Cause

A heart attack or unstable angina is caused by sudden narrowing or blockage of a coronary artery. This blockage keeps blood and oxygen from getting to the heart. A heart attack or unstable angina can happen when plaque in the coronary artery breaks open or ruptures. Blood then clots in the artery and blocks blood flow.

With a heart attack, lack of blood flow causes the heart's muscle cells to start to die. With unstable angina, the blood flow is not completely blocked by the blood clot. But a heart attack may soon follow, because the blood clot can quickly grow and block the artery.

See a picture of how a heart attack happens.

Atherosclerosis leads to plaque buildup in the coronary arteries, called coronary artery disease.

A stent in a coronary artery can also become blocked and cause a heart attack. The stent might become narrow again if scar tissue grows after the stent is placed. And a blood clot could get stuck in the stent and block blood flow to the heart.

Heart attack triggers

In most cases, there are no clear reasons why heart attacks occur when they do. But sometimes your body releases adrenaline and other hormones into the bloodstream in response to intense emotions such as anger, fear, and the "fight or flight" impulse. Heavy physical exercise, emotional stress, lack of sleep, and overeating can also trigger this response. Adrenaline increases blood pressure and heart rate and can cause coronary arteries to constrict, which may cause an unstable plaque to rupture.

In rare cases, the coronary artery spasms and contracts, obstructing blood flow and causing chest pain. If severe, the spasm can completely block blood flow and cause a heart attack. Most of the time in these cases, atherosclerosis is also involved, although sometimes the arteries are clear. Cocaine, cold weather, emotional stress, and other things can cause these spasms. But in many other cases, it is not known what triggers the spasm.