Does an SVC electrode further reduce DFT in a hot-can ICD system?

Pectorally implanted ICDs that defibrillate with the RV electrode and the ICD housing have gained clinical acceptance. However, it is still debatable whether adding an SVC electrode connected to the housing will further reduce the threshold of defibrillation (DFT). This study utilized eight pigs. DFTs were measured with a 50 V step-down protocol starting at 650 V (20 J). Shock strength for 50% success (E50) was estimated with the average of three reversals. In addition to alpha dummy device, Lead I (Pacesetter Models 1558 and 1538) or Lead II (Endotak 72) were used. Leads I are active fixation, true bipolar sensing with 5-cm shocking coils. Lead II has an integrated bipolar sensing with a 4.7-cm RV and 6.9-cm SVC shocking coils. A 95 microF defibrillation system was used to deliver a 44% tilt tuned biphasic 1.6/2.5 ms waveform, and to measure lead impedance. The RV electrode was the anode during phase I. With Lead IRV-->CAN the DFT was 531 +/- 75 V (13.6 +/- 3.8J) and the E50 was 496 +/- 89 (12 +/- 4.3 J). These were not significantly (NS) different than the DFT for RV-->CAN and SVC which was 518 +/- 84 V (13 +/- 4.2 J) or the E50 which was 476 +/- 84 V (11 +/- 3.9 J). Similar results were obtained with Lead II. Despite a decrease in lead impedance there was no apparent benefit from the addition of the SVC electrode. Lead I provided equivalent DFT performance to Lead II.