Elucidating the Mechanism of Cesium-Induced Sustained Monomorphic Ventricular Tachycardia in Rabbits

The mechanisms of sustained ventricular tachycardia (VT) induced by large cumulative dose of cesium chloride (Cs) remains unclear. Seven anesthetized rabbits were intravenously injected with Cs (1 mmol/kg) 3 time s at 20-min intervals. The surface ECG and monophasic action potential of the left ventricular endocardium were simultaneously recorded. In another 12 rabbits, transmembrane action potentials of right ventricular muscles were recorded with glass microelectrodes. In experiments in vivo, sustained monomorphic VT was induced after the third injection of Cs, whereas the early afterdepolarization (EAD)-related nonsustained polymorphic VT was induced after the second injection. Overdrive pacing during the sustained VT resulted in postdrive acceleration. The pacing and recovery cycle lengths showed an inverse relation. In experiments in vitro, preparations were superfused with Tyrode's solution containing 7.5 mM Cs. Cs initially induced EADs. Additional exposure to Cs depolarized the membrane potential, which consequently attained threshold, producing spontaneous activities. Further exposure resulted in an induction of sustained rhythms that were accelerated by overdrive pacing. Our results indicate that the sustained rhythms at low membrane potential induced by a long exposure to Cs in vitro may underlie an electrophysiologic mechanism for the sustained VT induced after large cumulative dose of Cs in vivo.