Background: Implantable cardioverter-defibrillators (ICD) are routinely programmed to pace after a shock to prevent possible asystole. In those with no prior history of bradycardia, there is little data regarding the prevalence and characteristics of those who use post-shock pacing (PSP).
Methods: We analyzed the occurrence of pacing within the first nine beats after the first successful ICD shock for ventricular tachycardia (VT) or ventricular fibrillation (VF) in the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT). All ICDs were single lead with the first PSP delivered at 1400msec and all subsequent stimuli delivered at 1200msec. We excluded patients with pacing during pre-shock rhythms and those who had pacing rates different than the protocol default rate of 50bpm (1200 msec).
Results: There were 2521 patients enrolled in SCD-HeFT, of which 811 received an ICD. A total of 153 shock events were examined; 36 (23.5%) had at least one of the first nine beats paced post-shock, though only 4 (2.5%) had greater than 4 out of the 9 beats paced. No subjects needed pacing for all nine beats and only 8 (5.2%) paced for greater than 5 seconds. There were no differences in age, gender, etiology of cardiomyopathy, or NYHA class between those with PSP or not. The prevailing heart rate pre-shock was predictive of PSP; the mean cycle length of the baseline rhythm pre-shock was longer (slower rate) for those who used PSP (735 ± 228msec vs. 624 ± 158msec, P=0.001). More often, VF (vs. VT) was the rhythm shocked in those using PSP (P=0.015). A trend also was seen toward increased frequency of PSP in those receiving 30J shocks (16 of 49) versus ≤20J shocks (20 of 104, P=0.068).
Conclusion: Patients infrequently require multiple paced beats post-shock for VT or VF. Patients using PSP have a slower baseline heart rate and are more likely to have VF as the shocked rhythm. While 1 or 2 paced beats out of the first nine occurred occasionally, these patients also had rapid return of their native rhythm for which the hemodynamic contribution of 1 or 2 paced beats is unclear. These data suggest that for most patients receiving a primary prevention ICD programmed for shock-only therapy, the need for PSP is limited. PSP use may reflect convention and the assumption that minor post-shock pauses are detrimental.
