Effect of electrode polarity on internal defibrillation with monophasic and biphasic waveforms using an endocardial lead system

Introduction: To test the hypothesis that the effect of shock polarity on defibrillation depends on waveform duration, this study determined strength-duration defibrillation curves of monophasic and biphasic truncated exponential waveforms for both polarities. Methods and Results: Defibrillation thresholds (DFTs) were obtained in 32 pigs for catheter electrodes in the right ventricle (RV) and superior vena cava (SVC) using a modified Purdue technique, Both electrode polarities were tested in five different protocols, In part 1, DFTs were determined with 1- to 14-msec monophasic waveforms, In parts 2, 3, and 4, DFTs were determined with two different sizes of SVC electrodes for biphasic waveforms with a phase 1 of 4 or 6 msec and a phase 2 ranging from 1 to 10 msec, In part 5, DFTs were tested for monophasic waveforms ranging from 2 to 11 msec and for biphasic waveforms with a phase 1 duration corresponding to each monophasic waveform and a phase 2 held constant at 1 msec, Mean DFTs for monophasic waveforms were significantly lower when the RV electrode was an anode than when it was a cathode for waveform durations 3 msec. For biphasic waveforms in which phase 2 was less than or equal to phase 1 in duration, no significant difference in mean DFT was observed when polarity was reversed. Even a phase 2 as short as 1 msec could eliminate the DFT difference between polarities observed with monophasic shocks. When phase 2 was 1 2 msec longer than phase 1, polarity did affect the DFT of biphasic waveforms; it affected the DFT similarly to a monophasic waveform of the same polarity as phase 2, Phase 1 duration and electrode size also affected the difference in DFT produced by changing the electrode polarity. Conclusions: For phase durations most commonly used clinically because of their low DFTs, reversing polarity changed defibrillation efficacy for monophasic but not biphasic shocks. For inefficient biphasic waveforms with phase 2 greater than or equal to 2 msec longer than phase 1, the DFT was lower when the RV electrode was an anode during phase 2, similar to the polarity difference for monophasic waveforms, suggesting that a long second phase of biphasic waveforms defibrillates in a similar fashion to monophasic waveforms.