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.

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.

Heart attacks occur because of problems inside the coronary arteries. The coronary arteries arise from the aorta and extend over the outside of the heart. Small branches of the arteries feed oxygen and nutrients to heart cells. If blood flow within one of the coronary arteries decreases suddenly or stops completely, a heart attack will likely occur. The two main reasons for a heart attack are:

• Sudden blockage of a coronary artery — Most often this is the result of a blood clot that forms at the site of the fatty build-up (called plaque). 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 fatty deposits. 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% 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.