Defibrillation threshold testing in hypertrophic cardiomyopathy: acute outcomes and long-term follow-up

Background

Current American and European guidelines do not recommend routine defibrillation threshold testing (DFT) during implantable cardioverter-defibrillator (ICD) implantation, based on registry data and randomized trials. However, high-risk groups such as patients with hypertrophic cardiomyopathy (HCM) were underrepresented. This study evaluated outcomes of routine DFT in HCM patients within a high-volume implantation program.

Methods

A retrospective analysis was performed on consecutive HCM patients who underwent transvenous or subcutaneous ICD implantation between January 2010 and February 2025. Baseline characteristics, procedural details, and follow-up outcomes were collected and analyzed.

Results

A total of 229 HCM patients (including 112 with left ventricular outflow tract obstruction) underwent ICD implantation. Among them, 211 received transvenous ICDs (207 left-sided and 4 right-sided) and 18 received subcutaneous ICDs.

DFT was omitted in 30 patients. Among the remaining 199 patients, VF was non-inducible in 2 and inducible in 197. Successful VF termination with a 10 J safety margin was achieved in 179 of 197 inducible cases (90.9%) with the first shock (mean energy 25.5 ± 14.4 J). In the remaining 18 cases (9.1%), the initial shock failed.

A second, higher-energy shock (mean 34.7 ± 9.2 J) successfully terminated VF in 11 of these 18 patients (8 with a 10 J margin, 3 without). In the remaining 7 cases, reverse shock polarity was applied, achieving successful termination with a 10 J margin in 5 cases. Three patients underwent system revision—two with lead repositioning or exchange and two with additional subcutaneous array implantation. Ultimately, VF was successfully terminated with a 10 J margin in 196 of 197 cases; in the remaining case, termination occurred at 40 J (Figure 1).

Periprocedural complications occurred in five patients, none related to DFT

During a mean follow-up of 1,636 ± 1,434 days, 9 patients received appropriate ICD therapies. In 2 patients, shocks failed to terminate ventricular arrhythmias. One of these patients had previously shown successful DFT at 14 J, while the other required intraoperative lead repositioning and subcutaneous array implantation following DFT failure.

Logistic regression identified secondary prevention as an indication for ICD implantation (odds ratio [OR] 3.24, 95% confidence interval [CI] 1.12–9.39, p = 0.03) and increased left ventricular septal thickness (OR 1.10, 95% CI 1.01–1.19, p = 0.03) as independent predictors of first-shock failure during DFT.

Conclusion

Routine DFT in HCM patients undergoing ICD implantation resulted in system revision in only a small proportion of cases. Ineffective ICD shocks during follow-up were rare and did not correlate with intraoperative DFT outcomes. These findings suggest that routine DFT may be safely omitted in most HCM patients, although selective testing could still be considered in high-risk individuals.