Reentry site during fibrillation induction in relation to defibrillation efficacy for different shock waveforms

Reentry Site and Defibrillation Waveform Efficacy. Introduction: Unsuccessf ul defibrillation shocks may reinitiate fibrillation by causing postshock r eentry. Methods and Results: To better understand why some waveforms are more effic acious for defibrillation, reentry was induced in six dogs with 1-, 2-, 4-, 8-, and 16-msec monophasic and 1/1- (both phases 1 msec) 2/2-, 4/4-, and 8 /8-msec biphasic shocks. Reentry was initiated by 141 +/- 15 V shocks deliv ered from a defibrillator with a 150-muF capacitance during the vulnerable period of paced rhythm (183 +/- 12 msec after the last pacing stimulus). Th e shock potential gradient field was orthogonal to the dispersion of refrac toriness. Activation was mapped with 121 electrodes covering 4 x 4 cm of th e right ventricular epicardium, and potential gradient and degree of recove ry of excitability were estimated at the sites of reentry, Defibrillation t hresholds (DFTs) were estimated by an up-down protocol for the same nine wa veforms in eight dogs internally and in nine other dogs externally. DFT vol tages for the different waveforms were positively correlated with the magni tude of shock potential gradient and negatively correlated with the recover y interval at the site at which reentry was induced by the waveform during paced rhythm for both internal (DFT = 1719 + 64.5VV - 11.1RI; R-2 = 0.93) a nd external defibrillation (DFT = 3445 + 150VV - 22RI; R-2 = 0.93). Conclusion: The defibrillation waveforms with the lowest DFTs were those th at induced reentry at sites of low shock potential gradient, indicating eff icacious stimulation of myocardium. Additionally, the site of reentry induc ed by waveforms with the lowest DFTs was in myocardium that was more highly recovered just before the shock, perhaps because this high degree of recov ery seldom occurs during defibrillation due to the rapid activation rate du ring fibrillation.