Myocardial infarction (MI) is the irreversible necrosis of heart muscle secondary to prolonged ischemia. This usually results from an imbalance of oxygen supply and demand. The appearance of cardiac enzymes in the circulation generally indicates myocardial necrosis. Myocardial infarction is considered, more appropriately, part of a spectrum referred to as acute coronary syndromes (ACSs), which also includes unstable angina and non–ST-elevation MI (NSTEMI). Patients with ischemic discomfort may or may not have ST-segment elevation. Most of those with ST-segment elevation will develop Q waves. Those without ST elevations will ultimately be diagnosed with unstable angina or NSTEMI based on the presence of cardiac enzymes.
Myocardial infarction may lead to impairment of systolic function or diastolic function and to increased predisposition to arrhythmias and other long-term complications.
Approximately 1.5 million cases of myocardial infarction occur each year in the U.S, and 500,000 cases involve significant blockage.