Myocardial infarction (MI)

infarction of a segment of heart muscle, usually due to occlusion of a coronary artery. SYN: cardiac infarction, heart attack.MI is the most common cause of death in the U.S. Each year about 800,000 people sustain first heart attacks, with a mortality rate of 30%, and 450,000 people sustain recurrent heart attacks, with a mortality rate of 50%. The most common cause of MI is thrombosis of an atherosclerotic coronary artery. Infarction of a segment of myocardium with a borderline blood supply can also occur because of a sudden decrease in coronary flow (as in shock and cardiac failure), a sudden increase in oxygen demand (as in strenuous exercise), or hypoxemia. Less common causes are coronary artery anomalies, vasculitis, and spasm induced by cocaine, ergot derivatives, or other agents. Risk factors for MI include male gender, family history of myocardial infarction, obesity, hypertension, cigarette smoking, prolonged estrogen replacement therapy, and elevation of total cholesterol, LDL cholesterol, homocysteine, lipoprotein Lp(a), or C-reactive protein. At least 80% of MIs occur in people without a prior history of angina pectoris, and 20% are not recognized as such at the time of their occurrence either because they cause no symptoms (silent infarction) or because symptoms are attributed to other causes. Some 20% of people sustaining MI die before reaching a hospital. Classical symptoms of MI are crushing anterior chest pain radiating into the neck, shoulder, or arm, lasting more than 30 minutes, and not relieved by nitroglycerin. Typically pain is accompanied by dyspnea, diaphoresis, weakness, and nausea. Significant physical findings, often absent, include an atrial gallop rhythm (4th heart sound) and a pericardial friction rub. The electrocardiogram shows ST-segment elevation (later changing to depression) and T-wave inversion in leads reflecting the area of infarction. Q waves indicate transmural damage and a poorer prognosis. Diagnosis is supported by acute elevation in serum levels of myoglobin, the MB isoenzyme of creatine kinase, and troponins. Unequivocal evidence of MI may be lacking during the first 6 hours in as many as 50% of patients. Death from acute MI is usually due to arrhythmia (ventricular fibrillation or asystole), cardiogenic shock (forward failure), congestive heart failure, or papillary muscle rupture. Other grave complications, which may occur during convalescence, include cardiorrhexis, ventricular aneurysm, and mural thrombus. Acute MI is treated (ideally under continuous ECG monitoring in the intensive care or coronary care unit of a hospital) with narcotic analgesics, oxygen by inhalation, intravenous administration of a thrombolytic agent, antiarrhythmic agents when indicated, and usually anticoagulants (aspirin, heparin), a beta-blocker, and an ACE inhibitor. Patients with evidence of persistent ischemia require angiography and may be candidates for balloon angioplasty. Data from the Framingham Heart Study show that a higher proportion of acute MIs are silent or unrecognized in women and the elderly. Several studies have shown that women and the elderly tend to wait longer before seeking medical care after the onset of acute coronary symptoms than men and younger people. In addition, women seeking emergency treatment for symptoms suggestive of acute coronary disease are less likely than men with similar symptoms to be admitted for evaluation, and women are less frequently referred for diagnostic tests such as coronary angiography. Other studies have shown important gender differences in the presenting symptoms and medical recognition of MI. Chest pain is the most common symptom reported by both men and women, but men are more likely to complain of diaphoresis, whereas women are more likely to experience neck, jaw, or back pain, nausea, vomiting, dyspnea, or cardiac failure, in addition to chest pain. The incidence rates of acute pulmonary edema and cardiogenic shock in MI are higher in women, and mortality rates at 28 days and 6 months are also higher. But because men experience MI at earlier ages, mortality rates are the same for both sexes when data are corrected for age.