Overview of Myocardial Infarction
In the United States, about a million people a year suffer a heart attack, and two-thirds of them are men. A heart attack (myocardial infarction) occurs when a part of the heart muscle loses its blood supply. The heart muscle gets its oxygen and nutrients from blood traveling through tiny vessels called coronary arteries. Over the years these vessels may accumulate a hard, sticky material in their walls called plaque. (See "What is Coronary Artery Disease?") If this plaque suddenly ruptures, its surface becomes rough. Blood may clot at this rough surface, clogging the artery. This blockage causes a portion of the heart muscle (myocardium) to cramp as it becomes starved for nutrients. The patient may feel a pain in his chest, jaw, or left arm. He or she may sweat, find it hard to breathe or move, and may faint. If the coronary-artery blockage continues for more than a few minutes, the portion of the heart muscle it feeds will begin to die (infarct). This tissue death is irreversible and may lead to chronic cardiac problems, even if the patient survives the initial attack. If a heart attack is suspected, the patient should get medical attention as soon as possible. Half of all heart-attack deaths occur in the first three to four hours; prompt treatment can mean the difference between life and death.
Symptoms of Myocardial Infarction
About two-thirds of heart-attack victims experience chest discomfort that comes and goes, shortness of breath, and/or fatigue a few days to a few weeks beforehand. This is known as angina, and it usually lasts a few minutes and stops if the patient rests. Prompt medical treatment of angina can sometimes avert a heart attack. The first sign of a heart attack is usually constant discomfort in the chest, often extending to the jaw and left arm. (Rarely, the right arm may be affected.) Some victims feel discomfort only in the arm and/or jaw, rather than in the chest. About one third of patients—primarily women and non-Caucasians—do not feel discomfort but instead experience secondary symptoms: fatigue, sudden heavy sweating, nausea, and shortness of breath. (see
Heart Disease in Women) Up to a fifth of patients will experience mild symptoms or none at all. Unlike angina symptoms, heart-attack symptoms usually last from 30 minutes to several hours and do not stop if the patient rests.
Causes of Myocardial Infarction
Heart attacks usually occur as a result of plaque rupturing in the coronary arteries, as described in the Overview. A portion of the heart muscle dies because it cannot get oxygen and nutrients from the blood supply, which has been cut off by a clot. Occasionally a heart attack will result not from a clot but from a blood-vessel spasm, which narrows the vessel. It is not known why spasms occur, but when they happen near areas of partial plaque blockage, blood flow can become completely blocked, causing a heart attack.
A related condition, called cardiac arrest, can also cause a heart attack. Cardiac arrest occurs when the reduced blood flow from narrowed coronary arteries disturbs the heart’s electrical pacemaker function. The disturbance of these pacemaker signals causes the heart to stop beating or to beat ineffectively. Cardiac arrest will cause death in several minutes if not reversed with CPR and external defibrillation. (A “defibrillator” is the name for the electrical pads that emergency medics use to “shock” a patient’s chest). A person who survives cardiac arrest may or may not have suffered a heart attack (infarction of the heart muscle), depending on how long the person's heart had stopped beating.
Diagnosis of Myocardial Infarction
Paramedics in the field or doctors in the emergency room will often make the diagnosis using an electrocardiogram (ECG) machine, which monitors the heart’s electrical impulses. Some types of heart attacks cannot be detected by an electrocardiogram and must be diagnosed by a blood test, which may take several hours. This test looks for proteins released by damaged heart muscle. In most cases, a diagnosis can be made within 24 hours.
Complications of Myocardial Infarction
The extent of damage to the heart depends on which artery is clogged and for how long. If a large amount of heart muscle does not receive blood, the heart usually cannot pump adequately and the patient may die. Even less extensive tissue death can produce lasting damage. Dead cardiac tissue is replaced by scar tissue, which cannot participate in pumping. To compensate for this scar tissue, the heart may enlarge. This enlargement gives more force to the weakened pumping of the damaged heart, but it may cause congestive heart failure or heart-rhythm disturbances. (Congestive heart failure is a chronic, progressive disease in which the weakened heart fails to pump enough blood to many parts of the body.) Despite these complications, medical science has made great advances in caring for cardiac patients. The chance of dying after admission to the hospital for a heart attack has declined about 30 percent over the last few decades.
Self Care of Myocardial Infarction
Cardiologists usually recommend that patients increase their physical activity after discharge from the hospital. Some patients accomplish this by enrolling in a medically supervised exercise program. It is also essential to quit smoking, because smoking worsens coronary artery disease and increases the risk of another heart attack.