Comparison of the effects of active left and right pectoral pulse generators on defibrillation efficacy

Abstract

P placement of implantable cardioverter-defibrillator (ICD) pulse generators is now routine because of the steady decrease in pulse generator size. With left pectoral positioning, the generator shell can be incorporated into the shocking pathway as an “active can,” resulting in a lowering of defibrillation thresholds (DFTs). Uniformly low DFTs can be achieved with the combination of biphasic waveforms, active cans, and advanced lead design, particularly dual-coil leads. Despite the ubiquitous use of active cans, the mechanism by which they reduce defibrillation energy requirements is controversial. Although some studies have suggested that the reduction in energy requirements is due to optimization of the shock vector, directing more current through the left ventricle, other studies indicate that reduction of shock impedance is more important. An active can in a right pectoral position, however, would be expected to reduce impedance but possibly worsen the current vector. A case report and 2 retrospective series have demonstrated that use of a right pectoral active can is feasible. The effect of such a shocking configuration on defibrillation efficacy has not been evaluated directly. The present study is a prospective comparison of the effect of an active can on DFTs in the right and left pectoral position. This was a study of 126 patients undergoing ICD implantation for standard clinical indications. Each patient gave written informed consent and the institutional review board of the University of Maryland approved this study. Right-sided implants were performed in 25 patients because of contraindications to left pectoral implantation. These contraindications included 7 patients with abandoned left-sided lead systems due to erosion, ICD incompatibility or infection, 5 patients with explanted left-sided lead systems for similar indications, 5 patients with left arm arteriovenous shunts or fistulas for hemodialysis, and 2 patients with left-sided pacemakers. Other indications for right pectoral placement included upgrading of a preexisting right-sided permanent pacemaker, left superior vena cava thrombosis, left mastectomy, and previous left arm vein harvest for coronary artery bypass surgery. These patients were compared with 101 patients who had routine left pectoral ICD implants performed over the same time period. All patients received an integrated dual-coil lead system. This was an Endotak lead (Guidant Corp., St. Paul, Minnesota) in 120 subjects including 20 with right-sided implants and a Sprint lead (Medtronic Inc., Minneapolis, Minnesota) in the remaining 6 patients. The lead was introduced via the axillary, cephalic, or subclavian vein, and the tip positioned in the right ventricular apex so that the distal end of the proximal coil was near the right atrium/superior vena cava junction. After lead implantation, a prepectoral pocket was created for the pulse generator emulator. All testing was performed using conscious sedation with midazolam and fentanyl. Two shocking configurations were tested in each patient in random order: (1) with the 2 coil transvenous lead only, called “lead alone”; and (2) with a pulse generator emulator (model 6967, Guidant Corp.) in the prepectoral pocket, connected electrically to the proximal transvenous coil, called “active can” configuration. The right ventricular coil was the cathode for the first phase of the biphasic shock in all testing. Testing was performed with an external defibrillator (ECD model 2815, Guidant Corp.), which delivers a 60%/50% tilt, biphasic waveform through a 150 F capacitance. Ventricular fibrillation was induced with high output ramp pacing through the defibrillator lead. The DFT was determined using a modified step-down protocol to first failure. Testing began at a 15-J delivered energy and decreased to 10, 8, 5, 3, and 1 J on subsequent trials. If the 15-J shock failed, the first shock energy was increased in 5-J steps on subsequent trials until defibrillation was successful. At least 3 minutes were allowed between shocks for full hemodynamic recovery. The DFT was defined as the lowest first shock energy that achieved successful defibrillation. After a failed shock, a higher energy rescue shock was delivered immediately. The patients were grouped on the basis of pectoral implantation position, left or right. Proportions were analyzed by Fisher’s exact test. Continuous data were expressed as mean values SD and analyzed by t tests. Paired t tests were used for comparing the effect of electrode configuration on defibrillation parameters. A p value 0.05 was considered significant. From the Department of Medicine, Division of Cardiology, University of Maryland School of Medicine, Baltimore, Maryland. Dr. Gold’s address is: Division of Cardiology, N3W77, University of Maryland, 22 South Greene Street, Baltimore, Maryland 21201. E-mail: Mgold@medicine.umaryland.edu. Manuscript received June 13, 2001; revised manuscript received and accepted August 6, 2001. TABLE 1 Clinical Characteristics of Patients with Leftand Right-Sided ICD Implants