Relationship of prior myocardial infarction to false-positive electrocardiographic diagnosis of acute injury in patients with chest pain.

Prompt management of patients suffering acute myocardial infarction requires accurate early diagnosis based on the electrocardiogram. To assess the predictive value of ST segment elevation and ST segment depression (both greater than or equal to 0.1 mV) for the diagnosis of evolving myocardial infarction, we studied 100 consecutive patients admitted to the coronary care unit of The New York Hospital with at least 30 minutes of chest pain. Of 31 patients with ST segment elevation, 26 patients (84%) evolved myocardial infarction (positive test results for serum creatine phosphokinase-MB isoenzyme fraction), while only 13 (48%) of 27 patients with ST segment depression had myocardial infarctions. Among patients with ST segment elevations with a history of prior myocardial infarction, only five (50%) of ten evolved myocardial infarction, compared with 21 of 21 with no prior infarction. False-positive diagnoses of acute injury were due to ST elevation in the area of prior Q wave infarction. Prior myocardial infarction did not alter the lower predictive value of ST segment depression for evolving infarction. We conclude that patients presenting with chest pain and ST segment elevation have approximately twice the likelihood of myocardial infarction than patients with ST segment depression; incorporation of historic information regarding prior myocardial infarction can improve the predictive value of ST segment elevations to 100% but does not improve prediction with ST segment depressions.