Effects of vagal stimulation on cesium-induced early afterdepolarizations and ventricular arrhythmias in rabbits

Previous evidence has shown that increased sympathetic tone enhances the cesium chloride (Cs)-induced early afterdepolarizations (EADs) and ventricular tachycardias (VTs).

We assessed the effects of vagal stimulation on Cs-induced EADs and ventricular arrhythmias in the rabbit heart. Monophasic action potentials (MAPs) of the left ventricular endocardium were recorded simultaneously with surface ECG. Two protocols were used: 1) While in their intrinsic sinus rhythm, 11 rabbits were given three intravenous Cs injections (1 mM/kg) 20 minutes apart, and the effects of vagal stimulation on the ventricular arrhythmias thus induced were examined. 2) Under constant atrial pacing (cycle length, 250 msec), EAD amplitude was measured after Cs injection (1 mM/kg) without (five rabbits, control group) or with (four rabbits, vagal stimulation group) vagal stimulation. We observed the following. 1) Cs produced EADs and VTs of polymorphic (PVT) and monomorphic (MVT) types. During PVT, the take-off potential of repetitive premature action potentials in MAP recordings was about the same as the peak level of EADs, and during MVT, the take-off potential was the level of full repolarization. Vagal stimulation suppressed PVT but not MVT. Vagal stimulation after spontaneous termination of MVT restarted MVT of the same morphology at a rate much slower than the preceding sinus rate. 2) EAD amplitude was significantly smaller in the vagal stimulation group than in the control group.

The results suggest that PVT originated from triggering by EADs, whereas MVT was of different origin, and that vagal stimulation suppressed PVT by decreasing the amplitude of EADs.