Influence of autonomic nervous system stimulation on the protective zone.

The effect of sympathetic and parasympathetic stimulation on the vulnerable period threshold and the protective zone was studied in chloralose-anaesthetised dogs. Sympathetic stimulation substantially decreased the repetitive extrasystole threshold and shifted the timing of the protective zone earlier into diastole. Vagus nerve excitation exerted the opposite effect on both electrophysiological properties. During concurrent sympathetic nerve stimulation, the changes produced by vagal activation were accentuated. These findings suggest that parasympathetic influences on the protective zone are due, in part, to an antagonism of adrenergic effects on ventricular electrical properties.