Abstract Body (Do not enter title and authors here): Idiopathic ventricular fibrillation (VF) arrest can be a challenging diagnosis, especially when the etiology is unclear. We present a case of a young male who was found to have VF originating from the right ventricular (RV) moderator band (MB).

A 19-year-old male with a history of idiopathic VF arrest with ICD placement presented with a complaint of sudden dizziness and lightheadedness. Upon device interrogation, he was found to have 17 seconds of fine-VF terminated by one out-of-hospital ICD shock. He reported non-adherence to daily nadolol. He was also noted to have a high burden of unifocal premature ventricular contractions (PVC) on telemetry. On EKG the morphology of PVCs was of a LBBB with a superior axis, suggesting origin from the MB. The patient then underwent an electrophysiological (EP) study, where it was felt that earliest site of activation was identified at the mid-inferior RV along a muscle band extending from mid inferior free wall, superiorly to the interventricular septum. He then had successful radiofrequency ablation at this site, with no post-ablation PVCs or ICD discharge, and was subsequently restarted on his home nadolol.

Idiopathic VF is defined as VF in the absence of clinical cause. Our patient’s PVCs and fine-VF were highly suspicious for RV MB origination. We proceeded with EP mapping and identified the earliest site of PVC activation at the MB. Pace mapping revealed 95% matching at this site. Following ablation, no further PVCs were induced even with isoproterenol. Though not fully understood, the MB is becoming increasingly noted as a source for Purkinje-mediated ventricular arrhythmias. Possible mechanisms of arrhythmogenesis include fibrosis, stretching of fibers, and ion channel variations. Arrythmias from the MB are potentially responsive to catheter ablation.

Early recognition of typical PVC morphology from the RV MB as the source of VF can provide an opportunity for timely intervention. Younger patients, especially, can be treated with catheter ablation to reduce their risk of sudden cardiac death.