The effect of biphasic defibrillation on the immediate pacing threshold of a dedicated bipolar, steroid-eluting lead.

It is apparent that pacing threshold increases following an ICD shock, although the degree of change observed is dependent on the method used to assess pacing and the lead design used. We previously demonstrated a rise in postshock pacing threshold using a lead with integrated bipolar pacing in which the distal shocking coil also serves as the pacing anode. In this study, we sought to investigate whether the postshock pacing threshold increased significantly in an endocardial, steroid-eluting lead with dedicated bipolar pacing electrodes. Twenty patients (16 men, 4 women; median age 73, ejection fraction [EF] 0.17-0.58) were studied during pectoral ICD implantation (Medtronic active can model 7221Cx or 7223Cx with model 6932-65 lead). The diastolic pulse width pacing threshold at 1 or 2 V was determined. Pacing rate was set > or = 100/min at twice diastolic threshold output to assess pacing immediately following the first DFT test shock. For subsequent shocks, the output was adjusted to establish postshock thresholds as 1, 2, 3, or 4 times the diastolic threshold. The postshock threshold was defined as the output yielding 100% capture > or = 2.5 seconds following a shock. In 8 of 20 patients (ratio 0.40 +/- 0.11), a rise in the post-shock threshold was shown by failure of consistent capture when pacing at 2 times diastolic threshold > or = 2.5 seconds after a DFT test shock. Two of these patients failed at 3 times threshold, but none failed at 4 x threshold. Five of 12 patients with successful capture of 2 times threshold failed to capture at threshold. The postshock threshold increased by a mean factor of 2.83 +/- 0.83 in the group of patients with a threshold rise. Following ICD shock in an active can, steroid-eluting lead system with dedicated bipolar pacing, the post-shock threshold increases significantly. Our studies suggest a need for postshock pacing to be set at least 4 x threshold regardless of the lead design.