POSTSHOCK POTENTIAL GRADIENTS AND DISPERSION OF REPOLARIZATION IN CELLS STIMULATED WITH MONOPHASIC AND BIPHASIC WAVE-FORMS

Introduction: Even though the clinical advantage of biphasic defibrill ation waveforms is well documented, the mechanisms that underlie this greater efficacy remain incompletely understood, It is established, th ough, that the response of relatively refractory cells to the shock is important in determining defibrillation success or failure, We used t wo computer models of an isolated ventricular cell to test the hypothe sis that biphasic stimuli cause a more uniform response than the equiv alent monophasic shocks, decreasing the likelihood that fibrillation w ill be reinduced, Methods and Results: Models of reciprocally polarize d and uniformly polarized cells were used, Rapid pacing and elevated [ K](o) were simulated, and either 10-msec rectangular monophasic or 5-m sec/5-msec symmetric biphasic stimuli were delivered in the relative r efractory period. The effects of stimulus intensity and coupling inter val on response duration and postshock transmembrane potential (V-m) w ere quantified for each waveform, With reciprocal polarization, biphas ic stimuli caused a more uniform response than monophasic stimuli, res ulting in fewer large gradients of V-m (only for shock strengths less than or equal to 1,25 X threshold vs less than or equal to 2,125 X thr eshold) and a smaller dispersion of repolarization (1611 msec(2) vs 18 35 msec(2)). The reverse was observed with uniform polarization: monop hasic pulses caused a more uniform response than did biphasic stimuli, Conclusion: These results show that the response of relatively refrac tory cardiac cells to biphasic stimuli is less dependent on the coupli ng interval and stimulus strength than the response to monophasic stim uli under conditions of reciprocal polarization. Because this may lead to fewer and smaller spatial gradients in V-m, these data support the hypothesis that biphasic defibrillation waveforms will be less likely to reinduce fibrillation, Further, published experimental results cor relate to a greater degree with conditions of reciprocal polarization than of uniform polarization, providing indirect evidence that interac tions between depolarized and hyperpolarized regions play a role in de termining the effects of defibrillation shocks on cardiac tissue.