A heart attack is caused when not enough blood and oxygen reach part of the heart muscle. The heart muscle starts to die. This lack of blood and oxygen is most often caused by a blockage in one or more of the coronary arteries.

This blockage is usually the result of coronary artery disease, in which fatty deposits called plaque (say "plak") build up inside the coronary arteries. Plaques are covered by a fibrous cap. If the cap breaks open or ruptures, the body will try to repair the injured artery lining. A blood clot may form and block blood flow.

Sometimes a blood clot that forms over a ruptured plaque may not completely block the artery. But the clot may block blood flow enough to cause angina symptoms. These symptoms may happen with rest and may not go away with rest or nitroglycerin. These symptoms are an emergency, because the blood clot can quickly grow and block the artery. If the blood clot dissolves and an immediate heart attack is avoided, the body will try again over time to repair the tear on the surface of the plaque. But this newly repaired plaque can also be very unstable. It is more likely to rupture again, putting you at even greater risk of a heart attack.