Abstract 1400: Origin and Mechanisms of Ventricular Tachycardia and Fibrillation in a Knock-in Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia
Patients affected by catecholaminergic polymorphic ventricular tachycardia (CPVT) present bidirectional (BVT), polymorphic ventricular tachycardia (PVT) and ventricular fibrillation (VF), resulting in sudden death. Since RyR2 gene mutations leading to CPVT result in abnormal Ca2+ release it has been inferred that VT/VF is due to delayed after-depolarization (DAD)-induced triggered activity. However, the origin and mechanisms of VT/VF in CPVT are unknown. Recently, a knock-in mouse carrying the RyR2-R4496C mutation (MUT) was shown to reproduce the human BVT, PVT and VF phenotype during adrenergic stimulation. We used volume-conducted ECGs and epicardial optical mapping in 12 MUT and 4 wildtype (WT) hearts to determine whether the arrhythmias originate at the Purkinje fiber (PF) network. Hearts were Langendorff-perfused with Tyrode’s solution containing 2.7–3.6 mM/L Ca2+ and 100–200 nM/L isoproterenol. Spontaneous VT occurred in 66% of MUT hearts (41 episodes). No VT was shown in WT hearts (p=0.02). In MUT hearts monomorphic VT (MVT) manifested as right (RV) or left (LV) ventricular epicardial breakthroughs at the same locations recorded in sinus rhythm that correspond to the endocardial insertion of major branches of the RV and LV PF networks. Maps showed that the characteristic alternating-QRS ECG pattern of BVT corresponded to epicardial breakthroughs whose origin alternated beat-to-beat between RV and LV. PVT manifested as multifocal breakthroughs in RV and LV. In a MUT heart (1mM/L caffeine and 1μM/L epinephrine), transition from multifocal PVT to reentrant VF was due to the formation of a stable rotor (frequency =39 Hz). Current-clamp analysis in 5 single MUT and 8 WT PFs during perfusion with 100 nM/L isoproterenol showed DADs in 50% of WT and 100% of MUT cells (p=0.057) and triggered activity in 12% of WT and 80% of MUT cells (p=0.01). These data provide the first evidence that altered Ca+ release in MUT hearts may give rise to abnormal excitation whose origin may be unifocal (MVT), bifocal (BVT), multifocal (PVT) or reentrant (VF). The highly focalized nature and reproducible location of the epicardial breakthroughs in MVT and BVT, strongly suggest that VTs in the setting of CPVT are the result of DAD-induced triggered activity originating in PF.