What is heart disease?

Causes and Symptoms

The heart
is a fist-sized organ located in the lower left quarter of the chest. It consists of four chambers: the right and left atria on top and the right and left ventricles at the bottom. The chambers are enclosed in three layers of tissue: the outer layer (epicardium), the middle layer (myocardium), and the inner layer (endocardium). Surrounding the entire organ is the pericardium, a thin layer of tissue that forms a protective covering for the heart. The heart also contains various nodes that transmit electrochemical signals, causing heart muscle tissue to contract and relax in the pumping action that carries blood to organs and cells throughout the body.

Signals from the brain cause the heart to contract rhythmically in a sequence of motions that move the blood from the right atrium down through the tricuspid valve into the right ventricle. From here, blood is pushed through the pulmonary valve into the lungs, where it fulfills one of its major functions: to pick up oxygen in exchange for carbon dioxide. From the lungs, the blood is pumped back into the heart, entering the left atrium from which it is pumped down through the mitral valve into the left ventricle. Blood is then pushed through the aortic valve into the main artery of the body, the aorta, from which it starts its journey to the organs and cells. As it passes through the arteries of the gastrointestinal system, the blood picks up nutrients which, along with the oxygen that it has taken from the lungs, are brought to the cells and exchanged for waste products and carbon dioxide. The blood then
enters the veins, through which it is eventually returned to the heart. The heart nourishes and supplies itself with oxygen through the coronary arteries, so called because they sit on top of the heart like a crown and extend down the sides.

The heart diseases collectively include all the disorders that can befall every part of the heart muscle: the pericardium, epicardium, myocardium, endocardium, atria, ventricles, valves, coronary arteries, and nodes. The most significant sites of heart diseases are the coronary arteries and the nodes; their malfunction can cause coronary artery disease and cardiac arrhythmias, respectively. These two disorders are responsible for the majority of heart disease cases.

Coronary artery disease occurs when matter such as cholesterol and fibrous material collects and stiffens on the inner walls of the coronary arteries. The plaque that forms may narrow the passage through which blood flows, reducing the amount of blood delivered to the heart, or may build up and clog the artery entirely, shutting off the flow of blood to the heart. In the former case, when the coronary artery is narrowed, the condition is called ischemic heart disease. Because the most common cause of
ischemia is narrowing of the coronary arteries to the myocardium, another designation of the condition is myocardial ischemia, referring to the fact that blood flow to the myocardium is impeded. Accumulation of plaque within the coronary arteries is referred to as coronary atherosclerosis.

As the coronary arteries become clogged and then narrow, they can fail to deliver the required oxygen to the heart muscle, particularly during stress or physical effort. The heart’s need for oxygen exceeds the arteries’ ability to supply it. The patient usually feels a sharp, choking pain, called angina pectoris. Not all people who have coronary ischemia, however, experience anginal pain; these people are said to have silent ischemia.

The danger in coronary artery disease is that the accumulation of plaque will progress to the point where the coronary artery is clogged completely and no blood is delivered to the part of the heart serviced by that artery. The rough, uneven texture of the plaque instead may cause the formation of a blood clot, or thrombus, which closes the artery in a condition called coronary thrombosis. The result is a myocardial infarction (commonly called a
heart attack), in which some myocardial cells die when they fail to receive blood.

Although coronary ischemia is usually thought of as a disease of middle and old age, in fact, it starts much earlier. Autopsies of accident victims in their teens and twenties, as well as young soldiers killed in battle, show that coronary atherosclerosis is often well advanced in young persons. Some reasons for these findings and for why the rates of coronary artery disease and death began to rise in the twentieth century have been proposed. While antibiotics and vaccines reduced the mortality of some bacterial and some viral infections, Western societies underwent significant changes in lifestyle and eating habits that contributed to the rise of coronary heart disease: high-fat diets, obesity, and the stressful pace of life in a modern industrial society. Furthermore, cigarette smoking, once almost a universal habit, has been shown to be highly pathogenic (disease-causing), contributing significantly to the development of heart disease, as well as lung cancer, emphysema, bronchitis, and other disorders. In the early and middle decades of the twentieth century, coronary heart disease was considered primarily an ailment of middle-aged and older men. As women began smoking, however, the incidence shifted so that coronary artery disease became almost equally prevalent, and equally lethal, among men and women.

