Rate-dependence of QT dispersion and the QT interval: comparison of atrial pacing and exercise testing

The study was done to determine whether variables of QT dispersion from the 12-lead electrocardiogram (ECG) are dependent on heart rate.

The dispersion of the QT interval is under evaluation as a risk marker in patients at risk for ventricular arrhythmias. Assuming that a similar rate correction is necessary as for the QT interval itself, investigators have frequently reported QTc-dispersion values utilizing the Bazett formula. It is not known whether there is a physiologic basis for such a rate correction in the human heart.

In 35 patients referred for evaluation of ventricular arrhythmias, digital 12-lead ECGs recorded at various heart rates during submaximal exercise testing and again during atrial pacing upon electrophysiologic testing were submitted to computerized interactive analysis of several ECG dispersion variables.

Data from 11 patients were excluded due to incomplete high-quality analysis possible at all heart rates. From the remaining 24 patients, a total of 193 ECG recordings at various heart rates (ranging from 76 +/- 17 beats/min to 117 +/- 14 beats/min during atrial pacing and from 78 +/- 18 beats/min to 110 +/- 14 beats/min during exercise testing) were available. A highly significant linear relationship with heart rate was found for both the QT interval and the Q-to-T-peak interval. By contrast, standard QT interval dispersion (QTmax - QTmin), the T-peak-to-T-end interval, and the average area under the T wave did not change with increasing heart rates.

Dispersion of the QT interval and other ECG variables of dispersion of ventricular repolarization are independent of heart rate. Therefore, it is not necessary to rate-correct these measurements.