Factors Associated With High-Voltage Impedance and Subcutaneous Implantable Defibrillator Ventricular Fibrillation Conversion Success

BACKGROUND: The ability to predict defibrillation efficacy at the time of subcutaneous implantable cardioverter-defibrillator implantation without the need to induce ventricular fibrillation might eliminate the need for defibrillation testing. The purpose of this study was to determine the association of high-voltage impedance and system implant position on ventricular fibrillation conversion success with a submaximal 65-J shock. METHODS: In the subcutaneous implantable cardioverter-defibrillator IDE study (Investigational Device Exemption), a successful conversion test required 2 consecutive ventricular fibrillation conversions at 65 J in either shock vector. Chest radiographs were obtained after implantation. Patients with imaging and impedance data were included. Suboptimal device position was defined as an inferior electrode or pulse generator or electrode coil depth >3 mm anterior to the sternum. Absence of suboptimal positional parameters was defined as appropriate position. Conversion success rate was calculated among all 65-J tests. RESULTS: Of 314 patients who underwent subcutaneous implantable cardioverter-defibrillator implantation, 282 patients were included in this analysis. There were 637 inductions to test defibrillation at 65 J. Sixty-two conversion failures (9.7%) occurred in 42 (14.9%) patients. Lower body mass index and lower shock impedance were associated with higher conversion success rate, whereas white race was associated with lower conversion success rate. Suboptimal position was more common in obese patients. Inferior electrode and greater distance between the lead and sternum were associated with a higher impedance. When appropriate system position was achieved, conversion failure was not associated with high body mass index. CONCLUSIONS: Subcutaneous implantable cardioverter-defibrillator shock efficacy is associated with system position and high-voltage system impedance. A high impedance is associated with inferiorly placed pulse generator and electrode system, inadequate coil depth, and a lower rate of defibrillator success. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov . Unique identifier: NCT01064076.