['The probability of survival decreases to 70% after 2 minutes of ventricular fibrillation. Biphasic shocks are more effective than monophasic shocks in terminating short-duration (<30 sec) ventricular fibrillation. We tested the hypotheses that developing ischemia changes the electrophysiologic characteristics of fibrillation and that the relative efficacy of biphasic shocks increases as electrophysiologic characteristics deteriorate.', 'Monophasic (12 msec) and biphasic (6/6 msec) shocks (1 to 4 A) were tested in random order in isolated rabbit hearts after 1-minute ischemic fibrillation. Monophasic action potentials showed only a sporadic occurrence of electrical diastole after 5 seconds of fibrillation (24% of action potentials in the right ventricle and 18% in the left ventricle). After 60 seconds of fibrillation, diastole (17.83+/-1.14 msec in the right ventricle and 21.52+/-1.16 msec in the left ventricle) appeared after almost every action potential (P < 0.0001 compared with 5 sec), despite a lack of change in fibrillation cycle length and dominant frequency. Monophasic I50 was 2.89 A, and biphasic I50 was 1.4 A (77% reduction in energy). Normalized curve width decreased 28%. Retrospective analysis showed that shocks delivered early in the fibrillation action potential had a greater probability of succeeding (89%) than shocks delivered late (30%; P < 0.001).'] Does electrophysiologic deterioration after one-minute fibrillation increase relative biphasic defibrillation efficacy?