A heart attack — or "myocardial infarction" as it's called by doctors — is one of the most feared and common medical problems. Each year, 865,000 people in the United States have a heart attack. Of these, roughly 180,000 die, and about hald of these die before ever getting any medical attention. But advances in treatment have meant that even more victims can survive and go on to live normal, healthy lives. Paramount to your chances of surviving a heart attack is knowing the early warning signs and getting immediate medical attention. Therapies for heart attacks, such as clot-busting drugs, balloon angioplasty and coronary stenting are most effective when begun within 6 hours of heart attack symptoms. The sooner blood flow is restored, the greater the amount of heart muscle that will be saved.

A heart attack occurs when muscle cells within the heart are starved for oxygen because of an inadequate blood supply to the heart muscle. Some of the heart's muscle cells become damaged or die, depending on how long the starvation lasts, and on how much of the heart muscle is affected. These dead or damaged cells then affect how efficiently the heart contracts and expands, thus limiting its ability to pump blood.

In a mild heart attack, pain or other symptoms are slight or do not develop at all--this event is known as a "silent" heart attack. A mild heart attack may pass unnoticed, only to be detected on subsequent examinations with electrocardiograms. A severe heart attack can kill immediately or within a few hours. It may also leave the heart muscle so damaged that heart failure or arrhythmias (irregular heart rhythms) develop.

A heart attack is caused by blood vessel disease within the heart. Coronary heart disease, often referred to as coronary artery disease, is a more general name for the diseased blood vessels that result in a heart attack and the chest pain known as angina. There are two major causes of a heart attack:

• Blockage of Coronary Arteries. A heart attack occurs when the blood supply to part of the heart muscle (the myocardium) is severely reduced or stopped. This occurs when one of the coronary arteries (the arteries that supply blood to the heart muscle) is blocked, most often by a blood clot that forms at the site of the build-up of plaque due to atherosclerosis. Such an event is called a coronary thrombosis or coronary occlusion.
• Spasm. Sometimes a coronary artery temporarily contracts or goes into spasm. When this happens the artery narrows and blood flow to part of the heart muscle decreases or even stops. What causes a spasm is unclear, but it can occur in normal blood vessels as well as vessels partially blocked by atherosclerosis. If a spasm is severe, a heart attack may result.

The coronary arteries rise from the aorta and branch off to envelop the heart. At rest, the blood flow through the coronary arteries averages slightly more than 7 ounces per minute. This represents about 4 to 5 percent of the blood pumped by the heart. The heart, which makes up less than 1 percent of your body's weight, requires this large amount of blood because it is the hardest working muscle you have.

Several medical conditions can increase your risk of having a heart attack by putting excess strain on the heart muscle, or by promoting the accumulation of plaque in coronary arteries. High blood pressure, diabetes, high cholesterol and smoking are conditions and activities that significantly increase your risk of a heart attack.