Beat-to-beat behavior of QT interval during conducted supraventricular rhythm in the normal heart

To assess beat-to-beat behavior of QT interval under different conditions, high resolution recordings and computerized beat-to-beat analysis of the electrocardiogram were performed at rest, during recovery after short exercise, and during atrial pacing. Beat-to-beat variations of QT interval during sinus rhythm at rest and after short exercise were measured in ten healthy men. In an additional three patients with supraventricular tachycardia, beat-to-beat QT changes were studied after abrupt sustained acceleration and deceleration of heart rate by atrial pacing. Beat-to-beat changes in RR interval at rest are followed by minimal changes of the QT interval. The measured proportional change of the QT interval compared with the change in RR interval (delta QT/delta RR) was 0.02. This value represents 10% of the value expected for QT changes from Bazett's formula. Following short exercise QT interval did not change for 15 seconds and reached a maximal value 80 seconds later as compared to the RR interval (192 vs 115 secs, P < 0.001). The steady state of the QT interval during sustained atrial pacing was achieved after 132, 135, and 133 seconds for pacing intervals of 600, 500, and 600 msec, respectively. Our data indicate a relatively slow adaptation of the QT interval to changes in heart rate.