Digitalis drugs and vulnerability to ventricular fibrillation

The effect of acetylstrophanthidine (AS), a rapid-acting digitalis-like agent, on the ventricular fibrillation (VF) threshold was examined in normal and denervated chloralose-anesthetized dogs. In neurally intact dogs an intravenous bolus of AS (0.075 mg/kg) increased the VF threshold up to a maximum 50% (P less than 0.01) within 30 min after injection. The augmented VF threshold following intravenous administration of AS was not altered by vagotomy. Bilateral stellectomy in vagotomized dogs, as well as carotid sinus and aortic arch denervations, however, prevented the AS induced increase in VF threshold. In neurally intact dogs beta-adrenergic blockade with propranolol (0.25 mg/kg) precluded AS effects. These data suggest that the increase in the VF threshold resulting from AS administration in the normal canine ventricle is due to withdrawal of sympathetic tone mediated via the baroreceptor reflex. The direct effect of AS on the myocardium is to decrease the VF threshold.