences the distribution of atheromatous obstructive lesions.

      The six principal risk factors for atherosclerosis include age, genetics (genetics, human), gender, serum cholesterol concentrations, smoking, and diabetes (diabetes mellitus). The prevalence of atherosclerosis increases with age, and young children with evidence of fatty streaks are very likely to develop the disease when they are older. In addition, occurrence of atherosclerosis in some families is linked to a genetic component, and family history serves as a useful predictor for the course of development of the disease, particularly if it is evident at a young age. There are several important genetic defects of lipid metabolism, one of which involves a defect of lipoprotein receptors and constitutes an extreme form of familial propensity for atherosclerosis.

      Men develop atherosclerosis more often then women, and complications generally appear at an earlier age. One reason for this difference has been ascribed to estrogen, a female hormone that induces high-density lipoproteins (HDL) which remove excess cholesterol from arterial cells, thus providing a protective effect against development of atherosclerosis in women. When estrogen levels decrease after menopause, the incidence of atherosclerosis and its complications rises. Though hormone replacement therapy was once considered an anti-atherosclerotic therapy for women, controversy surrounding the long-term effects of hormone therapy has greatly reduced their use for cardiovascular (human cardiovascular system) conditions.

 Epidemiological (epidemiology) observation has demonstrated that patients with high serum cholesterol and lipoprotein concentrations have a higher incidence of atherosclerosis. Increased cardiovascular risk lies mainly in cholesterol associated with low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL) that contain large amounts of cholesterol relative to HDLs and deliver cholesterol directly to the cells of peripheral tissues, including arteries. There are also some qualitative abnormalities in lipoproteins that appear to control the risk of atherosclerosis and are also associated with familial propensity.

      Smoking may increase the risk of developing atherosclerosis and its associated complications, with one pack of cigarettes a day doubling the risk and two packs a day tripling it. Smoking also increases the incidence of adverse coronary events in young women taking oral contraceptives. Complications of atherosclerosis are a significant cause of lowered life expectancy in smokers.

      Incidence of atherosclerotic disease and its complications in patients with either type 1 (insulin-dependent) or type 2 (non-insulin-dependent) diabetes (diabetes mellitus) is significantly higher than in healthy individuals. This appears to be so even when accounting for abnormal blood lipids frequently associated with diabetes and poor diabetic control, suggesting that high blood sugar may potentially injure the artery and facilitate atherosclerosis. Patients with diabetes also frequently have high levels of triglycerides (triglyceride), which appear to be associated with diabetes and its complications.

      In addition to these primary risk factors, there is substantial evidence that other factors, such as sedentary lifestyle, environmental stress, and obesity, increase risk. Atherosclerosis and its complications have also been associated with elevated levels of the blood amino acid homocysteine; however, this is unlikely to account for many cases of the disease. Great attention has been placed on the reduction of risk factors as therapeutic strategy for treatment and prevention of atherosclerosis.

Complications of atherosclerosis
      Complications of atherosclerosis constitute a major cause of death in many industrialized countries. The major complications of atherosclerosis are associated with occlusion (blockage) or inadequate blood flow to organs perfused by the affected artery. However, changes in the mechanical stability and characteristics of the artery itself may result in a series of nonocclusive complications. The aorta and the iliac arteries sometimes become mechanically unstable and dilate, forming aneurysms (aneurysm) (widening of an artery because of the destruction of the arterial wall). These aneurysms may favour the formation of blood clots that break off and occlude vessels downstream, or they may burst and hemorrhage, which may be fatal. The aorta also loses its elasticity and may actually calcify (harden). Blood ejected into a rigid aorta encounters increased flow resistance that is manifested by increased cardiac work and elevated systolic (systole) blood pressure. These factors may be important in the development of heart failure, high blood pressure, and stroke in elderly patients.

      Occlusive complications of atherosclerotic disease occur by two mechanisms that have strikingly different clinical pictures, even though both arise from the presence of atherosclerosis. Chronic occlusive disease develops over time as atherosclerotic deposition increases plaque size and tends to occlude the vessel. While this does not often occur in the aorta, chronic occlusive disease can significantly alter flow in very large aortic branches, such as the carotid (carotid artery) and iliac arteries. Occlusions of the coronary vessels may also occur slowly over time.

