Heart attacks most often occur as a result of coronary artery disease. The coronary arteries supply oxygen-rich blood to the heart, but in people with coronary artery disease, plaque builds up in the arteries.
If an area of plaque ruptures, or breaks open, a blood clot forms on the plaque's surface. A large blood clot can mostly or completely block the flow of oxygen-rich blood through a coronary artery. The result is a heart attack, which is also called myocardial infarction.
When a coronary artery becomes blocked, the section of heart muscle fed by that artery can't get enough oxygen and begins to die. Healthy heart tissue is replaced with scar tissue, which can cause severe or long-lasting problems, such as heart failure and life-threatening arrhythmias.
Although it's less common, a heart attack can also be caused by a severe spasm of a coronary artery that suddenly cuts off blood flow. Spasms can occur in coronary arteries that have no plaque buildup.