Incidence and prognostic significance of silent myocardial ischemia in patients after acute myocardial infarction

The incidence and prognostic significance of silent myocardial ischemia in 175 patients who survived a first acute myocardial infarction (AMI) were assessed by means of maximal exercise stress test and 24 hour continuous ECG monitoring performed before discharge. Thirteen patients showed silent ischemia with both techniques, 31 only with stress test, 5 only with ECG monitoring and 8 had a doubtful stress test but showed S-T segment depression during ECG monitoring, 118 were free from ischemia. Ten cardiac deaths and 31 coronary events including unstable angina, by-pass operation, and myocardial infarction occured during the one year follow-up period. S-T segment depression during ECG monitoring showed a lower sensitivity but a higher specificity and predictive accuracy for cardiac death and coronary events than did exercise stress test. When S-T segment depression is detected in the same patient with both techniques the sensitivity is lower and specificity higher. Whenever the presence of S-T segment depression is recorded, it shows a good sensitivity without significantly decreasing the specificity. Classifying patients according to the occurence of S-T segment depression on exercise test and/or ECG ambulatory monitoring, the Yates corrected chi-square test showed a significant pattern when cardiac deaths and coronary events are considered together (p< 0.01). However, the incidence of cardiac deaths and coronary events was comparable in patients with exercise S-T segment depression regardless of the occurence of S-T segment changes during continuous ECG monitoring. All 8 patients who stopped the exercise test before reaching target heart rate for dyspnea (5 patients), arrhythmias (2 patients), and hypertensive response (1 patient) had S-T segment depression during ECG monitoring: 2 of them died during follow-up and 3 suffered from unstable angina. Thus, ECG monitoring of these patients adds same helpful information but is less useful in patients who perform a maximal exercise stress test.