Biphasic waveforms prevent the chronic rise of defibrillation thresholds with a transvenous lead system

Objectives. The purpose of this study was to compare chronic changes in monophasic and biphasic defibrillation thresholds using a uniform transvenous lead system and testing protocol. Background. Defibrillation thresholds increase over time in patients with nonthoracotomy lead systems. This increase can result in an inadequate chronic defibrillation safety margin and could limit the safety of smaller pulse generators, which have a reduced maximal output. However, previous studies of the temporal changes of defibrillation thresholds evaluated complex lead systems or monophasic shack waveforms, neither of which are used with current technology. Methods. This study was a prospective, randomized assessment of the effects of shock waveforms on the changes of transvenous defibrillation thresholds over time. Paired monophasic and biphasic thresholds were measured bath at implantation and at follow-up (250 +/- 105 days) in 24 consecutive patients who were not receiving antiarrhythmic drugs. The lead system was a dual-coil Endotak C lead, and reverse polarity shocks (distal coil = anode) were delivered. Results. Monophasic defibrillation thresholds increased from (mean +/- SD) 13.7 +/- 6.0 J to 16.8 +/- 6.7 J (p = 4.02), whereas biphasic thresholds were unchanged (10.4 +/- 4.3 J to 10.2 +/- 4.8 J, p = 0.86) in the same patients. Shock impedance chronically increased (47.0 Ohm to 50.5 Ohm, p = 0.02) and was unaffected by waveform. Conclusions. These results indicate that biphasic shocks prevent the chronic increase in defibrillation thresholds with a transvenous lead system. (C) 1997 by the American College of Cardiology.