The Predictive Value of Impedance for Defibrillation Success After S-ICD Implantation

BACKGROUND: The subcutaneous implantable cardioverter-defibrillator is a completely extrathoracic device, making implantation position crucial. This can be evaluated using the PRAETORIAN score, but defibrillation testing (DFT) is currently recommended. There is a trend in replacing DFT with impedance. However, there are situations in which impedance can give a false sense of security. OBJECTIVES: This study sought to compare predictive values of impedance and the PRAETORIAN score for DFT success. METHODS: The multicenter PRAETORIAN-DFT (A Randomised Trial of S-ICD Implantation With and Without Defibrillation Testing) Trial randomized 965 patients between DFT and PRAETORIAN score calculation after subcutaneous implantable cardioverter-defibrillator implantation. DFT was successful if the ventricular arrhythmia converted to sinus rhythm or atrial fibrillation within 5 seconds from 65J shock delivery within 2 inductions. Predictive values of impedance and PRAETORIAN score were calculated, using a cutoff of 90 for both values. RESULTS: Overall, 457 patients underwent DFT according to protocol. DFT was successful in 445 patients and unsuccessful in 12. The failed DFT group had significantly higher impedance (median: 87 [Q1-Q3: 63-108] vs 68 [Q1-Q3: 57-78]; P = 0.014). Impedance <90 ohms had a negative predictive value (NPV) of 98% for successful DFT, but only one-half of failed DFTs had an impedance ≥90 ohms (sensitivity 50%). A PRAETORIAN score <90 had a NPV of 99% with a sensitivity of 70%. In addition, the PRAETORIAN score had a higher positive likelihood ratio and a lower negative likelihood ratio compared with impedance. CONCLUSIONS: Impedance <90 ohms predicts DFT success with a NPV of 98% but misses 50% of DFT failures and is therefore an insufficient surrogate for DFT. Impedance might serve as an intraoperative tool to estimate correct implant position, but this should always be confirmed with imaging to ensure effective shock delivery. (A Randomised Trial of S-ICD Implantation With and Without Defibrillation Testing [PRAETORIAN-DFT]; NCT03495297).