Other conditions such as hypertension or diabetes mellitus are considered precursors of coronary artery disease. Hypertension, or high blood pressure, is an extremely common condition that, if unchecked, can contribute to both the development and the progression of coronary artery disease. Over the years, high blood pressure subjects arterial walls to constant stress. In response, the walls thicken and stiffen. This “hardening” of the arteries encourages the accumulation of fatty and fibrous plaque on inner artery walls. In patients with diabetes mellitus, blood sugar (glucose) levels rise either because the patient is deficient in insulin or because the insulin that the patient produces is inefficient at removing glucose from the blood. High glucose levels favor high fat levels in the blood, which can cause atherosclerosis.

Cardiac arrhythmias are the next major cause of morbidity and mortality among the heart diseases. Inside the heart, an electrochemical network regulates the contractions and relaxations that form the heartbeat. In the excitation or contraction phase, a chain of electrochemical impulses starts in the upper part of the right atrium in the heart’s pacemaker, the sinoatrial or sinus node. The impulses travel through internodal tracts (pathways from one node to another) to an area between the atrium and the right ventricle called the atrioventricular node. The impulses then enter the bundle of His, which carries them to the left atrium and left ventricle. After the series of contractions is complete, the heart relaxes for a brief moment before another cycle is begun. On the average, the process is repeated sixty to eighty times a minute.

This is normal rhythm, the regular, healthy heartbeat. Dysfunction at any point along the electrochemical pathway, however, can cause an arrhythmia. Arrhythmias range greatly in their effects and their potential for bodily damage. They can be completely unnoticeable, merely annoying, debilitating, or frightening. They can cause blood clots to form in the heart, and they can cause sudden death.

The arrhythmic heart can beat too quickly (tachycardia) or too slowly (bradycardia). The contractions of the various chambers can become unsynchronized, or out of step with one another. For example, in atrial flutter or

atrial fibrillation, the upper chambers of the heart beat faster, out of synchronization with the ventricles. In ventricular tachycardia, ventricular contractions increase, out of synchronization with the atria. In
ventricular fibrillation, ventricular contractions lose all rhythmicity and become uncoordinated to the point at which the heart is no longer able to pump blood. Cardiac death can then occur unless the patient receives immediate treatment.

An arrhythmic disorder called heart block occurs when the impulse from the pacemaker is “blocked.” Its progress through the atrioventricular node and the bundle of His may be slow or irregular, or the impulse may fail to reach its target tissues. The disorder is rated in three degrees. First-degree heart block is detectable only on an electrocardiogram (ECG or EKG), in which the movement of the impulse from the atria to the ventricles is seen to be slowed. In second-degree heart block, only some of the impulses generated reach from the atria to the ventricles; the pulse becomes irregular. Third-degree heart block is the most serious manifestation of this disorder: No impulses from the atria reach the ventricles. The heart rate may slow dramatically, and the blood flow to the brain can be reduced, causing dizziness or loss of consciousness.

Disorders that affect the heart valves usually involve
stenosis (narrowing), which reduces the size of the valve opening; physical malfunction of the valve; or both. These disorders can be attributable to infection (such as rheumatic fever) or to tissue damage, or they can be congenital. If a valve has narrowed, the passage of blood from one heart chamber to another is impeded. In the case of mitral stenosis, the mitral valve between the left atrium and the left ventricle is narrowed. Blood flow to the left ventricle is reduced, and blood is retained in the left atrium, causing the atrium to enlarge as pressure builds in the chamber. This pressure forces blood back into the lungs, creating a condition called pulmonary edema, in which fluid collects in the air sacs of the lungs. Similarly, malfunctions of the heart valves that cause them to open and close inefficiently can interfere with the flow of blood into the heart, through it, and out of it. This impairment may cause structural changes in the heart that can be life-threatening.