      Under the circumstances of chronic occlusion, there are a variety of mechanisms by which the vessel can adapt and maintain blood flow. One adaptation is through the formation of new vessels (collateral circulation). In addition, the blood vessel itself may dilate in response to increased atherosclerotic obstruction (vessel remodeling). Chronic occlusion will not alter resting blood flow until lumen occlusion becomes greater than 70 percent. Prior to this stage the reduction of blood flow in a specific vascular bed occurs, symptoms of which are first noted under conditions of stress. For example, in coronary artery disease, the patient may be asymptomatic at rest and only have pain when exercising. Exercise testing is often used to diagnose chronic stable coronary artery disease. As the vessel becomes more occluded and resting blood flow is reduced, the patient becomes more susceptible to acute complications, such as myocardial infarction ( heart attack).

 The atherosclerotic plaque is also susceptible to disruptive influences that may result in the formation of a blood clot (thrombus (thrombosis)) on its surface, favouring an acute thrombotic occlusion in the vessel and acute atherosclerotic complications. Because the occlusion occurs very quickly, the body does not have an opportunity to compensate or respond to this occlusion, and, unless prompt treatment is available, distal tissues will be damaged as a result of inadequate blood flow (ischemia). When this occurs in the coronary circulation, the result is myocardial infarction; when it occurs in the cerebral circulation, the result is stroke. Acute thromboses cause irreversible damage to tissues and are associated with loss of function of portions of organs, such as occurs in the heart following myocardial infarction or in the brain following cerebral stroke.

      Strategies for prevention of atherosclerosis are aimed at alteration of risk factors. Patients are urged to improve their diet and to exercise in order to lower their cholesterol and better their lipoprotein profile. If lifestyle modifications are unsuccessful, statins, drugs that act primarily by inhibiting cholesterol synthesis and also possibly by stabilizing the atherosclerotic plaque, are often prescribed to reduce the chances of a thrombotic event. Similarly, cessation of smoking is clearly a mechanism to reduce incidence of atherosclerosis. People who have had a heart attack or stroke, or are at high risk for either, are often prescribed daily low-dose aspirin therapy. Maintaining a steady low level of aspirin in the blood prevents atherosclerotic complications by blocking the ability of platelets to stick together to form blood clots. It has also been shown that aspirin decreases the risk of death from heart attack if it is given during or immediately after the attack.

 Treatment of chronic occlusive disease is aimed at opening or bypassing the occluded vessel; however, vessels can also be replaced surgically via several techniques. In the carotid circulation, atherosclerotic plaques can be directly removed from vessels to reestablish open blood flow via a procedure called atherectomy, in which a tiny knife inserted into a vessel through a catheter is used to shave fatty deposits off the vessel wall. Occlusion of the coronary arteries is treated using coronary artery bypass surgery. This procedure relocates native vessels, such as the saphenous vein (from the leg) or the internal mammary artery (in the chest), to the heart, where they serve to bypass the flow of blood around the occlusion. Replacement of the large arteries and the aorta with Dacron™ (synthetic fibre tubing) grafts is quite common.

 Noninvasive methods have been developed to open chronic atherosclerotic occlusions using a catheter in a procedure called angioplasty. A balloon near the end of the catheter is inflated, thereby dilating the vessel and allowing blood to flow around the obstruction. This procedure is often followed by the insertion of an expandable stent (a wire mesh tube) that becomes permanently embedded in the wall of the artery to be sure that the vessel stays open. Today many stents have special coatings that gradually release drugs to prevent reclosure of the vessel (restenosis).

 Acute closure of vessels in the coronary circulation is treated in several ways. Proteolytic enzymes (proteolytic enzyme) (enzymes that break down proteins) or drugs that activate the proteolytic process are often used for clot dissolution. The most commonly used therapeutic agent is tissue plasminogen activator (t-PA), which is given intravenously and acts rapidly to dissolve clots. Acute coronary thrombosis can also be treated by angioplasty, almost always accompanied by insertion of a stent and resulting in rapid and complete restoration of blood flow. However, since the blood clotting mechanism is compromised by the use of thrombolytic agents, such as t-PA, there is a substantial incidence of bleeding during and following stent implantation that can be fatal. This complication has led to a relative reluctance to utilize these procedures in older patients because of the possibility of producing hemorrhagic stroke. Regardless of the method, time is a critical factor in the success of restoring proper blood flow, and early recognition of symptoms has become a major health education goal.

