Is 40 Joules Enough to Successfully Defibrillate With Subcutaneous Implantable Cardioverter-Defibrillators?

Objectives: This study evaluated the efficacy of conversion test performed at 40 J (defibrillation margin ≥40 J), and factors potentially associated with test failure were identified.

Background: Current subcutaneous implantable cardioverter-defibrillator (S-ICD) devices deliver a maximum of 80 J. Functional defibrillation testing is recommended at S-ICD implantation, and it is usually conducted by delivering a shock energy of 65 J to ensure a safety defibrillation margin ≥15 J. Although high rates of successful conversion were reported at 65 J, limited data exist on the defibrillation margin extent.

Methods: Ventricular fibrillation was induced and conversion test was performed by delivering a 40-J shock in 308 patients. Success was defined as termination of ventricular fibrillation by the first shock delivered in standard polarity. The S-ICD system positioning was evaluated with the PRAETORIAN score using bidirectional chest X-rays.

Results: The generator was positioned in an intermuscular pocket in 301 patients (98%) and the lead was implanted by means of a 2-incision technique. The PRAETORIAN score was <90 (low risk of conversion failure) in 293 (95%) patients. Overall, ventricular fibrillation termination occurred in 259 (84%) patients with 40 J. Male gender (odds ratio [OR]: 3.79; 95% confidence interval [CI]: 1.09 to 13.14; p = 0.036), body mass index (OR: 1.09; 95% CI: 1.01 to 1.19; p = 0.036), dilated cardiomyopathy with reduced ejection fraction (OR: 0.42; 95% CI: 0.20 to 0.87; p = 0.019), and PRAETORIAN score >50 (OR: 2.93; 95% CI: 1.26 to 6.83; p = 0.013) were independently associated with conversion failure.

Conclusions: The authors showed a high rate of defibrillation success with 40-J shocks in S-ICD systems implanted by means of modern surgical techniques. The variables associated with shock failure were male gender, higher body mass index, and suboptimal device position according to the PRAETORIAN score.