The new definition of myocardial infarction: diagnostic and prognostic implications in patients with acute coronary syndromes.

Background: The clinical implications of the recently revised criteria for diagnosis of acute myocardial infarction (AMI) in patients with suspected acute coronary syndromes are unknown.

Methods: To evaluate the prognostic implications of the new diagnostic criteria for AMI, we studied 493 consecutive patients with suspected acute coronary syndromes admitted to University of Michigan, Ann Arbor, between May 1, 1999, and January 1, 2000. Patients with positive cardiac enzymes and symptoms suggestive of coronary ischemia (n = 275) were divided into 2 groups: group A, with elevated peak creatine kinase-MB fraction and/or new electrocardiographic changes suggestive of AMI regardless of troponin status (diagnosed as AMI by old criteria), and group B, with normal peak creatine kinase-MB fraction but elevated troponin I level (additional patients diagnosed as having AMI by new criteria).