A heart attack, also known as a myocardial infarction (MI), occurs when blood flow to a part of the heart is blocked for a long enough time that part of the heart muscle is damaged or dies. The most common cause of a heart attack is the formation of a blood clot (thrombus) in one of the coronary arteries, which supply oxygen-rich blood to the heart muscle. This clot often forms in a coronary artery that has been narrowed by fatty deposits (atherosclerosis).