A prospective randomized evaluation of implantable cardioverter-defibrillator size on unipolar defibrillation system efficacy

Background: The active can unipolar implantable cardioverter-defibrillator (ICD) has been shown to defibrillate efficiently, but its current 80-cc size limits use in the pectoral position in many patients. Decreasing can size will facilitate pectoral insertion and will soon be feasible as an inevitable consequence of technological advancements. However, decreasing the can size has the potential to compromise unipolar defibrillation efficacy. It is the purpose of this study, therefore, to prospectively and randomly compare unipolar defibrillation efficacy with 80-cc, 60-cc, and 40-cc can sizes in patients immediately before ICD surgery in anticipation of advances in technology that will make smaller ICDs possible. Methods and Results: Twenty-four consecutive patients un-derwent prospective, randomized evaluation of the effect of ICD can size on defibrillation efficacy during standard ICD surgery. Each patient had the unipolar defibrillation threshold (DFT) measured with 80-cc, 60-cc, or 40-cc active can placed in the left subcutaneous infraclavicular region. The system included a 10.5F tripolar right ventricular electrode that served as the shock anode. The shock waveform used in each instance was a single capacitor biphasic 65% tilt pulse delivered from a 120-μF capacitor. Stored energy at the DFT for the 80-cc, 60-cc, and 40-cc cans were 8.1±4.7 J, 8.7±5.8 J, and 9.5±4.8 J, respectively. There was no statistical significant difference between the DFTs for the three unipolar can electrodes (P=.39). Leading edge voltage also did not differ significantly among the three unipolar cans (356±92 V, 365±110 V, and 387±94 V, respectively, P=.29). There was, however, a slight progressive increase in resistance with decreasing can size (57±7 Ω, 60±7 Ω, and 65±9 Ω, respectively, P<.001). Conclusions: Decreasing can volume from 80 cc to 60 cc to 40 cc does not compromise unipolar defibrillation efficacy despite a slight rise in shock resistance. These findings indicate that technological advances that allow for smaller-volume ICDs will not compromise defibrillation efficacy for unipolar systems.