Abstract 11923: Ventricular Parasystole: An Under-Recognized Risk Factor for Ventricular Fibrillation in Patients With Implantable Cardioverter-Defibrillators

Introduction: Ventricular parasystole is an uncommon characteristic of ventricular ectopic beats characterized by cyclical depolarization that is independent of the dominant rhythm. The relationship between ventricular parasystole and ventricular arrhythmias including ventricular tachycardia (VT) and fibrillation (VF) is unknown. We sought to quantify the prevalence of ventricular parasystole in patients with implantable cardioverter defibrillators (ICDs) with VT and VF. Methods: We reviewed all inpatient ICD interrogations between June 2019 and August 2020 at a single tertiary care hospital and identified patients with VT/VF. For each patient, we fully evaluate all available electrocardiograms, daily 7-lead rhythm strips during hospitalization, Holter monitors, stress test electrocardiograms, and ICD interrogations to identify evidence of ventricular parasystole. Results: Out of 468 ICD interrogations, 129 patients with VT/VF were identified (mean age 55.1 ± 20.3 years; 14 (11%) single chamber, 57 (44%) dual chamber, 12 (9%) subcutaneous, 46 (36%) biventricular). Of these, 88 (68%) patients had VT only, 31 (24%) VF only, and 10 (8%) both VT and VF. Ventricular parasystole was identified in 2/88 (2%) VT only patients, 7/31 (23%) VF only patients, and 3/10 (30%) patients with VT and VF. Patients with VF were significantly more likely than those with VT to have parasystole (p<0.001). Nine of the 10 patients with VF and parasystole (90%) had presumed premature ventricular contraction (PVC) induced VF of unclear etiology (no acute ischemia or channelopathy). Additionally, in 4 patients with VF and parasystole (3 with VF only, 1 VT and VF), all with dual chamber ICDs, we also identified intradevice interactions with ventricular parasystole which led to R-on-T pacing-induced VF episodes (this was the only cause of VF in 1 patient). Conclusions: Ventricular parasystole was found in 24% of ICD patients with VF, and was significantly more common in ICD patients with VF compared to VT. Ventricular parasystole can lead to pacing-induced VF due to intradevice interactions in dual chamber ICDs. The role of ventricular parasystole and PVC-induced VF requires further study to determine whether parasystole is a causative factor or an epiphenomenon.