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.