INFLUENCE OF BODY POSITION ON DEFIBRILLATION THRESHOLDS OF NONTHORACOTOMY IMPLANTABLE DEFIBRILLATORS - A PROSPECTIVE RANDOMIZED EVALUATION

Introduction: Defibrillation thresholds (DFTs) usually are determined with the patient in the supine position. However, patients may be in t he upright position when a shock is delivered during follow-up, which may explain some first shock failures observed clinically, This study investigated whether body posture affects defibrillation energy requir ements of nonthoracotomy implantable cardioverter defibrillators with biphasic shocks. Methods and Results: Using a step up-down protocol, D FTs were compared intraindividually in 52 patients (''active-can'' sys tems in 41 patients, two-lead systems in 11 patients) for the supine a nd upright positions as achieved by a tilt table. The mean DFT was 7.3 +/- 4.2 J in the supine versus 9.2 +/- 4.8 J in the upright position (P = 0,002), Repeated comparison in reversed order 3 months after impl antation in 22 patients revealed thresholds of 6.2 +/- 2.5 J (supine) versus 8.4 +/- 3.7 J (upright; P < 0.03) 1 week and 4.4 +/- 2.4 J (sup ine) versus 6.2 +/- 4.15 (upright; P < 0.04)3 months after implantatio n. DFTs decreased significantly for both body positions from 1 week to 3 months after implantation (P < 0.04), Conclusion: (1) DFTs for biph asic shocks delivered by nonthoracotomy defibrillators are higher in t he upright compared to the supine body position. (2) Differences remai n significant 3 months after implantation. For both body positions, DF T decreases significantly from 1 week to 3 months after implantation. These findings have important implications for programming first shack energy to lower than maximal values or for development of devices wit h lower maximal stored energy.