      In patients at risk for thrombotic events, antiplatelet or anticoagulant drugs can be used to decrease platelet aggregation and to prevent the formation of thrombi. Statins can also reduce the incidence of acute arterial events through an anti-inflammatory mechanism, although this is not well understood. In addition, several dietary habits have been associated with lower incidence of acute thrombotic events, including high intake of vitamin E and fish oil and drinking red wine; however, extensive studies are needed to confirm these effects.

       polyarteritis nodosa, also called periarteritis nodosa, is an uncommon disease of unknown cause; hypersensitivity may play an important role. It is more common in males and may occur at any age. Small arteries and veins in various parts of the body are affected, producing effects as a result of occlusion or bleeding (bleeding and blood clotting) or a combination of the two. The course may be rapid, involving only weeks or months, or it may be highly prolonged. The involvement of the blood vessels may affect blood flow to the skin, the gastrointestinal tract, the kidneys (kidney), and the heart. There may be associated symptoms of arthritis, and involvement of almost all organs has been noted. There is an associated fever in most instances, an increase in the number of leukocytes (leukocyte) in the blood, and evidence of inflammation. No recognized specific mode of therapy is available, although patients have responded to a number of anti-inflammatory therapies.

       arteritis is an inflammation in localized segments of arteries. One particularly notable type is cranial arteritis (temporal arteritis), a disease of variable duration and unknown cause that is accompanied by fever and involves the temporal and occasionally other arteries of the skull. In general, older persons are affected. Excision of the involved artery may be carried out, but the general symptoms may remain.

       frostbite may occur after exposure to subfreezing temperature momentarily or to less severe temperature for a longer period. It occurs more readily if blood vessels are diseased. Several degrees of frostbite produce thrombosis of the arteries and arterioles and also may involve veins (vein). Symptoms may vary from a mild stage of reddening to gangrene and eventual loss of the extremity.

Arterial embolism
      An embolus (embolism), a foreign or abnormal particle circulating in the blood, may block a vessel too small to permit further passage. The sources of emboli include blood clots from the chambers of the diseased or abnormally functioning heart. Mural thrombosis on the infarcted ventricular wall or clots in the atrium in atrial fibrillation are common sources. Fat emboli may occur after fracture of bones (bone) and discharge of fatty marrow. Air emboli may be suspected after major injury, especially when large veins are opened during accidents or during vascular surgery of the neck or chest cavity. Bacterial emboli occur in bacterial endocarditis and occasionally in other infections. Cancers (cancer) may produce minute emboli of tumour cells. fungus growth or foreign materials, such as fragments of bullets, may become emboli. These emboli may cause transient local symptoms from diminished blood flow and may result in death of tissue. Treatment may include anticoagulant therapy and surgical removal of the clot.

Nonocclusive disease
      A penetrating injury such as that caused by a bullet or a sharp instrument may result in an arteriovenous fistula, an opening between an artery and its immediately adjacent vein. Large amounts of blood may be shunted from the artery to the vein. Arteriovenous fistulas are particularly common in wartime, resulting from penetration of shell fragments and other types of injury (wound) involving the arms and legs. They may also occur as a complication of surgery. Others are congenital in origin.

      The physician may hear a loud murmur caused by the turbulent flow of blood from the artery to the vein. Enlargement of the heart and all the manifestations of congestive heart failure may occur if the amount of blood shunted is large. In the area around the site of the arteriovenous fistula, the blood vessels become dilated and bacterial infection of the artery lining may develop. A cure can usually be achieved by surgery, though in some situations the remaining arterial flow may be impaired.

      A special kind of arteriovenous fistula occurs from the pulmonary (pulmonary circulation) artery to the pulmonary vein. There the situation is complicated by the fact that unoxygenated venous blood is being shunted into a vessel normally containing oxygenated blood. Cyanosis results and produces a stimulation for the formation of red blood cells, leading to a form of secondary polycythemia, or abnormally high red-blood-cell level.

Physical injuries
      Physical injuries to arteries may lead to damage of the vascular wall, with consequent formation of blood clots and blockage. On other occasions, a form of inflammation can develop that may lead to rupture and may be the source of emboli in the peripheral arteries. Sudden disastrous external stress—as in a severe automobile accident, airplane crash, or underwater explosion—may cause death through rupture of the major arteries, such as the aorta, rupture of the heart valves, or rupture of the heart itself.

