High shock impedance during subcutaneous implantable defibrillator generator replacements

We would like to congratulate van der Stuijt et al.1 with their efforts on systematic evaluation of subcutaneous implantable cardioverter-defibrillator (S-ICD) replacements. In this single-centre study, they have performed 72 device replacements, a remarkably large population with this relatively new technique. Dutch centres have been instrumental in the development of S-ICD therapy2 and ‘early adopters’ with a large population with relatively long follow-up.3 The current study shows low complication rates of S-ICD device replacement but the actual strength of the manuscript is the systematic analysis of defibrillation testing. This was performed in 63 patients and the first shock efficiency was coined high with 91.4%. Shock impedance at first implant and during device replacement was available in 48 patients. It was higher during replacement (86 ± 26Ω vs. 77 ± 28Ω) with a very large variability. In some patients, shock impedance was almost doubled between the two...