Submuscular versus subcutaneous pectoral implantation of cardioverter-defibrillators: Effect on high voltage pathway impedance and defibrillation efficacy

Implantable cardioverter-defibrillator (ICD) pulse generators are now routi nely positioned in a pectoral location, either submuscularly (under the pec toralis muscles) or subcutaneously l(ver the pectoralis muscles). Furthermo re, in current ICDs, the generator shield usually participates in the defib rillation energy pathway ("hot can"). Consequently, the precise generator l ocation could affect defibrillation system efficacy. To assess this Issue, we compared high voltage pathway impedance and defibrillation threshold (DF T) in 20 patients undergoing submuscular and 46 patients undergoing subcuta neous pectoral implantation of an Angeion Sentinel(R) ICD and an AngeFlex(R ) dual-coil defibrillation lead. Measurements were performed at time of ICD implant, pre-hospital discharge, and 1, 3 and/or 6 months later. Following induction of ventricular fibrillation, 569 biphasic waveform shocks were d elivered between the generator shield and either the distal defibrillation coil (RV/can configuration) or both proximal and distal coils (RV/SVC/can c onfiguration). Impedance differences between submuscular and subcutaneous i mplants were approximately 3-4 Ohms (p value of 0.132 to < 0.001 depending on time of follow-up and lead configuration). A significant increase in imp edance over time was noted independent of implant location and lead configu ration. The DFT at implant or pre-discharge was assessed in 27 individuals, and was 9.9 +/- 3.8 J in 8 patients in the submuscular group, and 7.4 +/- 3.3 J in 19 patients in the subcutaneous group (p = 0.057). In conclusion, anatomic location of a "hot can" ICD generator (submuscular versus subcutan eous) influences impedance to defibrillation current, but the impact is of small magnitude and does not appear to result in clinically important diffe rences in DFT.