Influence of R-wave amplitude on exercise-induced ST depression: Need for a “gain factor” correction when interpreting stress electrocardiograms

Abstract

Does 2 mm of ST depression induced by exercise have the same clinical significance in a patient with a 30-mm R wave as a patient with a 10-mm R wave in the same monitored lead? To answer this question the exercise responses of 85 patients were compared by 2 quantitative methods of assessing myocardial ischemia. A computer-derived treadmill exercise score, based largely on the characteristics of exercise-induced ST-segment depression, was compared with a thallium exercise score. Both scores correlated well over a wide range of values (r = 0.71, p < 0.001). Then, the treadmill exercise score was corrected (by adjusting the magnitude of the ST depression to a standardized R-wave amplitude of 12 mm in V 5 and 8 mm in aVF) to determine if this would improve its correlation with the thallium exercise score. The patients were separated into 2 groups by R-wave amplitude: 53 had an R V5 of 9 to 17 mm and 32 had an R V5 < 9 or > 17 mm. Correction of the treadmill exercise score for R-wave amplitude did not change the slope and intercepts of the regression line for patients with an R V5 amplitude of 9 to 17 mm, but did for those with an r v5 amplitude < 9 or > 17 mm. In this latter group, R-wave correction changed the regression line from one that differed significantly from that of patients with less extreme R V5 voltage to one that was indistinguishable from it. Correction of the treadmill exercise score also increased the correlation coefficient from 0.54 to 0.68 in this group. In several patients an abnormal score became normal when it was corrected for R-wave voltage. This corrected score was consistent with the coronary arteriographic findings, the lack of symptoms, and the good exercise tolerance. Thus, as judged independently by a thallium exercise score, the degree of exercise-induced ST depression is influenced by R-wave amplitude, and if not normalized to a standard voltage, may either exaggerate or underestimate the degree of exercise-induced myocardial ischemia.