Effect of adrenaline on vagus nerve reflexes

Abstract

Dizziness or syncope may occur during treatment of nasal disease. These symptoms are considered imputable to the vagovagal reflex, which is partly involved in neurogenic syncope. In response to trigeminal nerve stimulation, the vagus nerve causes a sudden fall in heart rate and blood pressure. Elevation in blood adrenaline level during pain or tension was noted, and its effects on vagovagal reflexes were studied. Ten cats were used in the experiment. The vagus nerve was exposed on the right side of the neck by making an incision in the trachea, and a platinum electrode was attached to the vagus nerve on nerve's distal side. The head was tilted a 30 degree angle. The cats were divided into control and adrenaline-treated groups, and changes in cerebral blood flow, heart rate, and arterial pressure were compared between the two groups after electric stimulation of the vagus nerve. Cerebral blood flow was measured by the hydrogen clearance method. 1) Control group The vagus nerve was electrically stimulated for 1 minute. 2) Adrenaline-treated group The vagus nerve was electrically stimulated for 1 minute following 30 seconds of intravenous administration of adrenaline. Cerebral blood flow was significantly decreased in both the control and adrenaline-treated groups after electric stimulation, but the decrease was significantly greater in the latter group at all sites of measurement. Whereas heart rate and arterial pressure were significantly decreased in the control group, these variables in the adrenaline-treated group showed no significant change despite the greater decrease in cerebral blood flow. Heart rate, blood pressure, and cerebral blood flow were all significantly decreased after electric stimulation of the vagus nerve. These changes were considered owing to a fall in blood pressure due to vasodilation resulting from bradycardia and a relative decrease in sympathetic nervous tension resulting from electric stimulation. On the other hand, in the adrenaline-treated group, neither heart rate nor blood pressure showed any significant change, but cerebral blood flow was significantly decreased at all sites of measurement despite an adrenaline load. This contradictory results may be accounted for by the powerful beta 2-activity of adrenaline. The greater decrease in cerebral blood flow in the adrenaline-treated than in the control group can be attributed to decreased peripheral vascular resistance by its beta 2-activity. In the field of otorhinolaryngology the trigeminal region is often involved in the treatment of nasal disorders so that vagovagal reflexes are often encountered. The results of this study counsels caution in the treatment involving the trigeminal region.