Radiation injuries (radiation injury)
       X-rays, radium, and other radioactive substances in large dosages have marked effects on the vascular system. Initial reactions are inflammatory, and secondary changes caused by scarring and retractions may occur, which in turn lead to vascular occlusion (obstruction). The effects may be progressive for a period of years and are, at times, complicated by the development of cancer.

Functional disease
       Raynaud syndrome is said to occur when the extremities—including occasionally even the ears, nose, or cheeks—become pale, cyanotic, and numb under the influence of cold or emotion. Pain is also present at times. On cessation of the stimulus, redness develops, and there is a tingling or burning sensation lasting some minutes. This sequence of events is apparently caused by the excessive constriction of the small arteries and arterioles of the fingers upon stimuli that ordinarily cause only a minor degree of vasoconstriction (constriction of blood vessels). Raynaud syndrome, which is initially manifested by this phenomenon, involves spasmodic contraction of the blood vessels, usually beginning in early adulthood and affecting women about three times as often as men. The limb involvement is usually symmetrical (on both sides) and may lead to gangrene. Attacks may subside after the return to a warm environment or the release from tension.

      The symptoms associated with Raynaud syndrome may occur in people without other evidence of organic disease, especially in cold and moist climates. It may result from the operation of pneumatic hammers or may occur in individuals with various disorders, such as a cervical rib, a supernumerary (extra) rib arising from a neck vertebra (vertebral column). It may appear as a complication of arteriosclerosis and thromboangiitis obliterans (Buerger's disease). Various substances, such as nicotine, arsenic, ergot, and lead, have occasionally been blamed. Therapy includes treatment of the primary condition and avoidance of the precipitating cause.

       acrocyanosis is a similar condition, characterized by episodes of coldness and cyanosis of the hands and feet. It is often associated with profuse sweating and, at times, with local edema. It is a form of local sensitivity to cold and is frequently seen in mentally or emotionally disturbed people or in those with neurocirculatory asthenia (a symptom-complex in which there is breathlessness, giddiness, a sense of fatigue, pain in the chest over the heart, palpitation, and a fast and forcible heartbeat of which the affected person is conscious). Reassurance and avoidance of cold help to eliminate attacks.

      Erythermalgia (erythromelalgia) ( erythromelalgia) is an uncommon condition in which the extremities, especially the palms of the hands and the soles of the feet, are red, hot, painful, and often somewhat swollen. Dilation of the blood vessels (vasodilation) is the underlying factor. The condition is relieved by elevation of the extremity and cooling. Usually it occurs in middle and later life and is chronic in the primary form; it may occur as a secondary manifestation of underlying vascular disease. It may also occur as a manifestation of an abnormally high red-blood-cell level and, occasionally, as the result of injury or a variety of other disorders.

Diseases of the veins (vein)

Organic disease
      In thrombophlebitis there is thrombosis (clot formation) in the veins and a variable amount of inflammatory reaction in the vessel wall. In some instances, the inflammatory reaction is predominant and thrombosis is secondary. In other instances, thrombosis appears before reaction in the vein wall. Embolization—breaking loose of a blood clot—is most likely to occur during this period, though it may occur at any stage of the disease. A form of the disease in which little or no inflammatory reaction or pain develops is called phlebothrombosis.

      Thrombophlebitis most frequently involves the veins of the legs (leg). It may occur without apparent cause and tends to recur. At times it occurs as a result of local injury, either from a penetrating wound or from an external blow without a break in the skin. It may occur as a result of severe muscular effort or strain and in the course of infectious diseases (infectious disease), thromboangiitis obliterans, and a wide range of other underlying diseases. Thrombophlebitis may develop in various parts of the body if there is cancer, especially cancer of the pancreas. The presence of varicose veins (varicose vein) in the legs causes a tendency to the development of thrombophlebitis. Treatment includes bed rest and anticoagulant therapy.

       pulmonary embolism may occur in bedridden persons as a result of a clot from a thrombophlebitic lesion, or it may occur in an apparently healthy individual. If the embolus is small, it may not have any effect on the systemic circulation. With larger pulmonary emboli, there may be massive bleeding from the lungs (lung) and the development of a large area of pulmonary infarction, resulting in sudden death. Getting up and walking soon after an operation or after congestive heart failure is the best method for avoiding pulmonary embolism. Anticoagulant therapy is useful both as prevention and as therapy after the condition has developed. Surgical removal of a massive pulmonary clot has, on rare occasions, been spectacularly successful.

