Acute myocardial infarction (MI), commonly known as a heart attack, is a condition characterized by ischemic injury and necrosis of the cardiac muscle. Ischemic injury occurs when the blood supply is insufficient to meet the tissue demand for metabolism. More than two thirds of myocardial infarctions occur in lesions that are less than 60% severe. (See the images below.) Almost all MIs are caused by rupture of coronary atherosclerotic plaques with superimposed coronary thrombosis. Patients with MI usually present with signs and symptoms of crushing chest pressure, diaphoresis, malignant ventricular arrhythmias, heart failure (HF), or shock. MI may also manifest itself as sudden cardiac death, which may not be apparent on autopsy (because necrosis takes time to develop). MI is clinically silent in as many as 25% of elderly patients, a population in whom 50% of MIs occur; in such patients, the diagnosis is often established only retrospectively by applying electrocardiographic criteria  or by performing imaging with 2-dimensional (2-D) echocardiography or magnetic resonance imaging (MRI).