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The spectrum of ischemic myocardial injury ranges from very mild (minimal tissue damage) to severe (necrosis of a significant volume of myocardium, as occurs in classic myocardial infarction). Ischemia refers to a reduction in blood supply to a target tissue. The severity of ischemia and, therefore, the associated tissue injury, depends on the degree of blockage. This may range from slight and partial to total.

Myocardial infarction (MI): blockage of blood flow in a coronary artery caused by a coronary thrombus (solid mass). This may occur suddenly as a result of an embolism, a migrating clot that becomes lodged in a coronary artery; or gradually, the result of increasing interference of blood flow.

Events preceding MI: coronary artery disease begins with a fatty streak of cholesterol and its esters. The fatty streaks enlarge to become a fatty plaque (atheroma). If the plaque becomes large enough to restrict blood flow, then this may result in the first clinical signs of coronary artery disease, i.e., angina pectoris (chest pain). Stable angina lasts approximately 1 to 5 minutes and is caused by exposure to cold or physical/emotional stress. Frequent attacks of angina not necessarily associated with stress are referred to as unstable angina. These tend to presage AMI.

Subsequently, platelets may aggregate on the damaged surfaces of blood vessels, and release powerful vasoconstrictors that further reduce blood flow. The platelets often evolve further into a thrombus, which, if large enough, totally obstructs the vessel and causes classical MI.