Induced versus spontaneous ventricular fibrillation in subcutaneous ICD recipients: impact on sensing and therapy delivery

Introduction The subcutaneous implantable cardioverter-defibrillator (S-ICD) is an established alternative to transvenous systems for selected patients requiring defibrillation therapy. Defibrillation testing with ventricular fibrillation (VF) induction using a 50 Hz burst is routinely performed to validate device detection and therapy delivery. Purpose To compare induced versus spontaneous VF episodes in S-ICD recipients, focusing on sensing and therapy delivery times, and a post-induction residual time (PRT). Methods We conducted an observational case series including 10 consecutive patients implanted with an S-ICD. Implants were intermuscular in all cases. Baseline diagnoses were idiopathic VF (60%), idiopathic dilated cardiomyopathy (30%) and Brugada syndrome (10%). Mean age was 36.4 years (range 22–61), and 2 patients were male. One induced VF episode (implant test) and the first spontaneous VF episode per patient were analysed using stored subcutaneous ECGs. The following variables were measured: total episode duration, sensing time (from VF onset to detection). PRT is an original term introduced by the authors and was defined as the random delay between the end of the 50 Hz burst and the first valid VF sensing marker; it reflects a temporary suspension of S-ICD sensing imposed by the current induction protocol and is not present in spontaneous episodes. Results All induced and spontaneous VF episodes were correctly detected and successfully terminated by the first S-ICD shock. PRT was observed in all induced VF episodes and was absent in all spontaneous events. Spontaneous episodes showed shorter total duration (mean paired difference −2.94 sec; 95% CI −4.97 to −0.91) and shorter sensing time (mean difference −3.10 sec; 95% CI −4.82 to −1.38), both statistically significant. No episodes were associated with haemodynamic compromise attributable to delayed therapy. Conclusions These data indicate that PRT is a by-product of the current 50 Hz induction method rather than an intrinsic limitation of the S-ICD sensing algorithm. Future integration with leadless pacing technologies could enable alternative VF induction strategies (e.g. T-wave shock on sensed or paced rhythm), potentially reducing or eliminating this "stunning" period during which S-ICD sensing is effectively switched off.