Corrected QT in ventricular paced rhythms: what is the validation for commonly practiced assumptions?

Introduction: Ventricular pacing (VP) may impact the accuracy of QT interval measurement, as it increases the QT by increasing the QRS duration amongst other mechanisms. We aimed to investigate the accuracy of the commonly used clinical practice of subtracting 50 ms from the corrected QT (QTc) in ventricular paced rhythms.

Methods: We conducted a prospective observational study on 23 consecutive pacemaker patients. Four ECGs were recorded for each subject, 2 in their native rate and 2 following an atrial paced, atrial sensed and inhibited response to sensing and then a dual pacing, dual sensing and dual response pacing of 100 bpm to allow for an intrinsic and a ventricular paced QRS, respectively. The averaged QTc in the ventricular paced rhythm was then compared with the non-ventricular-paced QTc for individual subjects.

Results: At a mean spontaneous heart rate of 66 bpm (SD ±8), the mean difference in QTc between the ventricular paced and nonpaced QRS was 48.27 ms (95% CI = 32-64.6 ms, p < 0.001). At faster paced rates, the mean QTc difference was 81.3 ms (95% CI = 35.8-126.8 ms, p = 0.002).

Conclusions: The QTc measurement during VP confirms the current 50-ms subtraction assumption rule within a range of ±16 ms at an average heart rate of 66 bpm. However, at faster heart rates, the 50-ms adjustment may underestimate the QTc discrepancy between a wide and normal QRS.