Wavefront termination with monophasic and biphasic shocks: A simulation study

This modeling study examines the effect of monophasic and biphasic waveforms on the response of cardiac fiber to a defibrillation shock. Shocks of various strengths and coupling intervals (CIs) are delivered extracellularly during the relative refractory period. The results show that monophasic shock strengths of three times the diastolic threshold (DT) either elicit no response, or, for coupling intervals above 380 ms, reinitiate propagation. In contrast, biphasic shocks of same strength are capable of terminating the existing wavefronts by invoking either a graded response (CIs 370 to 382 ms) that prolongs the refractory period, or by inducing a propagation block (CIs above 400 ms) that renders the fiber absolutely refractory. Biphasic shocks have only a small `vulnerable' window of coupling intervals over which propagation is reinitiated. Thus, the results obtained in this study offer a possible explanation of the mechanisms underlying the increased efficacy of biphasic defibrillation waveforms.