Post-induction residual time: a novel sensing phenomenon during induced ventricular fibrillation in subcutaneous ICD recipients.

Background: Subcutaneous implantable cardioverter defibrillators (S-ICD) are extensively used in the primary and secondary prevention of sudden cardiac death. Ventricular fibrillation (VF) induction during implantation remains standard to confirm device functionality. Objective: To compare electrocardiographic (ECG) and technical variables between induced and spontaneous ventricular arrhythmias in S-ICD recipients. Methods: A retrospective observational study was performed on 10 consecutive patients with S-ICD. Variables included episode duration, sensing time, charge time, and shock impedance. For each patient, the implant induced VF episode was compared to at least one spontaneous event during follow-up. Results: The mean age was 36.4 years (range: 22-61); 2 patients were male. All patients received intermuscular S-ICD implants, with the lead placed in the left parasternal position in 7 patients and right parasternal in 3. Episode duration and sensing time were consistently shorter in spontaneous events, and shock impedance was lower. Charge time, however, was longer in spontaneous events (by 1.4 to 1.8 s). Conclusions: In this case series, spontaneous ventricular arrhythmic events in S-ICD patients demonstrated shorter sensing and therapy delivery times compared with induced ventricular fibrillation episodes. A measurable Post-Induction Residual Time (PRT) was identified in all induced episodes and was absent during spontaneous ventricular arrhythmias, suggesting that 50-Hz-induced VF may not fully reproduce spontaneous arrhythmic events from the perspective of device sensing performance. These findings support the hypothesis that part of the delay observed during implant testing may be related to the induction process itself rather than to the intrinsic sensing capabilities of the S-ICD. Further studies are warranted to confirm these observations and to evaluate alternative VF induction strategies in future device platforms. Future integration of S-ICD systems with leadless pacing technologies may provide alternative ventricular fibrillation induction strategies, including T-wave shock delivery during either sensed intrinsic rhythm or leadless-paced rhythm. Such approaches could potentially avoid the occurrence of Post-Induction Residual Time (PRT) and may provide a model that more closely reproduces spontaneous ventricular fibrillation than current 50-Hz induction techniques.