Time-domain analysis of exercise-induced ST-segment elevation in Q-wave myocardial infarction: a useful tool for the screening of myocardial viability.

Background: Exercise-induced ST-segment elevation in Q-wave leads has been traditionally associated with passive stretching of the infarct wall, perinecrotic ischemia and, according to recent scintigraphic studies, with myocardial viability. At present, however, no definitive conclusions are available. We evaluated the potential role of a time-domain analysis of exercise-induced ST-segment elevation for the identification of viable myocardium and residual ischemia in patients with previous Q-wave myocardial infarction.

Methods: Sixty patients with a previous Q-wave myocardial infarction underwent a bicycle exercise stress test, dobutamine stress echocardiography, coronary arteriography and left ventriculography.

Results: Patients with exercise-induced ST-segment elevation in Q-wave leads (n = 36) showed more severe impairment of resting left ventricular function, when evaluated in terms of wall motion score index at echocardiography (1.62 +/- 0.33 vs 1.41 +/- 0.22, p < 0.01) and in terms of wall motion score at ventriculography (5.9 +/- 1.6 vs 4.1 +/- 1.5, p < 0.03), compared to patients without ST-segment shift (n = 24). No differences between the two groups were seen in the severity and extension of coronary artery disease. The two groups of patients did not differ in the overall incidence of viability (50% in patients with vs 62% in those without ST-segment elevation, p = NS) and homozonal ischemia (39 vs 26%, p = NS), when evaluated with dobutamine echocardiography. However, a time-domain analysis of the ST-segment changes during exercise showed that the duration of exercise up to 0.1 mV ST-segment elevation was significantly lower in patients with viability (6.2 +/- 3.3 min) than in those without (10.2 +/- 2.2 min) (p < 0.001). Accordingly, ST-segment elevation occurred within 3 and 6 min of exercise in 7/18 and in 12/18 patients with viability respectively, but in only 0/18 (p < 0.01) and in 1/18 (p < 0.01) patients without viability. Thus, ST-segment elevation occurring within the first two stages of the exercise test was, respectively, 39 and 67% sensitive and 100 and 94% specific for viability. Early onset ST-segment elevation (within 3 and 6 min) was also more frequent in patients with high-dose dobutamine-induced homozonal ischemia than in those without (sensitivity for ischemia 50 and 67%; specificity 95 and 74%, respectively).

Conclusions: After myocardial infarction, ST-segment elevation in Q-wave leads at the peak of exercise is associated with severe resting left ventricular dysfunction but fails to identify patients with a viable myocardium or residual ischemia. Instead, ST-segment elevation occurring in the early phases of exercise is a highly specific, although not very sensitive marker of dobutamine-assessed viability in the infarct area and may be indicative of residual ischemia.