DISPARATE EFFECTS OF BIPHASIC AND MONOPHASIC SHOCKS ON POSTSHOCK REFRACTORY PERIOD DISPERSION

The magnitude by which a defibrillation shock extends the refractory p eriod immediately postshock (refractory period extension, RPE) does no t explain why biphasic shocks defibrillate with greater efficacy than monophasic shocks. It may be that spatial heterogeneity of RPE is a mo re important regulator of defibrillation efficacy We measured RPE in 1 5 pentobarbital-anesthetized swine using 400-V biphasic and monophasic shocks of equal pulse duration at three discrete myocardial sites. Sp atial heterogeneity of RPE was calculated as the difference between th e maximum and minimum values of the three recording sites. Monophasic shocks produced greater magnitude of RPE than biphasic shocks at all s ites tested (82 +/- 6 to 99 +/- 13 and 64 +/- 6 to 68 +/- 5 ms, respec tively; P < 0.05). However, RPE dispersion was significantly less with biphasic shocks versus monophasic shocks (29 +/- 4 and 48 +/- 7 ms, r espectively; P < 0.05). This suggests that one potential mechanism by which biphasic shocks defibrillate with greater efficacy is limiting p ostshock spatial heterogeneity of refractoriness. Thus these data supp ort our hypothesis that RPE heterogeneity is a more likely predictor o f defibrillation efficacy than magnitude of RPE.