The effects of isoflurane on arterial pressure, preganglionic sympathetic activity, vagal nervous activity, pulse rate, and barostatic reflexes were studied in cats. When the end-expired isoflurane concentration did not exceed 1.0 per cent tachycardia was observed which was abolished when the anaesthetic concentration was increased to 1.8 per cent. Arterial pressure, sympathetic nervous activity and vagal activity declined progressively with increasing depth of anaesthesia, but the reduction of vagal activity at all levels of anaesthesia always significantly exceeded the reduction of sympathetic activity. Barostatic reflexes, measured as the reduction in arterial pressure, pulse rate and sympathetic nervous activity when an aortic depressor nerve was stimulated electrically, were found to be normal, except that the response of arterial pressure was depressed at high isoflurane concentration.
The authors therefore conclude that isoflurane depresses the cardiovascular system not only by direct action, but also by depression of the sympathetic nervous system. The tachycardia seen during isoflurane anaesthesia can be explained in part by a greater depression of parasympathetic than of sympathetic tone. The fact that barostatic reflexes were active during isoflurane anaesthesia while the reflex response of arterial pressure was diminished leads the authors to believe that isoflurane depresses the response of vascular smooth muscle to sympathetic impulses.