Heart in Health & Disease
The Heart Muscle
The heart is a very specialized muscle consisting of four chambers: a right and a left upper chamber (atrium) and a right and a left lower chamber (ventricle). The atria are thin-walled reservoirs for blood and empty directly into the thick walled, muscular lower chambers, which contract and pump the blood out of the heart. The heart is really "two hearts that beat as one." The left atrium and ventricle receive red, oxygenated blood from the lungs and pump it to the body. The right atrium and ventricle receive blue, de-oxygenated blood from the body and pump it to the lungs, where the blue blood is oxygenated.
The arteries are blood vessels that carry blood away from the heart, whereas veins carry blood back towards the heart. The heart muscle itself receives blood supply from two arteries: the left coronary and right coronary artery. "Coronary" means "crown" and the coronary arteries get their name by the fact that they encircle the top portion of the heart much like a crown. Specialized nerve cells in the heart generate and conduct electrical impulses that initiate the heart beat and coordinate the timing of contraction of the various chambers and allow the heart to beat in an organized fashion. A heart beat cycle consists of systole, when the heart is contracting and ejecting blood, and diastole, when the heart is relaxing and filling with blood for the next contraction.
Congestive Heart Failure
The heart is a muscular pump. In health, the heart circulates blood through the body by contracting vigorously in systole and ejecting blood into the great vessels, then relaxing in diastole and allowing blood to fill the ventricle in preparation for the next systolic contraction. The proper function of the heart depends both on normal contraction in systole and normal relaxation in diastole. Many disease states can weaken the heart muscle and affect its ability to contract normally. Some of these diseases include coronary atherosclerosis, heart attack, valvular heart disease, severe hypertension, thyroid disease, alcoholism, drug abuse, chemotherapy treatment, and viral infection. These disease states vary in mechanism, but all lead to the final common result of a heart that is enlarged and that weakly contracts. This is known as dilated cardiomyopathy, or enlarged heart, and is the cause of congestive heart failure. In left-sided congestive heart failure, a weakened left ventricle is unable to pump blood efficiently to the body. As a result, the blood backs up in the lungs, causing symptoms of shortness of breath. In right-sided congestive heart failure, a weakened right ventricle results in blood backing up in the lower extremities and causing leg swelling.
Improper relaxation of the left ventricle in diastole may also lead to left-sided congestive heart failure by not allowing the blood returning from the lungs to easily flow into the left ventricle. This is known as diastolic dysfunction, and results in the lungs becoming congested with blood. Congestive heart failure due to diastolic dysfunction frequently is associated with chronic hypertension but also occurs in the absence of hypertension as the normal heart ages.
Congestive heart failure is treated medically with digitalis, diuretics, ACE inhibitors, nitrates, and beta blockers. If a reversible underlying cause can be identified, more specific treatment is initiated. For example, congestive heart failure due to coronary disease is treated by coronary bypass or angioplasty.
In recent years, the area of congestive heart failure has itself become a subspecialty within cardiology, and congestive heart failure clinics are now open in many parts of the country.
Early symptoms of coronary artery disease include chest pain on exertion, or "angina." As the coronary artery disease progresses, the angina may occur with less and less exertion. This exertional angina ( also called stable angina) is significant with regards to symptoms but is also significant because the presence of cholesterol plaque in the coronary artery has the potential to cause myocardial infarction, or heart attack. Under certain conditions, a coronary plaque may become unstable and rupture. The various factors that lead to plaque rupture are not well understood and are currently actively being investigated. Coronary plaque rupture exposes the fatty contents of the plaque to the circulating blood, leading to a blood clot that occludes the channel of the coronary artery and prevents blood from flowing to the heart muscle. This results in "unstable angina" and heart attack, which are characterized by angina at rest.
Stable exertional angina is treated with a three-tiered approach of risk factor modification, medications, and coronary revascularization. Risk factor modification includes weight loss, adoption of a diet low in saturated fat, cessation of smoking, and initiation of an exercise program. These measures are aimed at reducing the coronary risk factors. Medications include aspirin, beta blockers, ACE inhibitors, and cholesterol-lowering drugs, all of which have been shown to reduce the risk of heart attack. Treatment of hypertension is done using a variety of antihypertensive agents nitrates are prescribed for controlling symptoms of angina. Coronary revascularization--with angioplasty or coronary artery bypass surgery--is performed in selected patients for relief of symptoms and, in some patients, for survival benefit.