      Varicose veins (varicose vein) are permanently tortuous (twisted) and enlarged. The medium and large veins, especially in the legs, are most likely to be affected. The condition may occur without obvious cause or as a result of postural changes, occupation, congenital anomaly, or localized causes of increased venous pressure. The veins may be near the surface and easily seen, or they may be hidden and unrecognized. Without complication they rarely cause symptoms, but they may become the site of thrombophlebitis with inflammatory changes and the production of emboli in the peripheral circulation. The veins may rupture on occasion, with bleeding into the surrounding tissues. Varicose veins may occur around the rectum and anus, producing hemorrhoids (hemorrhoid). If they occur within the scrotal sac in the region of the testes (testis), they are called varicocele. In all forms of varicose veins, the walls of the veins become hardened, and a certain amount of inflammation develops through the years. Dilated veins in the legs may be supported by appropriate elastic-type stockings or bandages, or they may be treated by surgery.

Functional disease
      Direct mechanical injury or an infection or other disease process in the neighbouring tissues may produce spasms in the veins (venospasms). Local venospasm is usually of relatively minor significance because of the adequacy of alternate pathways for the blood. If venospasm is widespread, however, involving an entire extremity or the veins in the lungs, it may impair blood flow and therefore be of greater significance.

Diseases of the capillaries (capillary)
      The capillaries (capillary) are the smallest blood vessels. Through their thin walls oxygen and nutrients pass to the tissue cells (cell), in exchange for carbon dioxide and other products of cellular activity. Despite the small size and thin walls of the capillaries, the blood pressures (blood pressure) may be quite high, as, for instance, in the legs of a person in a motionless upright position. In certain disease states there is increased fragility of the capillary wall, with resultant hemorrhages into the tissues. These hemorrhages are referred to as petechiae when small; if large, they may become a large area of discoloration of the skin. vitamin C deficiency and a variety of blood disorders may be associated with increased capillary fragility. Small petechial hemorrhages occur in bacterial endocarditis and certain other infectious processes. In some instances petechiae are caused by minute emboli; in others they appear to be directly related to capillary fragility itself. Treatment is of the underlying disorder.

      The capillaries are freely permeable to water and small molecules but ordinarily are not highly permeable to proteins (protein) and other materials. In some pathological situations, such as in certain allergic states (e.g., hives) or because of local injury, as in burns (burn), there may be local areas of permeability, with escape of fluid high in protein into the surrounding tissues. If the disease affects the entire body, a significant amount of plasma (the blood minus its cells) leaks into the nonvascular spaces, with resultant loss in blood volume. Again, treatment is of the underlying disorder.

Mark L. Entman

Hemodynamic disorders

      Hypertensive heart disease is discussed in the section Acquired heart disease (cardiovascular disease).

      Moderate hypotension (low blood pressure) may occur in persons who are weak and enfeebled but more often does not represent a diseased state. Indeed, life insurance figures demonstrate that the life expectancy of people with such a condition is greater than average. Hypotension of a severe degree may develop in heart failure, after hemorrhage, in overwhelming infections, and in a variety of circumstances that lead to the development of the clinical picture of shock. In shock the circulation is inadequate, blood pressure is low, heart rate is rapid, and irreversible tissue damage from insufficient blood supply may occur if the condition is not terminated (see below Physiological shock (cardiovascular disease)). Transient hypotension may occur as a normal reaction in certain forms of syncope but is not necessarily associated with organic disease.

      Syncope is the sudden loss of consciousness associated with a transient disorganization of circulatory function, as differentiated from other brief losses of consciousness associated with abnormal central nervous system (nervous system, human) activities, as in certain forms of epilepsy.

      The most common kind of syncope is ordinary fainting. Some individuals are more susceptible than others. Blood loss, exhaustion, the presence of other illness, and psychological factors may contribute to a tendency to faint. An affected person is usually in the upright position, becomes weak, pale, and sweaty, and may have nausea. The heart rate at this time is usually relatively rapid, but, with the abrupt onset of syncope, the heart rate often falls to below the normal level, and the person collapses as if dead. There is usually a rapid recovery without complications.

