Prognostic Implications of Defibrillation Threshold Testing in Patients With Hypertrophic Cardiomyopathy

INTRODUCTION: In hypertrophic cardiomyopathy (HCM) patients the need for defibrillation threshold (DFT) testing at the time of ICD implantation is debated. Moreover, its prognostic implications have never been explored. In a cohort of HCM patients we sought to (a) investigate factors prompting DFT testing, (b) evaluate ICD efficacy by testing DFT, (c) compare DFT in patients with and without massive LVH, and (d) assess whether DFT testing predicts shock efficacy for spontaneous VT/VF. METHODS AND RESULTS: We retrospectively analyzed a cohort of HCM patients implanted with an ICD. DFT was tested at the discretion of the implanting physician with a 10 J safety margin. During follow-up, ICD interventions were evaluated. The study population included 66 patients. DFT was determined in 25 (38%) patients. Age (HR: 0.95; 95%CI: 0.92-0.98; P = 0.004) and massive LVH (HR: 6.0; 95%CI: 2.03-18.8; P = 0.001) affected the decision to test DFT. DFT was at least 10 J less than maximal ICD output in 25/25. Safety margin was similar among patients with and without massive LVH (15 ± 3 J vs. 14 ± 2 J; P = 0.42). During follow-up (median 53 months) 15 shocks were delivered for 12 VT/VF in 7 patients. One VF ended spontaneously after a failed shock. Of 4 unsuccessful shocks, 2 occurred in 1 patient with DFT testing and 2 were delivered in 2 patients without. All unsuccessful shocks were ≤35 J. CONCLUSION: Young age and massive LVH prompt DFT testing. Contemporary ICDs are safe and effective in HCM patients independently from the magnitude of LVH. DFT testing does not predict shock efficacy for spontaneous VT/VF.