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Figure 2 Effects of exercise, atropine and isoproterenol on the electrocardiogram (ECG) complex at 100 beats/min. As heart rate increases in response to each experimental intervention, less decrement in the QT interval was observed during isoproterenol as compared to exercise or atropine. The changes in the ECG complex of lead V2 are shown for one typical subject at 100 beats/min.