Exercise-induced ventricular ectopy with RBBB morphology suggests underlying myocardial fibrosis in inherited left ventricular cardiomyopathy

INTRODUCTION AND OBJECTIVES Inherited left ventricular cardiomyopathies (iLVC) are genetic disorders characterized by myocardial remodeling and fibrosis, predisposing patients to ventricular arrhythmias. The prevalence of exercise-induced right bundle branch block-pattern ventricular ectopy (EI-RBBB-VE) and its relationship with myocardial fibrosis in iLVC remain poorly characterized. We aimed to assess the prevalence and characteristics of EI-RBBB-VE in iLVC and its association with myocardial fibrosis detected by late gadolinium enhancement (LGE) on cardiac magnetic resonance (CMR). METHODS In this multicenter, cross-sectional study, 77 patients with genetically characterized iLVC underwent CMR and treadmill exercise testing. The presence, morphology, and timing of EI-RBBB-VE were assessed. RESULTS EI-RBBB-VE was present in 48.1% of patients and was significantly more frequent in those with LGE (LGE(+)) than in those without LGE (LGE(-)) (61.0% vs 33.3%; P = .015). Patients with LGE exhibited a lower arrhythmic threshold, with earlier ectopy onset (median 36 [IQR, 7-174] vs 117 [85-414] seconds; P = .008) and ectopy occurring at a lower percentage of predicted maximal heart rate (52% vs 67%; P = .022). The prevalence of LGE did not differ between low- and high-burden ectopy groups (62.5% vs 66.7%; P = 1.0). On multivariable analysis, EI-RBBB-VE was independently associated with myocardial fibrosis (OR, 3.92; 95%CI, 1.28-12; P = .017), whereas resting Holter ectopic burden and genotype positivity were not. CONCLUSIONS EI-RBBB-VE is common in iLVC and is independently associated with myocardial fibrosis on CMR. Earlier exercise-induced ectopy also identifies patients with fibrosis, supporting exercise testing as an accessible tool for detecting an underlying fibrotic substrate beyond resting ECG, Holter monitoring, and genotype.