CARDIOVASCULAR DISEASE

any of the diseases, whether congenital or acquired, of the heart and blood vessels. Among the most important are atherosclerosis, rheumatic heart disease, and vascular inflammation. Cardiovascular diseases constitute one of the major human health problems of modern times. 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. 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 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. 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. 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 for diagnosis became widespread. In 1912 James Bryan Herrick, a Chicago physician, first described what he called coronary thrombosis (he was describing symptoms actually caused by myocardial infarction; see below Acquired heart disease: Myocardial infarction). Angina pectoris (described and discussed below Acquired heart disease: Angina pectoris) had been recorded centuries earlier. Cardiovascular surgery in the modern sense began in the 1930s, and open-heart surgery 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 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 heart disease (heart disease resulting from thickening and hardening of the artery walls; see below) 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 cardiovascular disease, disease involving high blood pressure, constitutes a major health hazard. James V. Warren The Editors of the Encyclopdia Britannica Acquired heart disease Atherosclerosis Atherosclerosis is a thickening and hardening of the medium- and large-sized arteries. When this disease affects the coronary arteries, it accounts for a large proportion of heart attacks (seizures of the heart that cause a sudden reduction in the blood supply to the heart muscle). Atherosclerosis also causes ischemic heart disease (a disease characterized by an inadequate blood supply to the heart because of a constriction or obstruction of a blood vessel). Finally, atherosclerosis also accounts for many strokes, numerous instances of peripheral vascular disease, and most aneurysms (dilation in the wall of a blood vessel to form a blood-filled sac) of the aorta, which can rupture and cause fatal hemorrhage. The basis of coronary artery disease is the slow development of areas of thickening in the coronary arteries. These thickenings are called atherosclerotic plaques, or atheromatous lesions, and they develop early in life, progressing over a period of many years with phases of quiescence or even regression interspersed with periods of progression. These coronary lesions are found in virtually all adults in the industrialized world. Although most people who have these lesions will never develop signs or symptoms of heart disease, in others the lesions intrude into the lumen (cavity) of the coronary arteries, progressively impeding blood flow to the myocardium (heart muscle) and leading to the clinical syndromes of coronary heart disease. Two major factors determine the growth of atheromatous lesions. One is the accumulation of cholesterol at the areas where the thickening occurs and the other is the incorporation of minute clots, or thrombi, into the endothelial (inner) surface of the artery. Accumulation of cholesterol in atherosclerotic lesions is related to the concentration of cholesterol-carrying lipoproteins in the blood that flows through the coronary arteries. Elevation of the concentration of these lipoproteins is primarily determined by genetic factors but can also be influenced by environmental factors, such as a high-fat diet. There are alternative views on what actually causes the irreversible accumulation of cholesterol-carrying lipoproteins to form atheromatous lesions. One theory is that the degree of elevation of plasma cholesterol, in association with defective receptors on the lining of the arteries and arterioles, determines whether cholesterol accumulates. The second is that there is a low-grade inflammatory process in this lining (endothelium) of the artery wall that leads to the formation of microthrombi (minute clots) on the surface and the trapping of cholesterol-carrying lipoproteins in the artery. These two divergent theories of the development of atherosclerosisthe accumulation of cholesterol-carrying lipoproteins and endothelial thrombosis (formation of a clot of blood products)may be combined by accepting that both occur. Under most conditions the incorporation of cholesterol-rich lipoproteins is the predominant factor in determining whether or not plaques progressively develop. Then, the endothelial injury that results (or that may occur independently) leads to the involvement of two cell types circulating in the bloodplatelets and monocytes. 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, monocytes, and other elements of the blood release hormones, called growth factors, that stimulate proliferation of muscle cells in arteries. Atherosclerotic lesions are focal in nature, and their distribution is determined by the interrelation of hemodynamic physical forces (forces of blood movement) such as blood pressure, blood flow, and turbulence within the lumen. These lead to physical forces of parallel strain, or shear, on the endothelial lining, giving rise to areas of relatively positive and negative pressure. These 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. Arterial atheromatous lesions may undergo reversal, although much of the evidence for this reversal has been observed in experimental circumstances in primates or in early lesions in human femoral arteries. The extent to which it is possible to induce regression of advanced obstructive lesions in human coronary arteries is not yet understood with certainty. Coronary artery disease 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. The same symptoms are also called 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 speed of their development (acute thrombosisthat is, 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 anastamose (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 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. 