The effects of halothane on the interactions between myocardial contractility, aortic impedance, and left ventricular performance. IV. Haemodynamic responses to vagus nerve stimulation.

The effects of electrical stimulation of the vagus nerves have been studied in anaesthetized dogs, before and after bilateral interruption of the cardiac sympathetic nerves. During constant heart rates, maintained by right atrial pacing, vagus nerve stimulation caused small but significant reductions of myocardial contractility, but these were minor compared with the effects of other negative interventions such as halothane anaesthesia and coronary artery ligation. When atrial pacing was withdrawn during vagus nerve stimulation, the ensuing bradycardia was associated with further marked reduction of myocardial contractility and in cardiac output. Vagus nerve stimulation depressed left atrial systole and left ventricular filling, and this might be expected to have greater effect in the halothane depressed heart than in the undepressed heart of a conscious animal. Halothane decreased the spontaneous heart rate of sympathectomized and vagotomized dogs, probably by a direct effect on the sino-atrial pacemaker. It is unlikely that increased vagal activity during halothane anaesthesia could significantly depress myocardial contractility, but these many mechanisms may contribute to the overall impairment of left ventricular performance at slow heart rates.