      Syncope can occur with any cardiac rhythm disturbance that compromises circulation, such as a transient cessation of circulatory activity due to heart block. Other forms of syncope occur as a result of lowered blood pressure upon assumption of an upright position, a condition often called orthostatic hypotension. In some individuals, disease of the autonomic nervous system prevents appropriate postural adjustments for the upright stance. The disorder may be caused by vascular or central nervous system involvement of the autonomic system. In other instances, postural hypotension may occur as a result of inadequate blood volume, of taking various drugs (drug) that affect the nervous control of the circulation, and from a wide variety of other causes. Transient hypotension also may result from hypersensitivity of the carotid sinus. Patients with stenotic (narrowed) aortic or mitral valves may experience syncope during exercise; these patients are at high risk for sudden cardiac death.

James V. Warren Michael Francis Oliver

Physiological shock
      Physiological shock may be defined as acute progressive circulatory failure, in which the tissues receive an inadequate supply of blood and its components (such as nutrients and oxygen) and an inadequate removal of wastes. The result is cell damage and, eventually, cell death. This definition is derived from the one constant feature of physiological shock: the failure of adequate blood flow through the capillaries (capillary), the smallest of the blood vessels. Shock may be so severe as to impair organ function or create a state of blood flow deficiency that grows progressively more dangerous.

Shock due to inadequate blood (blood disease) volume
      Hemorrhage is the most common cause of shock. In the “average American man” (weighing 86 kg, or about 190 pounds) the blood volume is about 78 ml per kilogram (about 6.7 litres [7 quarts] for a man weighing 86 kg), and the loss of any part of this will initiate certain cardiovascular (human cardiovascular system) reflexes. Hemorrhage results in a diminished return of venous blood to the heart, the output (cardiac output) of which therefore falls, causing a lowering of the arterial blood pressure. When this occurs, pressure receptors (baroreceptors) in the aorta and carotid arteries (carotid artery) will initiate remedial reflexes either through the autonomic (autonomic nervous system) (nonvoluntary) nervous system by direct neural transmission or by epinephrine (epinephrine and norepinephrine) (adrenaline) secretion into the blood from the adrenal gland.

      The reflexes consist of an increase in the rate and power of the heartbeat, increasing its output; a constriction of arterioles leading to nonessential capillary beds (notably the skin and some viscera); and a constriction of the veins (vein), diminishing the large proportion of the blood volume normally contained therein. By these means arterial blood pressure will tend to be maintained, thus preserving blood flow to the vital areas, such as the brain and the myocardium. After continued acute blood loss of 20 to 30 percent of the blood volume, the compensatory mechanism will begin to fail, the blood pressure will begin to fall, and shock will ensue.

      Increased sympathetic (autonomic) nervous activity thus accounts for the fast pulse rate, pallor, and coldness of the skin in shock and, in addition, is the cause of increased sweating and dilation of the pupils (pupil) of the eyes. Air hunger and mental confusion are caused by the inadequate carriage of oxygen, and decreased urine flow stems from a decrease in the renal (renal system) ( kidney) blood flow, which, if severe, can lead to kidney failure. If acute blood loss continues beyond about 50 percent, the inadequacy of flow through vital circulations will lead to death. Loss of whole blood is not necessary for the blood volume to be low; plasma loss through burnt areas of the skin, dehydration following inadequate intake of fluid, or exceptional fluid loss can lead to contraction of the blood volume to levels capable of causing shock.

      When it is possible to anticipate blood loss and to measure it accurately—e.g., during an operation—losses may be immediately replaced before significant volume depletion can occur. More often, however, hemorrhage is unexpected; it may not be possible to measure the amount of blood lost. If shock occurs in an otherwise healthy person, the lost blood generally is replaced by transfusion into a vein. But, since a preliminary matching between recipient serum and donor cells must be carried out and cannot be done in less than 20 minutes, other fluid is usually given intravenously during the delay. This fluid, such as plasma or a solution of the carbohydrate dextran, must contain molecules large enough so that they do not diffuse through the vessel walls. Since the main loss from burns (burn) is plasma and electrolytes (electrolyte), these require replacement in proportion to the area of the burn and the size of the patient.

Shock due to inadequate cardiac output
      Sudden interference with the blood supply to the heart muscle, as by a thrombosis in a coronary artery, causes damage to the muscle with resultant diminution in its contractile force. The output of the heart falls; if the decline is severe, a fall in blood pressure stimulates the baroreceptors and, in the way just described, cardiogenic shock results. This occurs uncommonly after myocardial infarction. But low heart output alone may not account for the shock, for in chronic heart failure the cardiac output may be low without such a response in the peripheral circulation.

Shock due to increased circulatory capacity
      If widespread dilation of the veins or of the capillary beds occurs, the blood volume is no longer sufficient to fill the larger space and shock ensues.

