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The subcutaneous implantable cardioverter-defibrillator (S-ICD) is approved as an alternative to the traditional transvenous implantable cardioverter-defibrillator (T-ICD). The S-ICD has a higher average defibrillation threshold (DFT) than the T-ICD, related primarily to greater distance from shocking coil to ventricular myocardium of the S-ICD. Therefore, the S-ICD utilizes a larger device capable of generating an 80-J shock compared to maximal shock output of 35–40 J typical for T-ICDs, and defibrillation efficacy testing is generally recommended for the S-ICD.
We previously reported computer modeling of predictors of defibrillation efficacy with the S-ICD, which determined that minimal fat underlying the S-ICD coil and generator (reducing shock impedance) and posterior generator position (better directing the shock vector across the ventricles) are predictors of lower DFT.
In this study, we performed computer modeling to determine whether design modifications to the S-ICD coil could result in reductions to S-ICD DFT.
A finite element computer model was created from a publicly available magnetic resonance imaging dataset (University of Minnesota) taken at end-diastole from a 63-year-old man (body mass index 31 kg/m2) as previously described.
Three experimental electrode configurations were compared to a standard electrode: (A) large flattened coil with elliptical cross-section, dimensions 2.3 × 10 × 85 mm; (B) L-shape arrangement of 2 coils in series: vertical in the standard left parasternal position and horizontal left inframammary coil (both coils 3-mm diameter, 80-mm long); (C) 2 coils in parallel at right and left parasternal positions, each 3-mm diameter, 80-mm long; and (D) standard left parasternal coil, 3-mm diameter, 80-mm long. The S-ICD generator was placed in the standard left, midaxillary position. Comsol simulation software version 5.6 (Comsol AB, Stockholm, Sweden) was used to solve for electric fields. Based on the critical mass theory (described in greater detail in Heist et al
), successful defibrillation is predicted at the voltage when 95% of the ventricular myocardium receives >4 V/cm. The required DFT energy (in joules) and shock impedance are reported.
The standard S-ICD configuration had a predicted DFT of 32.7 J and impedance of 87 Ω, well within typical clinical results from S-ICD studies. Both DFT and impedance were reduced at all experimental electrode configurations tested: large flat coil (22.1 J, 71 Ω); dual in series (22.7 J, 62 Ω), and dual in parallel (16.2 J, 61 Ω). Electric fields in the inferior aspect of the heart (which typically has the lowest electrical gradients during S-ICD shocks) were correspondingly increased in these coil modifications (Figure 1).
In this study, we demonstrated that design modifications to the S-ICD coil intended to reduce shock impedance and better direct the shock vector across the ventricular myocardium could reduce S-ICD DFT. In the case of dual-in-parallel vs dual-in-series coil designs, shock impedance is nearly identical (61 vs 62 Ω, respectively), but DFT is substantially lower with dual in parallel (16.2 vs 22.7 J) due to better shock vector coverage of the ventricles. The reduction in predicted S-ICD DFT based on coil modifications is substantial, and in the case of dual-in-parallel design results (compared to the standard coil) in approximately 50% reduction in DFT to 16.2 J, which is within range of typical results with a T-ICD.
Limitations of this study include use of a single patient anatomy for modeling. Results could vary for different anatomies, including high body mass index, unusual chest anatomies, or variations in lead position. S-ICD electrical performance could be affected by energy discharge in the setting of reduced shock impedance, potentially impacting translation of these results into the clinical setting. Additionally, the lead configurations analyzed could increase procedural complexity and/or complication rate compared to standard leads.
Modifications to the S-ICD coil could substantially reduce DFT and result in clinical benefit to patients, such as improved defibrillation efficacy, reduced S-ICD output requirement allowing for a smaller/lighter generator, and reduced need to perform ventricular fibrillation induction and defibrillation efficacy testing during implantation.
2015 HRS/EHRA/APHRS/SOLAECE expert consensus statement on optimal implantable cardioverter-defibrillator programming and testing.
Funding Sources: The authors have no funding sources to disclose.
Disclosures: Dr Heist reports being a consultant for Biotronik, Boston Scientific, and Pfizer; and consultant for/equity in Oracle Health. Dr Knops reports being a consultant for Abbott, Boston Scientific, Cairdac, Kestra, and Medtronic; and equity in AtaCor. Dr Yap reports being a consultant for Boston Scientific; and institutional research grants from Biotronik and Medtronic. Dr Boersma reports being a consultant for Abbott, Adagio, Biosense Webster, Boston Scientific, Medtronic, and Philips. Dr Friedman reports research grants from Abbott, American Heart Association, Biosense Webster, Boston Scientific, Medtronic, Merit Medical, National Cardiovascular Data Registry, and the National Institutes of Health; and consulting fees from Abbott, AtriCure, NI Medical, MicroPort, and Sanofi. Dr Poole reports research grants from Biotronik, Boston Scientific, Medtronic, and AtriCure. Wyatt Stahl is an employee of Boston Scientific. Dr Belalcazar reports being a consultant for Boston Scientific and Cardionomic.