long term follow up of Implantable Cardioverter Defibrillators (ICD) in hypertrophic cardiomyopathy: is subcutaneous the way forward?
Authors: Monkhouse C , Maclolmson J , Sevier L , Platten J , Dhinoja M , Chow A , Honarbakhsh S , Schilling R , Hunter R , Lambiase P D , Ahsan S , Mohiddin S 
Published: October 2024

Abstract

Introduction
Young patients have greater risks of longer-term complications including inappropriate therapy (IT) from Implantable Cardioverter Defibrillators (ICD) than older ICD patients. In prevention of sudden cardiac death (SCD) in hypertrophic cardiomyopathy (HCM), these complications are critical in determining treatment thresholds. When comparing transvenous ICDs (TV), subcutaneous ICDs (SICD) may have different risks & benefits that define their relative utility. We investigated TV & SICD outcomes in a propensity matched cohort of HCM patients.

Methods
We used electronic health records in a retrospective study of HCM patients receiving an ICD between 2006-22 at tertiary centre. Indications for ICD implantation, including type of device, were determined by the treating cardiomyopathy team according to ESC guidance. All TV device types were included & eligibility for SICD included passing screening in at least 1 sensing vector. Outcome data included patients lost to follow-up until the last device check & patients who had both device types were included twice. Patients were censored after their first IT for an analysis of IT, & their first ‘ICD complication’ for that analysis. ICD complications are defined as all other complications that resulted in a re-intervention procedure (excl SICD battery advisory), or extra procedure at box change. The audit was registered with local institutional review board for ethical approval.
Propensity scoring was used for stratification and covariate adjustment. Matching was conducted using the 12 baseline variables. Associations with IT, ICD complications and their composite were assessed using proportional hazard regression (Cox) models, with time to event starting at implant date. Univariable & multivariable regression analyses were used as appropriate. Hazard ratios (HRs) of 95% confidence intervals were estimated by fitting a model with the propensity score and dichotomous variable (TV vs SICD). A p value of &lt;0.05 was considered statistically significant. Analyses were conducted using R- Version 2023.06.0+421.

Results
Mean age was 55±14.2 years with follow-up of 73±63 months; 70.7% were male. Of a total of 611 patients, with 469 in the TV group & 164 in the SICD. SICD patients had fewer complications, with no differences in IT across all and propensity matched analyses (CI 0.64), fig 1.

Conclusion
In HCM, when compared to TV, SICDs are associated with fewer ICD complications & not associated with more frequent IT. Intriguingly, there was a higher rate of IT with the first generation SICD, causing the wide IT errors at longer durations. Complications over long term follow up require more assessment, but the absence of TV lead intervention creates reduced risks for SICD complications.
In HCM patients without pacing needs, SICDs may be preferred for HCM-SCD prevention. In addition, the lower risks associated with SICD could lead to lower treatment thresholds for people with elevated lifetime HCM-SCD risk.