Contribution of heart rate to QT interval shortening during exercise.

The contributions of the intrinsic effect of heart rate and factors other than heart rate, to exercise-induced QT interval shortening were assessed by studying a group of 24 patients with implanted, programmable P wave synchronised pacemakers and a group of 10 patients undergoing atrial and ventricular pacing at rest. In each patient with an implanted pacemaker, the relation between atrial rate and QT interval was studied during exercise in both atrial synchronised and asynchronous (fixed-rate) ventricular pacing modes. In three patients the exercise tests were repeated after beta-adrenergic blockade. There was a close linear correlation between atrial rate and QT interval reduction in each exercise test. With asynchronous ventricular pacing, QT shortening did occur but to a lesser extent than during atrial synchronised pacing and could be abolished by beta-adrenergic blockade. When the heart rate was increased at rest by either atrial or ventricular pacing QT interval shortening did occur but again to a lesser degree than with atrial synchronised ventricular pacing during exercise. The results suggest that the heart rate is only one of the determinants of the QT interval duration, and other factors, presumably associated with sympathetic activity, also contribute to QT interval shortening during exercise. By comparing the QT interval changes during atrial synchronised and asynchronous ventricular pacing on a within-patient basis, we determined that the contribution of the intrinsic effect of heart rate to QT interval shortening during exercise varied from 26 to 75%.