The heart receives its blood supply from two coronary arteries: left coronary artery and right coronary artery, which are the first arteries to branch from the aorta. These principal arteries branch many times and finally become capillaries that supply the heart muscle. The left coronary (left main) branches into the left anterior descending artery, which supplies the anterior, or "front wall" of the left ventricle, and the circumflex, which supplies the lateral, or "side wall" of the left ventricle. The right coronary supplies the right ventricle as well as the posterior and inferior walls, or "back side," of the left ventricle. Blood flow to the heart muscle occurs during diastole, or between contractions, when the heart muscle is resting.
Coronary Artery Disease
The coronary arteries, can become diseased, a condition known as coronary insufficiency. Various conditions--such as elevated serum cholesterol, hypertension, diabetes, cigarette smoking--cause cholesterol deposits to accumulate in the inner lining of the coronary arteries, a process called atherosclerosis. Atherosclerosis is a slow process, occurring over decades, and results in a gradual narrowing of the channel of the coronary artery as well as diminished arterial capacity to dilate under work load conditions. The result is a compromise to the blood flow to the heart muscle. This compromise may not manifest when the body is at rest but becomes much more significant under work conditions.
Even under "normal" conditions, the workload demands on the heart vary significantly. When the body is at rest, the heart's oxygen demand is relatively low and the coronaries supply enough blood for baseline heart function. During exercise, the body's blood demands rise greatly and the heart must increase it's output of blood to meet these needs. In order to do this, the heart itself demands greater blood supply. In a healthy heart under work load conditions, the coronary arteries have a tremendous capacity to dilate, increasing by several fold their delivery of blood to the working heart muscle. However, when the coronary arteries become diseased, they lose the capacity to dilate and increase delivery of blood to meet the body's needs. The result is coronary insufficiency.
Blood flows through the heart in one direction. This unidirectional flow is made possible by the action of four valves inside the heart: tricuspid, mitral, aortic, and pulmonic. The tricuspid valve is located between the right atrium and right ventricle and opens during diastole, allowing blood to pass from right atrium to right ventricle. When the right ventricle contracts in systole, the higher pressure in the right ventricle forces the tricuspid valve to close and the pulmonic valve to open. Since blood cannot pass backwards through the closed tricuspid valve, it is forced out of the right ventricle through the open pulmonic valve into the pulmonary artery and on to the lungs for oxygenation. The oxygenated blood returns to the heart from the lungs via the pulmonary vein, which empties into the left atrium. Separating the left atrium from the left ventricle is the mitral valve. During diastole, the mitral valve opens and blood from the left atrium fills the left ventricle. When the left ventricle contracts during systole, the mitral valve closes, the aortic valve opens, and blood is vigorously ejected through the aortic valve and into the aorta, which carries oxygenated blood to the body. The "lub-dub" sound that one hears when listening through a stethoscope is actually the sound of moving blood striking against the valves that "slam" shut as the heart contracts and relaxes. A heart murmur is an extra sound that may be heard if there is turbulence of blood flow across a valve.
Valvular Heart Disease
In a normal, healthy heart, the blood flows freely through the valves when they are open but cannot leak through them when the valves are closed. Heart valves can become diseased, however, leading either to improper opening or improper closing of the valve.
In some disease states, the normal valve elasticity and compliance is lost over time and the valve becomes stiff, immobile, and unable to open completely, causing an obstruction to the free flow of blood (stenosis). Examples of diseases in which this occurs are mitral valve stenosis ensuing from rheumatic heart disease and aortic stenosis due to calcification and thickening of the aortic valve.
Alternatively, valves may close improperly and become leaky (regurgitant). This may occur because the heart has become enlarged or because the valve tissue has become degenerative for various reasons (valve infection, severe mitral valve prolapse, phen-fen use). Both valvular stenosis and regurgitation may cause an audible heart murmur.
Typical symptoms of valvular disease consist of shortness of breath and fatigue. In severe cases, dizziness or fainting may occur. Valvular disease may also lead to heart rhythm disturbances.