Heart failure

may be a consequence of many disease conditions. It occurs primarily in the elderly. In this condition, the heart becomes inefficient at pumping blood. If the failure is on the right side of the heart, blood is forced back into the body, causing edema in the lower legs. If the failure is on the left side of the heart, blood is forced back into the lungs, causing
pulmonary edema. There are many manifestations of heart failure, including shortness of breath, fatigue, and weakness.

Numerous diseases afflict the tissues of the heart wall—the epicardium, myocardium, and endocardium, as well as the pericardium. They are often caused by bacterial or viral infection, but they may also result from tissue trauma or a variety of toxic agents.

Treatment and Therapy

The main tools for diagnosing heart disease are the stethoscope, the ECG, and the x ray. With the stethoscope the doctor listens to heart sounds, which provide information about many heart functions such as rhythm and the status of the valves. The doctor can determine whether the heart is functioning normally in pumping blood from one chamber into the other, into the lungs, and into the aorta. The ECG gives the doctor a graph representation of heart function. Twelve to fifteen electrodes are placed on various parts of the body, including the head, chest, legs, and arms. The activities of the heart are printed on a strip of paper as waves or tracings. The doctor analyzes the printout for evidence of heart abnormalities, changes in heart function, signs of a heart attack, or other problems. Generally, the electrocardiographic examination is conducted with the patient at rest. In some situations, however, the doctor wishes to view heart action during physical stress. In this case, the electrodes are attached to the patient and the patient is required to exercise on a treadmill or stationary bicycle. The physician can see what changes in heart function occur when the cardiac workload is increased. The x ray gives the doctor a visual picture of the heart. Any enlargements or abnormalities can be seen, as well as the status of the aorta, pulmonary arteries, and other structures.

Another standard diagnostic tool is the echocardiograph. High-frequency sound waves are pointed at the heart from outside the body. The sound waves bounce against heart tissue and are shown on a monitor. The general configuration of the heart can be seen, as well as the shape and thickness of the chamber walls, the valves, and the large blood vessels leading to and from the heart. Velocity and direction of blood flow through the valves can be determined.

Various procedures can help the doctor assess the degree of ischemia within the heart. In one test, a radioactive isotope is injected into a vein and its dispersion in the heart is read by a scanner. This procedure can show which parts of the heart are being deprived of oxygen. In another test using a radioactive isotope, the reading is made while the patient exercises, in order to detect any changes in expansion and contraction of the heart wall that would indicate impaired circulation. The coronary angiogram gives a picture of the blockage within the coronary arteries. A thin tube called a catheter is threaded into a coronary artery, and a dye that is opaque to x rays is released. The x-ray picture will reveal narrowings in the artery resulting from plaque buildup.

The main goals of therapy in treating heart diseases are to cure the condition, if possible, and otherwise help the patient live a normal life and prevent the condition from becoming worse. In coronary artery disease, the physician seeks to maintain blood flow to the heart and to prevent heart attack. Hundreds of medications are available for this purpose, including vasodilators (agents that relax blood vessel walls and increase their capacity to carry blood). Chief among the coronary vasodilators are nitroglycerin and other drugs in the nitrate family. Also, calcium-channel blockers are often used to dilate blood vessels. Beta-blocking agents are used because they reduce the heart’s need for oxygen and alleviate the symptoms of angina. In addition, various support measures are recommended by physicians to stop plaque buildup and halt the progress of the disease. These include losing weight, reducing fats in the diet, and stopping smoking. The physician also treats concomitant illnesses that can contribute to the progress of coronary artery disease, such as hypertension and diabetes.

