Causes
Every organ needs oxygen-carrying blood to stay alive, and the myocardium (the muscle that forms the wall of the heart) is no exception. It has its own oxygen supply, via the coronary arteries. In coronary artery disease, fatty deposits (plaques) form in the inner walls of the coronary arteries, narrowing them and reducing blood flow to the heart. This process is called atherosclerosis.
Most heart attacks occur when the atherosclerotic plaque lining an artery ruptures. Blood then forms a clot on the damaged artery, which may partially or completely obstruct blood flow. If the blockage gets severe enough, heart attack symptoms appear, and heart muscle cells may start to die. This is now considered a heart attack.
Rarely, a coronary artery spasm stops blood flow through an apparently healthy coronary artery, causing a heart attack. In most of these cases there's no identifiable cause.