Valvular disease is quantified and graded as mild, moderate, or severe. Mild or moderate valve stenosis or regurgitation are usually treated with medications. However, severe valvular stenosis or regurgitation requires surgical treatment. This involves open heart surgery with repair or removal of the diseased valve and replacement with a prosthetic valve. Two main types of prosthetic valves are currently used: mechanical tilting disc valves (made of metal and plastic) and tissue bioprosthetic valves (made of porcine or bovine tissue). Mechanical valves require that the patient be placed on lifelong blood thinner (warfarin), whereas bioprosthetic valves do not. However, mechanical valves in general are more durable and less likely to degenerate with time.
Patients with significant valvular disease or prosthetic valves are instructed to take prophylactic antibiotics prior to having any dental work, in order to prevent infection of the diseased/prosthetic valve.
Electronic Conduction System
The organized beating of the heart takes place because of the heart's sophisticated conduction system, or "electrical wiring." The electrical impulse for a heartbeat begins in the sinoatrial (SA) node, a specialized group of pacemaker cells located in the right atrium which initiate a regular, clockwork-like series of electrical impulses that ultimately lead to a heartbeat. The SA node is responsive to the physiologic needs of the body and can increase or decrease its rate of firing accordingly. The electrical impulse that riginates in the SA node travels rapidly via nerve fibers through both atria to the atrioventricular (AV) node. As the impulse passes through the atria, it is dispersed to the muscle tissue in the atria and stimulates both atria to contract simultaneously and empty their contents into their respective ventricles. The AV node is like a "grand central station" of nerve fibers, and here the electrical impulse is slowed down and organized. This allows time for blood to pass from atria to the ventricles. From the AV node the impulse passes to the nerve bundle of His and Purkinjie fibers (named after the scientists who discovered them). These nerve fibers are like a system of "freeways" that rapidly carry the impulse and distribute it to both ventricles. The impulse reaches various regions of the ventricle in a specific order that leads to a choreographed ventricular contraction, such that contraction begins at the bottom (apex) of the heart and ends at the top (base) of the heart. This results in efficient ejection of blood out of the heart.
Disturbances in the sophisticated electrical wiring of the heart lead to abnormalities of the heart rhythm, or arrhythmia. The study and treatment of these arrhythmias is called electrophysiology. Arrhythmias can be divided into bradycardia (slow heart beat) and tachycardia (fast heart beat).
Bradycardia usually result from degenerative disease of the conduction nerves and nerve centers in the heart, such as the sinus node, AV node, and bundle of His. For example, the sinus node may fire too slowly or not fire at all, causing sinus arrest. In this setting, one of the backup pacemaker centers of the heart takes over, usually at a very slow heart rate. Alternatively, the AV node may conduct too slowly or not at all, causing a breakdown in communication between the atria and ventricles, or AV dissociation. Severe disease in the Bundle of His may also cause AV dissociation. Some bradycardia may also result from high dose of medications, such as beta blockers. Bradycardia may cause symptoms such as weakness, dizziness, or fainting spells. Severe bradycardia are treated with a pacemaker.
Tachycardia are characterized by an erratic, rapid beating of the heart that overrides the normal SA node pacemaker. Tachycardia that have their origin in the atria are called atrial tachycardia. These include atrial fibrillation, atrial flutter, and atrial tachycardia.
Atrial fibrillation is a very common arrhythmia characterized by a very rapid and irregular beating of the heart. This arrhythmia is usually treated with digoxin or a beta blocker for control of the heart rate. Other antyrrhythmic drugs, such as stall, disopyramide, or amiodarone may be added to suppress the fibrillation rhythm itself. In some cases, electrical shock can be used to convert the patient back to sinus rhythm. Since atrial fibrillation may lead to the formation of blood clots in the heart, patients with atrial fibrillation are usually treated with coumadin, a potent blood thinner, to prevent the occurrence of stroke.
Atrial flutter and atrial tachycardia typically cause symptoms of palpitations or lightheadedness.
Tachycardias that originate in the ventricles, termed ventricular tachycardias, are usually more dangerous in that they may cause severe drops in blood pressure, leading to loss of consciousness. Some of these arrhythmias can be controlled with medications. However, some severe tachycardias cause repeated lapses of consciousness and are life-threatening. These are treated with an automatic implantable cardiac defibrillator (AICD), a device implanted under the skin that detects a dangerous tachycardia and terminates it with an electrical shock. Some difficult-to-treat tachycardias that do not respond to medicine may be ablated by radio-frequency catheter ablation therapy, which is done through a catheter in the groin.