Bacteremic shock
      Infection anywhere in the body may spread to the circulation, and the presence of organisms in the bloodstream— bacteremia—may lead to shock. bacteria are conveniently divided into “gram-positive” and “gram-negative” groups according to their reaction to a special staining method called the Gram stain.

  Gram-negative bacteremia is the more common and more lethal type of bacteremic shock. It is frequently caused by Escherichia coli, Proteus, Pseudomonas, or Klebsiella organisms; the first of these normally inhabits the intestine. The clinical picture of gram-negative bacteremia is much like that of hemorrhage, although no blood has been lost. This type of shock typically causes a rapid, thready pulse; cold, sweaty skin; and low blood pressure. A fever may occur, in addition to the local signs of the associated infection. The cause of the type of reaction is uncertain. The response to bacteremia from gram-positive organisms such as staphylococcus and Streptococcus is different: widespread dilation of the blood vessels results in warm, dry skin and a full-volume pulse in spite of lowered blood pressure.

      In both types of bacteremia the condition may be exacerbated by contraction in blood volume. This follows fluid loss—e.g., in the peritoneal cavity in peritonitis (inflammation of the peritoneum, the membrane that lines the abdominal cavity), in the tissues in streptococcal infection, or through the intestine in enteritis (inflammation of the intestine).

      Exceeded in frequency only by cardiogenic and hemorrhagic shock, bacteremic shock is most often caused by gram-negative organisms. There are three aspects of treatment: collections of pus are drained as soon as possible; the circulatory volume is increased to compensate for enlargement of the vascular bed; and appropriate antibiotics (antibiotic) are administered.

      Anaphylactic shock is discussed in detail in the article immune system disorder. An anaphylactic reaction is the direct result of the entrance of a specific foreign material into the bloodstream of a person whose body has become sensitized against it as a result of previous exposure and subsequent formation of antibodies (antibody). During an anaphylactic reaction, lung bronchi constrict intensely, narrowing the airways and interfering seriously with respiration (respiration, human); blood pressure may fall precipitously because of the release of substances ( serotonin, histamine, and bradykinin) that cause dilation of the arterioles and venules and an increase in the capillary wall permeability. Thus, the circulatory capacity is increased, and fluid is lost into the tissues.

      The essence of treatment of anaphylactic shock is the injection of epinephrine—a powerful stimulatory drug also found naturally in the body, whose effects include an increase in the heart rate and constriction of the blood vessels—followed by an antihistamine to counteract the reaction to the foreign substance and a bronchodilator to ease breathing.

Psychogenic shock
      Psychogenic shock causes fainting (syncope), probably by initiating dilation of the blood vessels that perfuse the muscles (muscle). In this type of shock, blood pressure falls, the skin becomes cold and sweaty, and the pulse rate increases. A decrease in the amount of blood that is supplied to the brain leads to light-headedness and loss of consciousness. A person who is suffering from psychogenic shock should be placed flat or even with the head slightly lower than the rest of the body in order to restore a good flow of blood to the brain and to bring about recovery from the fainting.

Drugs and shock
      Most anesthetic drugs (drug)— nitrous oxide is a notable exception—have a profound effect on the circulation. They are able to decrease the contractility of the heart muscle as well as increase the circulatory capacity by dilating the blood vessels. In addition, the normal postural circulatory reflexes are lost, so pooling of blood in the legs is liable to occur if the affected person is tilted to a head-up position. This is of particular importance after surgery; if a person is made to sit up too soon, it can lead to low blood pressure and an insufficient flow of blood to the brain. Overdosage of certain drugs—notably barbiturates (barbiturate), narcotics (narcotic), and tranquilizers (tranquilizer)—blocks normal circulatory reflexes and causes dilation of the blood vessels, leading to a fall in blood pressure that often is accompanied by a slow, full-volume pulse.

      A blood pressure that is dangerously low may be raised to safer levels by affecting the activity of the offending drug in one of many different ways. A therapeutic approach might entail, for instance, decreasing the dosage of the drug (such as an anesthetic agent), speeding up its elimination from the body, or administering a substance that is able to constrict the blood vessels. The choice of approach depends on the individual circumstance.