Sometimes medications and diet are not fully successful, and the ischemia continues. The cardiologist can unblock a clogged artery by a procedure called

angioplasty. The physician threads a catheter containing a tiny balloon to the point of the blockage. The balloon is inflated to widen the inner diameter of the artery, and blood flow is increased. This procedure is often successful, although it may have to be repeated. In atherectomy, a miniature drill shaves off the plaque, which is then removed. If neither procedure is successful, coronary bypass surgery may be indicated. In this procedure, clogged coronary arteries are replaced with healthy blood vessels from other parts of the body.

When coronary artery disease progresses to a heart attack, the patient should be treated in the hospital or similar facility. The possibility of sudden death is high during the attack and remains high until the patient is stabilized. Emergency measures are undertaken to minimize the extent of heart damage, reduce heart work, keep oxygen flowing to all parts of the body, and regulate blood pressure and heartbeat.

Cardiac arrhythmias can be managed by a variety of medications and procedures. Digitalis, guanidine, procainamide, tocanamide, and atropine are widely used to restore normal heart rhythm. In acute situations, the patient’s heart rhythm can be restored by electrical cardioversion, in which an electrical stimulus is applied from outside the body to regulate the heartbeat. When a slowed heartbeat cannot be controlled by medication, a pacemaker may be implanted to regulate heart rhythm.

Treatment of heart valve disorders and disorders of the heart wall is directed at alleviating the individual condition. Antibiotics and/or valve replacement surgery may be required. In many cases, valve disorders can be completely corrected. Cardiac transplantation remains a possible treatment for some heart patients. This is an option for comparatively few patients because there are ten times as many candidates for heart transplants as there are available donor hearts.

Perspective and Prospects

Heart disease became a major killer in the United States in the twentieth century. In the early decades, the best that the medical community could do was to treat symptoms. Since then, the emphasis has shifted to prevention. Hundreds of investigative studies have been undertaken to determine the causes of the most prevalent heart dysfunction, coronary artery disease. Many of these studies have involved tens of thousands of subjects, and they point to a general consensus that coronary artery disease is a multifactorial disorder, the primary elements of which are cholesterol and other fatty substances circulating in the bloodstream, smoking, diabetes, high blood pressure, stress, and obesity.

The reasons that mortality from heart disease is declining include improved medications and treatment modalities, and much credit has to be given to the success of preventive measures. Millions of Americans have stopped smoking and have begun watching their diets. Entire industries are devoted to helping Americans eat more intelligently. While fast-food outlets continue to offer high-fat standards, such as hot dogs and hamburgers, they have also added salads and leaner selections.

Perhaps most important, medical and sociological authorities have turned their attention to children. Because advanced atherosclerosis has been detected in young men and women, cholesterol-watching has become a major preoccupation with parents and school dieticians. In addition, national programs have been instituted to discourage smoking among the young. Whether the rates of coronary heart disease will be lower in these individuals than in their parents remains to be seen, but the success of these measures in the older populations indicates that the prognosis is good.

The prognosis is also good for other heart diseases. New drugs continue to be licensed for the treatment of arrhythmias, and more versatile and reliable pacemakers increase the prospects of a normal life for many patients. Improvements in heart surgery have been particularly impressive, especially those for managing congenital heart defects in neonates and infants. Heart transplants have been successfully performed on these patients, and numerous other procedures promise significant improvement in the prospects of young people with heart disease.

Rheumatic fever, however, one of the major causes of heart disease in children, remains a threat. <No vaccine is available for immunization against the streptococcus strains that cause rheumatic fever, but fortunately there are effective antibiotics to control infection in these patients. Rheumatic fever usually develops subsequent to a streptococcal throat infection that has not been treated adequately with antibiotics. Careful evaluation of the child with a sore throat and prompt, complete antibiotic treatment of those with streptococcal infection can avoid progression of the infection to rheumatic fever.

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