What is the minimal pacing rate that prevents torsades de pointes? Insights from patients with permanent pacemakers.

In the acquired long QT syndrome, torsades de pointes (TP) is invariably preceded by pauses or bradycardia. Thus, it has been proposed that out-of-hospital initiation of drugs that prolong repolarization should be safe in patients with permanent pacemakers. However, a minimal protective pacing rate has not been identified. The authors reviewed published reports of acquired TP in patients with permanent pacing. Those providing documentation of tachycardia onset and pacemaker programming were included in the analysis. Events occurring < or = 1 month after AV nodal ablation were excluded. Eighteen cases were identified (age 74 +/- 5; 10 women). QT prolonging drugs were present in 15 patients (quinidine 5, sotalol 3, disopyramide 3, amytriptiline, chloroquine, cisapride + haloperidol, and monopride + flecainide 1 each). Hypokalemia was present in eight patients. At the time of TP, the programmed lower rate was 63 +/- 13 beats/min. However, the effective pacing rate was lower (55 +/- 11 beats/min) due to invocation of pause-promoting features (hysteresis [4 patients]; + PVARP extension on PVC [1 patient]) or ventricular oversensing (2 patients). No patient developed TP with an effective pacing rate > 70 beats/min. TP is possible in the presence of a functional permanent pacemaker. Programmed lower rates < or = 70 beats/min are not protective. At programmed lower rates > 70 beats/min, TP may occur only when facilitated by programmable pause-promoting features or oversensing. It remains to be seen if rate smoothing algorithms can prevent TP when the baseline rate is programmed < or = 70 beats/min.