EARLY CHANGES IN DEFIBRILLATION THRESHOLD FOLLOWING IMPLANTATION OF ANONTHORACOTOMY SYSTEM IN DOGS

Background: Nonthoracotomy systems are rapidly becoming the preferred surgical method for implantation of cardioverter defibrillators. Testi ng is performed at the time of implantation to insure an adequate marg in of safety for defibrillation. However, this safety margin may chang e with lead maturation. This study evaluated changes in defibrillation threshold following implantation of a nonthoracotomy system. Methods and Results: Ten dogs underwent implantation of a nonthoracotomy syste m consisting of a single catheter with a distal coil electrode in the right ventricular apex and a proximal coil electrode in the superior v ena cava forming a common anode with a subcutaneous patch over the lef t thorax. Defibrillation threshold testing, using a biphasic waveform, was performed on each animal under general anesthesia at implantation (day 1) and subsequently on postoperative days 3, 7, 10, 17, 24, 31, 38, and 45. E50, the energy associated with a 50% likelihood of succes sful defibrillation, was determined at each setting. The mean E50 was 12.2 +/- 1.1 J at the time of implantation, increasing 36% to 16.8 +/- 2.0 J by day 38 (P < 0.01), individual increases in E50 of 10-12 J we re observed in fo ur animals. Conclusions: Energy requirements for def ibrillation with a nonthoracotomy system increase during te early post operative period, with the highest defibrillation threshold observed a t 38 days. This increase may be applicable to humans and should be con sidered when selecting an adequate energy safety margin for defibrilla tion at time of implantation.