Vascular Responses to Anaesthetic Agents* Stewart

ALTHOUGH EFFECTS Of anaesthetic agents on cardiac haemodynamics have been defined 1,'' until recently few data have been available concerning their actions on the peripheral circulation. In this regard, probably the best studied agent is halothane. Among the pertinent observations are decreased hepatic artery resistance in dogs, 3 decreased resistance in the canine hind limb, 4 decreased total peripheral resistance in man 5 and increased forearm blood flow in man. ~,~ All of these observations argue for peripheral vasodilatation, although Sawyer et al. 7 have shown peripheral vascular resistance to be increased by halothane in the miniature swine. On the other hand chloroform and eyelopropane have been shown to increase hepatic artery resistance '~ while cyclopropane has been reported to increase splanehnic resistance in man ~ and to increase forearm vascular resistance in man. ~).1. There is, therefore, a body of information which suggests that some general anaesthetic agents may produce peripheral vasodilatation while others may produce peripheral constriction. Those general anaesthetic agents which produce hypotension may do so through dilatation of peripheral blood vessels, either directly, through stimulation of betaadrenergic receptors, or through interference with the action of endogenous noradrenaline. To explore these possibilities, selected agents were studied in clinically relevant doses on isolated rabbit veins and aortae. Possible interference with the actions of endogenously released noradrenaline was studied by inference through noting the effects of these drugs on the response to exogenously administered noradrenaline, and also by ascertaining their effects on H:~-noradrenaline washout curves; possible effects on beta-adrenergic receptors were examined by testing these agents in the presence and absence of adrenergic blocking drugs. In the ease of those drugs which, in vivo, produced not dilatation but constriction, the possibility of mediation through the autonomic nervous system was examined by assaying their effects in reserpine treated tissues and also by determining whether H:~-noradrenaline washout curves might give any indication of exaggerated release of endogenous noradrenaline.