Introduction: Subcutaneous implantable cardioverter defibrillators (SICD) are an attractive option for sudden death (SCD) prevention in younger hypertrophic cardiomyopathy (HCM) patients. Conversely, SICDs have higher rate of inappropriate shock (IAS) when compared to transvenous devices. Objective: We characterize incidence of appropriate shock (AS) and IAS and analyze predictors of IAS in HCM SICD patients. Methods: Data was collected from HCM SICD patients from 2013 to 2021. We used multivariable logistic regression to assess for predictors of IAS in patients with > 6 mo follow up. Results: 94 HCM patients (age 47 ± 15 years) underwent SICD implant with mean follow up of 3.7 ± 2.0 year. Maximal LV thickness 20.5 ± 5.8 mm with massive hypertrophy (> 30 mm) in 10 patients (11.8%). Initial DFT with 65J was successful in 88 patients, with 5 more successful after device adjustment. 5 patients (5.9%) had 10 AS (3.2 AS per 100 pt-years). 10 patients (11.8%) had 19 IAS (6.0 IAS per 100 pt-years) due to T wave oversensing (n = 13), P wave oversensing (n = 2), atrial arrhythmia (n=2), and external noise (n = 1). IAS rate decreased over time, with IAS occurring in 8 patients of the initial half of the cohort and in only 2 of the second half. Time to IAS from implant was 11 ± 10.8 mo. QRS duration (OR 1.025, 95% CI 0.997-1.053; P = 0.084) showed trend to prediction of IAS, but no characteristic proved independently significant. Conclusions: This data adds to increasing evidence that in high-risk HCM patients, SICD represents a reliable treatment option for SCD prevention. IAS mainly due to cardiac oversensing, was seen in 11.8% of patients within this HCM cohort. IAS decreased over time, possibly due to improved patient selection, implant technique and device programming (SMART pass filter). A larger dataset is likely necessary to better understand this trend.

Introduction: Defibrillation threshold testing (DFT) is generally reserved for populations with perceived concern for defibrillation failure, including in hypertrophic cardiomyopathy (HCM). Higher shock impedance has been associated with risk of DFT failure with subcutaneous defibrillators (SICD). The PRAETORIAN score is a chest radiograph based tool that evaluates device positioning to predict risk of DFT failure. Objective: We assessed predictors of impedance during DFT in a single center HCM SICD cohort. Methods: Data was collected from HCM patients who underwent SICD implantation from 2013 to 2021. Predictors of impedance were evaluated with linear and logistic regression. Results: Impedance was reported in 75 HCM SICD patients during DFT with 65J. DFT was successful in 74 patients with mean impedance of 72.7 ohm and septal thickness of 20.6 mm. Ten patients had massive HCM (> 30 mm) with mean impedance of 66.4 ohm. Septal thickness was not predictive of impedance (β 0.37, 95% CI 1.24-0.50; P = .40). Independent predictors of impedance were BMI (β 1.41, 95% CI 0.61-2.21; P = .0008) and PRAETORIAN score (β 0.14, 95% CI 0.05-0.22; P = .0015). Multivariable regression revealed BMI no longer significant and PRAETORIAN score remaining significant (β 0.10, 95% CI 0.01-1.66; P = .038). Conclusion: In this HCM SICD cohort BMI and PRAETORIAN score were predictive of impedance. However, in a multivariable model, BMI was no longer predictive, highlighting importance of implant technique to improve impedance and DFT. Additionally, patients with massive hypertrophy all had successful DFT and septal thickness was not predictive of impedance. HCM has historically been portrayed as a high DFT population. Further prospective trials are necessary to establish whether standard DFT practice can be applied in HCM.