A heart attack (myocardial infarction, or AMI) is an emergency. It can happen suddenly from a blockage or sudden narrowing of your arteries. This reduces blood flow to your heart muscle. It's the result of years of damage to your blood vessels. Over time, fatty substances collect on the walls of the heart's arteries. This is from your genes and lifestyle choices. Many of these can lead to chronic disease. The arteries become narrower as the deposits build up within the artery walls. A heart attack can happen once one of these arteries narrows enough to stop oxygen supply to an area. It can also happen if the buildup breaks open and forms a blood clot.