A manual synchronous low energy shock impedance as a predictor of successful defibrillation testing during subcutaneous ICD implantation

Background Guidelines recommend defibrillation testing (DFT) during subcutaneous implantable cardioverter‐defibrillator (S‐ICD) implantation. Implant position, patient characteristics and device factors, such as shock impedance, influence defibrillation success. To evaluate the shock impedance, a manual synchronous 10J shock (low energy synchronous shock [LESS]) can be delivered, without the need to induce ventricular fibrillation (VF). Objective To compare LESS and DFT impedance values and to evaluate the diagnostic accuracy of LESS impedance for predicting a successful DFT during S‐ICD implantation. Methods Consecutive S‐ICD implantations were included. Shock impedances were compared by paired t ‐tests. Univariate analysis was performed to investigate factors associated with successful DFT. A prediction model of successful DFT based on LESS impedance was assessed by logistic regression. Receiver operating characteristic (ROC) curve, area under the ROC curve and the Hosmer–Lemeshow tests were used to evaluate the accuracy of LESS impedance. Results Sixty patients were included (52 ± 14 years; 69% male). LESS and DFT impedance values were highly correlated ( r 2 = 0.97, p < .01). Patients with a failed first shock had higher body mass index (BMI) (30 ± 3 vs. 25.7 ± 4.3, p = .014), higher mean LESS (120 ± 35Ω vs. 86. ± 23Ω, p = .0013) and DFT impedance (122 ± 33Ω vs. 87 ± 24Ω, p = .0013). ROC analysis showed that LESS impedance had a good diagnostic performance in predicting a successful conversion test (AUC 84% [95% CI: 0.72–0.92]) with a cutoff value of <94Ω to identify a successful DFT (sensitivity 71%, specificity 73%). Conclusion LESS impedance values without the need to induce VF can intraoperatively predict a successful DFT.