Ventricular Fibrillation Conversion Testing After Implantation of a Subcutaneous Implantable Cardioverter Defibrillator

Abstract

Background: Compared with transvenous implantable cardioverter defibrillators (ICDs), subcutaneous (S)-ICDs require a higher energy for effective defibrillation. Although ventricular fibrillation conversion testing (CT) is recommended after S-ICD implantation to ensure an adequate margin between the defibrillation threshold and maximum device output (80J), prior work found that adherence to this recommendation is declining. Methods: We studied first-time recipients of S-ICDs (between September 28, 2012, and April 1, 2016) in the National Cardiovascular Database Registry ICD Registry to determine predictors of use of CT, predictors of an insufficient safety margin (ISM, defined as ventricular fibrillation conversion energy >65J) during testing, and inhospital outcomes associated with use of CT. Multivariable logistic regression analysis was used to predict use of CT and ISM. Inverse probability weighted logistic regression analysis was used to examine the association between use of CT and inhospital adverse events including death. Results: CT testing was performed in 70.7% (n=5624) of 7960 patients with S-ICDs. Although deferral of CT was associated with several patient characteristics (including increased body mass index, increased body surface area, severely reduced ejection fraction, dialysis dependence, warfarin use, anemia, and hypertrophic cardiomyopathy), the facility effect was comparatively more important (area under the curve for patient level versus generalized linear mixed model: 0.619 versus 0.877). An ISM occurred in 6.9% (n=336) of 4864 patients without a prior ICD and was more common among white patients and those with ventricular pacing on the preimplant ECG, higher preimplant blood pressure, larger body surface area, higher body mass index, and lower ejection fraction. A risk score was able to identify patients at low (<5%), medium (5% to 10%), and high risk (>10%) for ISM. CT testing was not associated with a composite of inhospital complications including death. Conclusions: Use of CT testing after S-ICD implantation was driven by facility preference to a greater extent than patient factors and was not associated with a composite of inhospital complications or death. ISM was relatively uncommon and is associated with several widely available patient characteristics. These data may inform ICD system selection and a targeted approach to CT.