Study objective: Electrocardiographic abnormalities are common after transthoracic defibrillation. ECG ST-segment changes are especially problematic after defibrillation and may indicate ischemic or shock-induced cardiac dysfunction after resuscitation. Biphasic defibrillation waveforms, compared with monophasic waveforms, diminish shock-induced cardiac dysfunction in laboratory preparations. This effect has not been validated in human subjects. We therefore evaluated in a prospective, blinded fashion the effect of biphasic and monophasic transthoracic defibrillation on the ECG ST segment in 30 consecutive patients during surgery for the implantation of a cardioverter-defibrillator. Methods: In each patient two low-energy truncated biphasic transthoracic defibrillation shocks (115 and 130 J) were compared with a standard clinical 200 J monophasic damped-sine wave shock. The biphasic shocks and the damped-sine wave shock have been demonstrated to have equal defibrillation efficacy of 97%. Fifteen-second ECG signals recorded across transthoracic defibrillation electrodes were digitized before ventricular fibrillation induction and immediately after each defibrillation attempt. The ST segments 80 msec after the J point were analyzed in a blinded fashion by two reviewers. The ST-segment deflection, QRS-interval duration, QT interval, and heart rate after each therapy were compared with baseline values. Results: ECG ST-segment elevation was significantly greater with the 200-J damped-sine waveform than with either biphasic waveform. The ECG ST- segment levels were -.55±36 at baseline, -.76±36 mm after internal shock, - .02-.36 mm after 115-J biphasic shock, .21±.38 mm after 130-J biphasic shock, and 2.09±37 mm after 200-J damped-sine wave shock (P<.0001). QRS- interval duration, QT interval, and heart rate did not change significantly with any waveform. Conclusion: Transthoracic defibrillation with biphasic wave forms results in less postshock ECG evidence of myocardial dysfunction (injury or ischemia) than standard monophasic damped sine waveforms without compromise of defibrillation efficacy.