Heart attacks occur when blood flow to a portion of the heart is severely reduced or cut off. The result is death of heart muscle cells (called an infarct). Hardening and narrowing (atherosclerosis) of the coronary arteries that feed the heart is usually the underlying problem. In some cases, a blood clot blocks blood flow; other times, the narrowing is caused by atherosclerosis alone. Spasm of the coronary arteries may also cause a heart attack.