Myocardial infarction (MI) is usually caused by a blood clot in the coronary artery that stops blood flow to the heart. The lack of blood supply (Ischemia) and thus poor oxygen supply to the heart muscles can lead to death of its cells (infarction). So it needs immediate medical attention. Most MIs occur in people over 50, and become more common with increasing age. Sometimes younger people can be affected. An MI is three times more common in men than women.See more..