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 Diseases of the arteries There are many types of arterial disease. Some are generalized and affect arteries through 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 Arteriosclerosis The various types of arteriosclerosis are by far the most common occlusive diseases of the arterial system and a major cause of death in one form or another. Atherosclerosis of the coronary arteries has already been discussed. Arteriosclerotic lesions of the cerebral vessels may lead to formation of blood clots and stroke. Medial (Mnckeberg's) arteriosclerosis is the condition affecting the media (middle coat) of the main arteries. There are deposits of calcium salts in the media, but in general the channel (lumen) is not blocked. This disease is quite common and may contribute to the other major form of widespread arteriosclerosis, atherosclerosis obliterans, in which there is complete obliteration of the vessel channel. The classic symptom of arteriosclerosis affecting the legs is called intermittent claudication (intermittent lameness), which results from inadequate blood flow to the muscles involved in walking. In such individuals a cramplike discomfort associated with a limp occurs in the calf of the leg on exercise and is relieved by rest. The vascular disease may lead to changes in the extremities caused by reduced blood supply. These may be manifested as ulcerated areas and eventually gangrene, and most frequently involve the lower extremities. The lack of adequate blood flow renders the extremities susceptible to infection, and, therefore, the manifestations of reduced blood supply (ischemia) are frequently combined with those of infection. There is no specific therapy available. Disturbances in rhythm and conduction The heart's rhythmical beat is initiated and regulated from centres within the organ. The primary pacemaker, the sinoatrial node, is a small mass of specialized muscle cells located at the juncture of the upper vena cava and the right atrium. Electrical impulses are emitted by this group of cells. The excitation spreads through the two atria and, by way of a band of fibres called the bundle of His, into the ventricles. The bundle of His has its beginning in a small mass of cells, the atrioventricular node, located beneath the lining of the right atrium. Normally initiated heart rhythm, originating in the sinoatrial node, is called sinus rhythm. Under stimulation from the central nervous system and other metabolic factors, heart rate may normally rise and fall, with a slight variation, in part related to respiratory activity. In young individuals in excellent physical condition, the resting heart rate may fall as low as 40 to 50 beats per minute, and, under stressful psychological stimulation, the heart rate may rise to as high as 200 beats per minute. These situations are to be differentiated from pathological variations in heart rate. Abnormal slowing of heart rate, or sinus bradycardia (a slow sinus rhythm with a rate below 60, caused by disturbance of the sinoatrial node) or acceleration of heart rate, or tachycardia (excessive rapidity in the action of the heart with a pulse rate of above 100 beats per minute) may occur in a wide variety of disease states and be symptomatic of the underlying disease. Extra beats arising from the atrium, the nodal tissues, or the ventricle are not in themselves abnormal, though beats arising from the ventricle are more often associated with organic heart disease. Occasional extra systoles (contractions) occur in many normal individuals. In cardiovascular disease they are much more common. They do not interfere with normal cardiovascular function if infrequent. It has been noted that continued psychological stress, excessive smoking, and drinking of large amounts of tea and coffee enhance the tendencies for premature contractions of the atria. Premature contractions of the ventricles are more ominous, especially those that occur after exercise. They have been found to be associated with coronary artery disease in a large percentage of instances. If frequent enough, they may be the harbinger of more serious ventricular arrhythmias. Atrial arrhythmias Abnormalities in the rhythm of atrial contractions include atrial tachycardia, atrial flutter, and atrial fibrillation. As noted earlier, atrial tachycardia may be a manifestation of underlying disease such as thyrotoxicosis or may be merely a matter of stressful stimulation in a normal individual. Some individuals have characteristic episodes of paroxysmal atrial tachycardia of varying frequency and duration, with a rapid onset and termination. Ordinarily the episodes occur in the absence of any other heart abnormality. Occasionally, especially when the episodes are prolonged or when they occur in the presence of organic cardiovascular disease, they may be accompanied by evidence of heart failure or, in rare instances, of the shock state. Atrial flutter represents another form of arrhythmia associated with very rapid atrial activity. The atrial activity is regular but so rapid that the conduction of the impulses to the ventricle may be delayed and impaired so that only one of two, three, or four impulses excite ventricular activity. This disorder is most often, but not always, seen in persons with organic heart disease. Atrial fibrillation is another form of arrhythmia, in which there is wildly