Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Heart Attack (Myocardial Infarction)

What Is It?

A heart attack occurs when one of the heart's coronary arteries is blocked suddenly, usually by a tiny blood clot (thrombus). The blood clot typically forms inside a coronary artery that already has been narrowed by atherosclerosis, a condition in which fatty deposits (plaques) build up along the inside walls of blood vessels. A heart attack also is called a myocardial infarction or coronary thrombosis.

Each coronary artery supplies blood to a specific part of the heart's muscular wall, so a blocked artery causes pain and malfunction in the area it supplies. Depending on the location and amount of heart muscle involved, this malfunction can seriously interfere with the heart's ability to pump blood. Also, some of the coronary arteries supply areas of the heart that regulate heartbeat, so a blockage sometimes causes potentially fatal abnormal heartbeats, called cardiac arrhythmias. The pattern of symptoms that develops with each heart attack and the chances of survival are linked to the location and extent of the coronary artery blockage.

In 25% of adults, the first sign of heart disease is sudden death from a heart attack. Heart attacks strike approximately 865,000 people in the United States each year, causing more than 179,000 deaths. Because most of these heart attacks result from atherosclerosis, the risk factors for heart attack and atherosclerosis are basically the same:

• An abnormally high level of blood cholesterol (hypercholesterolemia)

• An abnormally low level of HDL (high-density lipoprotein), commonly called "good cholesterol"

• High blood pressure (hypertension)

• Diabetes

• Family history of coronary artery disease at an early age

• Cigarette smoking

• Obesity

• Physical inactivity (too little regular exercise)

In early middle age, men have a greater risk of heart attack than women. However, a woman's risk increases once she begins menopause. This could be the result of a menopause-related decrease in levels of estrogen, a female sex hormone that may offer some protection against atherosclerosis.

Although most heart attacks are caused by atherosclerosis, there are rarer cases in which heart attacks result from other medical conditions. These include congenital abnormalities of the coronary arteries, hypercoagulability (an abnormally increased tendency to form blood clots), a collagen vascular disease, such as rheumatoid arthritis or systemic lupus erythematosus (SLE, or lupus), cocaine abuse, a spasm of the coronary artery, or an embolus (small traveling blood clot), which floats into a coronary artery and lodges there.