Shock timing lowers transvenous defibrillation energy requirement.

Previous studies suggested that time periods exist during ventricular fibrillation when defibrillation shocks are more effective. However, there is no agreement on the amount of energy that can be saved or whether an implantable defibrillator can time shocks to these time periods. We conducted a study having two parts to investigate if there was any advantage to synchronizing internal defibrillation shocks to morphological patterns in ventricular fibrillation (VF). VF electrograms were recorded from the same three-electrode lead system used for internal defibrillation. In Part 1, we found no difference in the probability of successful defibrillation between shocks that were delivered into coarse and fine VF (48% vs 46%). However, shocks that were delivered to the upslope of coarse VF electrograms were more efficacious than those to the downslope of the waveform (67% vs 39%, P < .001). In the second study, we developed a real time computer system to prospectively deliver shocks on the upslope feature we identified in the first study. We found that the energy requirements at E50 and E80 were significantly lower for shocks delivered on the upslope of coarse VF than those delivered randomly at the end of 10 sec. We estimated a probability of success (POS) defibrillation curve using a maximum likelihood method for the timed and random shocks. The POS curve width was significantly narrower for shocks that were delivered to the upslope feature than the control treatment (7.1 +/- 0.9 vs. 10.8 +/- 1.7 J, P < 0.01). If these findings extend to clinical defibrillation, they may allow programming of internal defibrillators at lower energies. This could reduce potential postshock cardiac dysfunction, allow production of smaller devices, and improve battery life.