erratic and ineffective atrial contraction. The ventricular response is also erratic, so that the pulse is irregular without a basic underlying rhythm. This disorder is seen most frequently in persons with organic heart disease, such as rheumatic heart disease with mitral involvement and thyrotoxicosis, but also occurs in normal individuals, often on a paroxysmal basis. It renders the atrium ineffective, and therefore the contribution of this chamber to the normal pumping of blood is negated. This condition may be of no real functional importance in the normal heart but may have a significant detrimental effect in the failing heart. The circulation functions reasonably well, though in the case of severe mitral stenosis a rapid heart action is not well tolerated because of the limited speed of ventricular filling through a small mitral opening. A complication of atrial fibrillation is the development of blood clots within the walls of the fibrillating atria. Because these clots may eventually fragment and pass into the circulation (arterial embolism), atrial fibrillation is a condition that should be treated if possible. The abnormal rhythm may be slowed by digitalis or terminated by the use of electrical shocks (electrical defibrillation). Heart failure Congestive heart failure, a syndrome resulting from disease that has caused the heart to be inadequate as a pump, is characterized by manifestations distant from the heart, predominantly related to salt and water retention in the tissues. It may vary from the most minimal symptoms to sudden pulmonary edema (abnormal accumulation of fluid in the lungs) or to a rapidly lethal shocklike state. Chronic states of varying severity may last years. The symptoms are related predominantly to secondary manifestations resulting from retention of fluid and vascular congestion throughout the body, rather than to the direct effect of lessened blood flow. In most instances, failure results from a diseased heart, though on occasion the burden of other systemic diseases may exceed the capacity of the previously adequate heart and produce the condition. A physiological characteristic of the normal heart is its ability to meet the fluctuating demands of the body for blood flow. The diseased heart may no longer be able to respond in this way. This failure may be the result of severe and acute damage, such as an acute myocardial infarction, or of chronic and lesser impairment, such as scarring of a valve in the course of a long-standing rheumatic heart disease. The possible causes of the underlying heart disease are numerous. One is coronary heart disease that has resulted in profound myocardial damage. Other common causes are rheumatic valvular disease, hypertensive vascular disease with involvement of the heart, or one of the multitudinous but less common types of primary disease of the myocardium. Regardless of the cause, the common denominator leading to heart failure is altered function of the heart muscle, with lessened ability to pump blood. In many sophisticated studies, clear-cut disturbances in the mechanics of heart muscle contraction have been demonstrated. The presence of cardiac failure becomes apparent largely by signs and symptoms not directly related to the heart. With early and untreated heart failure, the person usually has a normal salt and water intake, but his ability to regulate and promptly excrete excess sodium and water is impaired. The nature of this impairment is not entirely clear. Salt and water accumulate in the body as extracellular fluid that manifests itself as clinically detectable edema. If the person is active and in the upright position, the fluid may gather particularly about the ankles and legs. If he is in bed, it may accumulate in the back, overlying the sacrum. In heart failure, there may also be low blood sodium levels. Left ventricular failure The pulmonary circuit (the blood vessels in the lungs) usually becomes congested in heart failure, because heart diseases most frequently affect the left ventricle in one way or another. If disease impedes the left ventricle's pumping of blood into the systemic circulation, the left side of the heart is unable to receive the normal flow of oxygenated blood from the lungs; consequently there is back pressure, and blood accumulates in the lung's blood vessels. If this congestion of the pulmonary vessels occurs, it lessens the amount of space available in the lungs for air and tends to stiffen the lungs. Finally, pulmonary capillary pressure may reach the point at which fluid flows into the tissues outside the vessels, a condition called pulmonary edema. These phenomena account for the frequency of difficulty in breathing, inability to breathe except in an upright position, attacks of respiratory distress without apparent cause during sleep at night (paroxysmal nocturnal dyspnea), and other respiratory symptoms in congestive heart failure. Hemodynamic disorders Hypertension Hypertensive heart disease has been discussed above. Hypotension 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). Transient hypotension may occur as a normal reaction in certain forms of syncope but is not necessarily associated with organic disease. Surgical treatment of the heart Cardiopulmonary bypass Cardiopulmonary bypass serves as a temporary substitute for a patient's heart and lungs during the course of open-heart surgery. The patient's blood is pumped through a 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, the large veins 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, 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. Congenital 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; and those, such as intracardiac abnormalities, that require a heart-lung machine.