Early Changes in Defibrillation Threshold Following Implantation of a Nonthoracotomy System in Dogs

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Abstract
Nonthoracotomy systems are rapidly becoming the preferred surgical method for implantation of cardioverter defibrillators. Testing is performed at the time of implantation to insure an adequate margin of safety for defibrillation. However, this safety margin may change with lead maturation. This study evaluated changes in defibrillation threshold following implantation of a nonthoracotomy system. Ten dogs underwent implantation of a nonthoracotomy system consisting of a single catheter with a distal coil electrode in the right ventricular apex and a proximal coil electrode in the superior vena cava forming a common anode with a subcutaneous patch over the left 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 successful 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 were observed in four animals. Energy requirements for defibrillation with a nonthoracotomy system increase during the early postoperative period, with the highest defibrillation threshold observed at 38 days. This increase may be applicable to humans and should be considered when selecting an adequate energy safety margin for defibrillation at time of implantation.