BackgroundA unipolar defibrillation system using a single right ventricular (RV) electrode and the active shell or “CAN” of the implantable cardioverter-defibrillator itself situated in a left infraclavicular pocket has been shown to be as efficient in defibrillation as an epicardial lead system. The purpose of this study was to determine whether defibrillation efficacy can be improved further by adding a superior vena cava (SVC) electrode to this already efficient defibrillation system.Methods and ResultsWe prospectively and randomly compared the defibrillation efficacy of a simplified unipolar defibrillation system, RV→CAN, with that of one incorporating a high SVC electrode, RV→SVC+CAN, in 15 consecutive cardiac arrest survivors undergoing implantation of a presently available transvenous defibrillation system. The RV defibrillation electrode was a 5-cm coil located on a 10.5F lead used as the anode in both lead configurations examined. The active CAN was a 108−cm2surface area shell of a titanium alloy pulse generator used as the cathode in both configurations and placed in a left infraclavicular pocket. The SVC electrode was a 6F 5-cm-long coil and was used as an additional cathode positioned at the junction of the SVC and the left innominate vein. The defibril-lation pulse used was a 65% tilt, asymmetric biphasic waveform delivered from a 120-μF capacitor. The defibrillation threshold (DFT) stored energy, leading edge voltage, current, and pulsing resistance were measured for both lead systems. The single-lead unipolar system, RV→CAN, resulted in a stored energy DFT of 7.4±5.2 J, and the three-electrode dual pathway system, RV→SVC+CAN, resulted in a DFT of 6.0+3.4 J (P= .20). There was no difference in defibrillation efficacy with the more complicated three-electrode system over the unipolar system despite a decrease in pulsing resistance to 48.6±3.5 Ω compared with 61.2±5.9 Ω for the unipolar system (P< .0001) and a slight rise in delivered current to 6.3±1.8 A compared with 5.5 ±2.0 A for the unipolar system (P= .062).ConclusionsThe unipolar single-lead transvenous defibrillation system provides defibrillation at energy levels comparable to that reported with present epicardial lead systems. Coupling of this lead system to a third SVC electrode increases system complexity but offers little defibrillation advantage despite a large decrease in pulsing resistance and a modest increase in delivered current.