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 conditi ons, high resolution recordings and computerized beat-to-beat analysis of the electrocardiogram were performed at rest, during recovery afte r 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 sup raventricular 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 m inimal changes of the QT interval. The measured proportional change of the QT interval compared with the change in RR interval (Delta QT/Del ta RR) was 0.02. This value represents 10% of the value expected for Q T changes from Bazett's formula. Following short exercise QT interval did not change for 15 seconds and reached a maximal value 80 seconds l ater 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 ach ieved after 132, 135, and 133 seconds for pacing intervals of 600, 500 , and 600 msec, respectively. Our data indicate a relatively slow adap tation of the QT interval to changes in heart rate.