Rate-dependence of QT dispersion and the QT interval: Comparison of atrialpacing and exercise testing

OBJECTIVES The study was done to determine whether variables of QT dispersi on from the 12-lead electrocardiogram (ECG) are dependent on heart rate. BACKGROUND The dispersion of the QT interval is under evaluation as a risk marker in patients at risk for ventricular arrhythmias. Assuming that a sim ilar rate correction is necessary as for the QT interval itself, investigat ors have frequently reported QTc-dispersion values utilizing the Bazett for mula. It is not known whether there is a physiologic basis for such a rate correction in the human heart. METHODS In 35 patients referred for evaluation of ventricular arrhythmias, digital 12-lead ECGs recorded at various heart rates during submaximal exer cise testing and again during atrial pacing upon electrophysiologic testing were submitted to computerized interactive analysis of several ECG dispers ion variables. RESULTS Data from 11 patients were excluded due to incomplete high-quality analysis possible at all heart rates. From the remaining 24 patients, a tot al of 193 ECG recordings at various heart rates (ranging from 76 +/- 17 bea ts/min to 117 +/- 14 beats/min during atrial pacing and from 78 +/- 18 beat s/min to 110 +/- 14 beats/min during exercise testing) were available. A hi ghly significant linear relationship with heart rate was found for both the QT interval and the Q-to-T-peak interval. By contrast, standard QT interva l dispersion (QT(max) - QT(min)), the T-peak-to-T-end interval, and the ave rage area under the T wave did not change with increasing heart rates. CONCLUSIONS Dispersion of the QT interval and other ECG variables of disper sion of ventricular repolarization are independent of heart rate. Therefore , it is not necessary to rate-correct these measurements. (C) 2000 by the A merican College of Cardiology.