QTc compared to JTc for monitoring drug-induced repolarization changes in the setting of ventricular pacing

Background QT prolongation is a risk factor for proarrhythmia when beginning antiarrhythmic drug therapy (AAD). However, there are no data regarding monitoring repolarization changes during a ventricular paced (VP) rhythm. Objective The purpose of this study was to compare serial changes in corrected QT and JT intervals, during native conduction (NC) and VP rhythms when initiating Class III AADs. Methods Twenty-two patients (73% men; mean age 65 ± 11 years) with an implantable device and with <10% VP were monitored during AAD initiation (16 sotalol, 6 dofetilide). QTc and JTc were measured from ECGs obtained during NC and VP at baseline (pre-AAD) and then after each AAD dose. Results During AAD loading, mean QTc increased significantly during NC (431 ± 28 ms to 463 ± 33 ms, P =.002) but not with VP (520 ± 48 ms to 538 ± 45 ms, P =.07). Mean percent increase in peak QTc during NC was significantly greater than during VP (12% vs 7%, P =.003). In contrast, peak JTc during AAD loading was not significantly different between NC and VP (P =.67). Conclusion When initiating AAD, the change in QTc during VP does not correlate with the change in QTc during NC; thus, the VP QTc is inadequate for monitoring repolarization changes. However, VP JTc correlates well with JTc during NC. When initiating Class III AADs in patients with VP rhythms, the JTc, and not the QTc, interval is the useful marker for assessing repolarization.