Assessing shock efficacy as a function of arrhythmia complexity in a slab of the canine heart

Cardioversion of a ventricular tachycardia typically requires less energy than defibrillation. It is likely that this is due to the more complex nature of reentrant wavefronts associated with fibrillation. In the present study, we compare current threshold for termination of simple and complex patterns of reentry to assess the effects of preshock myocardial activity on shock efficacy. We use an up-down protocol to determine the termination threshold of either a single or multiple reentry arrhythmia in a bidomain model of a 4 mm thick slab of canine myocardium. We find that the more complex arrhythmia requires 48% more energy than the single reentry to terminate all activity. Examination of postshock activity reveals that a weaker shock is less effective at terminating reentries which are not located directly between the cathode and anode. We conclude that better estimates of defibrillation thresholds must take into account both the chaotic nature of ventricular fibrillation and the possible locations of reentrant pathways.