Time to therapy delivery and effectiveness of the subcutaneous implantable cardioverter-defibrillator

Background At the time of subcutaneous implantable cardioverter-defibrillator (S-ICD) implantation, successful termination of ventricular fibrillation (VF) is confirmed. A previous study reported cases of prolonged time to therapy during defibrillation testing. Objectives We sought to evaluate the time to therapy, identify possible predictors of delay, and investigate the impact of delayed therapy on VF conversion. Methods We analyzed consecutive patients with S-ICDs who underwent initial conversion testing at a shock energy of 65 J in 53 Italian centers. Results We analyzed 570 patients (467 [82%] male; mean age 48 ± 15 years; mean body mass index 25 ± 6 kg/m2; mean ejection fraction 47% ± 17%). General anesthesia was used in 165 (29%) of patients, with sub- or intermuscular positioning of the generator in (422; 74%). Cardioversion was successful at 65 J in 557 (97.7%) of patients. In 12 patients (2.1%) the shock did not convert VF, and in 1 patient the shock was not delivered because of noise from entrapped subcutaneous air. All failures were successfully managed by reprogramming or repositioning the device. The mean time to therapy was 15 ± 3 seconds, and it exceeded 18 seconds in 51 patients (9%). Independent predictors of delayed therapy (18 seconds) were ejection fraction (odds ratio [OR] 0.98; 95% confidence interval [CI] 0.96–0.99; P = .016) and a 2× gain programmed (OR 3.66; 95% CI 1.44–9.30; P = .006). Effectiveness at 65 J was not associated with time to therapy (OR 1.13; 95% CI 0.97–1.32; P = .122). Conclusion In this analysis of a large population of patients with S-ICDs, delayed therapy during defibrillation testing occurred less frequently than previously reported and had no effect on VF conversion success. Delayed therapies seemed more common when a vector with a 2× gain was programmed.