Influence of R-wave amplitude on exercise-induced ST depression: need for a "gain factor" correction when interpreting stress electrocardiograms.

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 less than 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 V5 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 RV5 of 9 to 17 mm and 32 had an RV5 less than 9 or greater than 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 RV5 amplitude of 9 to 17 mm, but did for those with an RV5 amplitude less than 9 or greater than 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 RV5 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.(ABSTRACT TRUNCATED AT 250 WORDS)