Time to therapy delivery and effectiveness of 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. OBJECTIVE: 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 S-ICD patients who underwent initial conversion testing at a shock energy of 65J in 53 Italian centers. RESULTS: We analyzed 570 patients (82% male, 48±15 years, BMI 25±6Kg/m2, ejection fraction 47±17%). General anesthesia was used in 29% of patients, with sub-/inter-muscular positioning of the generator in 74%. Cardioversion was successful at 65J in 97.7% of patients. In 12 patients the shock did not convert VF and in 1 patient the shock was not delivered due to noise from entrapped subcutaneous air. All failures were successfully managed by reprogramming or repositioning the device. The mean time to therapy was 15±3s and it exceeded 18s in 51 (9%) patients. Independent predictors of delayed therapy (>18s) were ejection fraction (OR, 0.98; 95%CI, 0.96-0.99; p=0.016) and a 2x gain programmed (OR, 3.66; 95%CI, 1.44-9.30; p=0.006). Effectiveness at 65J was not associated with the time to therapy (OR, 1.13; 95%CI, 0.97-1.32; p=0.122). CONCLUSIONS: In this analysis of a large population of S-ICD patients, 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 2x gain was programmed.