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Background: Idiopathic ventricular arrhythmias (VAs) originating from the pulmonary sinus cusp (PSC) have not been sufficiently clarified.Objectives: The goal of this study was to investigate the prevalence, electrocardiographic characteristics, mapping, and ablation of idiopathic VAs arising from the PSC.Methods: Data were analyzed from 218 patients undergoing successful endocardial ablation of idiopathic VAs with a left bundle branch block morphology and inferior axis deviation.Results: Twenty-four patients had VAs originating from the PSC. In the first 7 patients, initial ablation performed in the right ventricular outflow tract failed to abolish the clinical VAs but produced a small change in the QRS morphology in 3 patients. In all 24 patients, the earliest activation was eventually identified in the PSC, at which a sharp potential was observed preceding the QRS complex onset by 28.2 ± 2.9 ms. The successful ablation site was in the right cusp (RC) in 10 patients (42%), the left cusp (LC) in 8 (33%), and the anterior cusp (AC) in 6 (25%). Electrocardiographic analysis showed that RC-VAs had significantly larger R-wave amplitude in lead I and a smaller aVL/aVR ratio of Q-wave amplitude compared with AC-VAs and LC-VAs, respectively. The R-wave amplitude in inferior leads was smaller in VAs localized in the RC than in the LC but did not differ between VAs from the AC and LC.Conclusions: VAs arising from the PSC are not uncommon, and RC-VAs have unique electrocardiographic characteristics. These VAs can be successfully ablated within the PSC. [ABSTRACT FROM AUTHOR]