Protective effect of vagal stimulation on reperfusion arrhythmias in cats

The role of the autonomic nervous system in modulating reperfusion arrhythmias is still unclear. Experiments with sympathetic denervation or α- and β-adrenergic blocking agents have provided mixed results, while the effect of parasympathetic activation has not been investigated extensively. The effect of bilateral vagotomy and of vagal stimulation was studied, with and without attendant bradycardia, on the incidence of reperfusion arrhythmias in α-chloralose anesthetized cats. The left anterior descending coronary artery was occluded for 20 minutes, followed by reperfusion in 105 animals. The incidence and severity of reperfusion arrhythmias was compared in 1) neurally intact animals (heart rate 208 ± 24 beats/min), 2) animals with acute bilateral vagotomy (heart rate 233 ± 25 beats/min), 3) animals with vagal stimulation adjusted to maintain heart rate at 90-100 beats/min, and 4) animals with vagal stimulation + ventricular pacing to maintain heart rate at prestimulation values. All the neurally intact and vagotomized animals developed complex reperfusion arrhythmias, but these arrhythmias occurred in only 60 and 72%, respectively, of the animals with vagal stimulation and vagal stimulation + pacing (p <0.005 vs. neurally intact and p <0.02 vs. vagotomy). The incidence of ventricular fibrillation was similar in neurally intact (62%) and vagotomized (58%) animals; it was strikingly lower (7%, p <0.01) in animals with vagal stimulation when heart rate was allowed to decrease, and it was 48% when heart rate was kept constant during vagal stimulation. A selective protection from sustained (> 30 seconds duration) ventricular tachycardia was observed in animals with vagal stimulation independent of heart rate changes. These data indicate that 1) suppression of tonic vagal activity, produced by bilateral vagotomy, does not modify the occurrence of complex reperfusion arrhythmias; 2) vagal stimulation initiated shortly before reperfusion significantly reduces the occurrence of complex reperfusion arrhythmias; 3) the protective effect against ventricular fibrillation is mediated primarily by the attentdant decrease in heart rate; 4) sustained ventricular tachycardia is prevented by vagal stimulation independently of heart rate changes.