Myocardial infarction (MI) is also known as heart attack.
A heart attack usually occurs as a result of a clot or prolonged spasm in a narrowed coronary artery, which restricts or cuts off the blood flow to your heart. If the blood supply is cut off or greatly reduced for more than a few minutes, your heart tissue will be damaged or will die (infarction). As damage occurs, your heart muscle releases chemicals or enzymes into the blood. Your doctor can measure these enzymes (CPK or troponin) to determine the size or degree of heart muscle injury. In some cases your heart may actually enlarge, either to compensate for its decreased pumping ability or due to the amount of heart muscle damage.
A less common cause of heart attack is a blood clot that forms in the heart (embolus) and breaks away to lodge in a coronary artery. Spasm of a coronary artery also stops blood flow to your heart, and may be caused by drugs such as cocaine or by smoking. Sometimes the cause of an MI is unknown.