Acute myocardial infarction refers to two subtypes of acute coronary syndrome, namely non-ST-elevated and ST-elevated MIs, which are most frequently (but not always) a manifestation of coronary artery disease. The most common triggering event is the disruption of an atherosclerotic plaque in an epicardial coronary artery, which leads to a clotting cascade, sometimes resulting in total occlusion of the artery. Atherosclerosis is the gradual buildup of cholesterol and fibrous tissue in plaques in the wall of arteries (in this case, the coronary arteries), typically over decades. Bloodstream column irregularities visible on angiography reflect artery lumen narrowing as a result of decades of advancing atherosclerosis. Plaques can become unstable, rupture, and additionally promote the formation of a blood clot that occludes the artery; this can occur in minutes. When a severe enough plaque rupture occurs in the coronary arteries, it leads to MI (necrosis of downstream myocardium). It is estimated that one billion cardiac cells are lost in a typical MI.