Synchronized shocks reduce defibrillation threshold

The extension of refractoriness hypothesis predicts that defibrillation success can be improved by synchronizing shocks to action potentials from cells in low intensity regions. Threshold should be lower for shocks delivered “early” in these action potentials than for shocks delivered “late”. To test this hypothesis, defibrillating shocks were synchronized to monophasic action potentials recorded from a low intensity region in isolated rabbit hearts (N=6). Probability-of-success curves were generated for “early” and “late” shocks. The authors found that I₅₀ for “early” shocks was 17% lower than I ₅₀ for “late” shocks (p<0.05). This corresponds to a 31% decrease in E₅₀ The relative standard deviation of I₅₀ decreased from 32% for “late” shocks to 18% for “early” shocks. Standard deviations at all success percentiles were larger for “late” shocks. These results suggest that synchronizing shocks to action potentials from a low intensity region may reduce defibrillation threshold and inter-subject variability