ELUCIDATING THE MECHANISM OF CESIUM-INDUCED SUSTAINED MONOMORPHIC VENTRICULAR-TACHYCARDIA IN RABBITS

The mechanisms of sustained ventricular tachycardia (VT) induced by la rge cumulative dose of cesium chloride (Cs) remains unclear. Seven ane sthetized rabbits were intravenously injected with Ca (1 mmol/kg) 3 li mes at 20-min intervals. The surface ECG and monophasic action potenti al of the left ventricular endocardium were simultaneously recorded. I n another 12 rabbits, transmembrane action potentials of right ventric ular muscles were recorded with glass microelectrodes. In experiments in vivo, sustained monomorphic VT was induced after the third injectio n of Cs, whereas the early after depolarization (EAD)-related nonsusta ined 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 expe riments 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 overdri ve pacing. Our results indicate that the sustained rhythms at low memb rane potential induced by a long exposure to Ca in vitro may underlie an electrophysiologic mechanism for the sustained VT induced after lar ge cumulative dose of Cs in vivo.