Neurogenic shock
      The maintenance of the tone of the blood vessels by the autonomic nervous system may be affected by severance of one of these nerves or by its interruption of the flow of nervous impulses. Thus, spinal anesthesia—injection of an anesthetic into the space surrounding the spinal cord—or severance of the spinal cord results in a fall in blood pressure because of dilation of the blood vessels in the lower portion of the body and a resultant diminution of venous return to the heart.

      Neurogenic shock does not usually require specific therapy; indeed, spinal anesthetics may be given with a view to producing a low blood pressure so as to diminish bleeding during an operation. If blood pressure becomes critically low, the legs are sometimes elevated and a vasoconstrictor administered.

      The endocrine (endocrine system, human) glands (gland) play a vital role in the regulation of normal metabolic (metabolism) processes through the actions of their hormones (hormone). It is not surprising, therefore, that a malfunction in an endocrine gland or in its hormones has an effect on circulation. Inadequate secretion by the adrenal (epinephrine and norepinephrine) cortex, the outer substance of the adrenal gland, leads to shock both by the diminution of myocardial efficiency and by a decrease in the blood volume. Functional disorders of the pituitary (pituitary gland), the adrenal medulla (the inner substance of the adrenal gland), the thyroid (thyroid gland), and the parathyroids (parathyroid gland) can all lead to circulatory upset and shock.

Refractory and irreversible shock
      The terms refractory shock and irreversible shock are widely used by physicians and other medical workers to refer to types of shock that present particularly difficult problems. The term refractory shock is applied when, in spite of apparently adequate therapy, the shock state continues. Commonly, the treatment later proves to have been inadequate, in which case the shock was not true refractory shock. This often occurs following a major injury in which there is internal bleeding, leading to underestimation of true blood loss and therefore to inadequate transfusion (blood transfusion). In certain cases, however, even if the therapy (therapeutics) actually is appropriate, the shock state persists; if patients in such cases respond to further special treatment, then this is true physiological refractory shock.

      In severe or prolonged shock states, the myocardial blood supply is sufficiently diminished to damage the heart's pumping action temporarily or permanently. Also, noxious products of inadequately perfused tissues may circulate and affect the heart muscle.

      While the flow of blood through major vessels (blood vessel) is under the control of the nerves, circulation through the capillary beds is of a more primitive type and is under the influence of local metabolic products. In shock, arteriolar constriction causes inadequate flow through the tissues, and local waste products increase. These cause dilation of the capillary sphincters and opening of the whole capillary bed, which thus contains an increased proportion of the blood volume. The capillaries become further engorged with slowly flowing blood, and fluid leaks through the vessel walls into the tissues. Thus, the body is further deprived of circulating blood volume.

      Widespread clotting of the blood can occur during capillary stagnation. This leads to severe damage to the cells unsupplied by flowing blood. Later, when enzymes (enzyme) dissolve the fibrin of the clots, the flow through these areas carries toxic metabolic products to vital organs—such as the heart, kidneys, or liver—and the ensuing damage leads to irreversibility of shock.

W.G. Prout

Additional Reading
J. Willis Hurst et al. (eds.), The Heart, Arteries, and Veins, 6th ed. (1986); Harry A. Fozzard et al. (eds.), The Heart and Cardiovascular System: Scientific Foundations, 2 vol. (1986); Wrynn Smith, Cardiovascular Disease (1987); Arthur J. Moss, Moss' Heart Disease in Infants, Children, and Adolescents, 3rd ed., edited by Forrest H. Adams and George C. Emmanouilides (1983); Gail G. Ahumada (ed.), Cardiovascular Pathophysiology (1987); Robert H. Anderson et al. (eds.), Paediatric Cardiology, 2 vol. (1987); Edward K. Chung (ed.), Quick Reference to Cardiovascular Diseases, 3rd ed. (1987); and Anders G. Olsson (ed.), Atherosclerosis: Biology and Clinical Science (1987).Treatment and prevention of vascular problems are the subject of Jeanette Kernicki, Barbara L. Bullock, and John Matthews, Cardiovascular Nursing: Rationale for Therapy and Nursing Approach (1970); George E. Burch and Travis Winsor, A Primer of Electrocardiography, 6th ed. (1972); Joseph K. Perloff, Physical Examination of the Heart and Circulation (1982); Jeffrey W. Elias and Phillip Howard Marshall (eds.), Cardiovascular Disease and Behavior (1987); and Eugene Braunwald (ed.), Heart Disease: A Textbook of Cardiovascular Medicine, 2nd ed. (1984).Michael Francis Oliver

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Universalium. 2010.

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