Myocardial infarction ("heart attack") is the irreversible damage of myocardial tissue caused by prolonged ischemia and hypoxia. This most commonly occurs when a coronary artery becomes occluded following the rupture of an atherosclerotic plaque, which then leads to the formation of a blood clot (coronary thrombosis). This event can also trigger coronary vasospasm. If a vessel becomes completely occluded, the myocardium normally supplied by that vessel will become ischemic and hypoxic. Without sufficient oxygen, the tissue dies. The damaged tissue is initially comprised of a necrotic core surrounded by a marginal (or border) zone that can either recover normal function or become irreversibly damaged. The hypoxic tissue within the border zone may become a site for generating arrhythmias. Collateral blood flow is an important determinant of infarct size and whether or not the border zone becomes irreversibly damaged.