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CARDIOVASCULAR ANESTHESIA

Halothane Attenuates Myogenicity in the Rabbit Ear Artery

Ross D. MacPherson, MSc, PhD, FANZCA^*, and Anthony W. Quail, MD, FANZCA

*Department of Anaesthesia and Pain Management, Royal North Shore Hospital, St. Leonards, NSW, Australia; and Discipline of Human Physiology, Faculty of Medicine and Health Sciences, University of Newcastle, Callaghan, NSW, Australia

Address correspondence and reprint requests to Ross MacPherson, MSc, PhD, FANZCA, Department of Anaesthesia and Pain Management, Royal North Shore Hospital, St. Leonards, NSW 2065, Australia. Address e-mail to rmacpher{at}doh.health.nsw.gov.au

The aim of this study was to test the hypothesis that halothane interferes with the myogenic response to an increase in intraluminal pressure. Myogenic responsiveness refers to the intrinsic property of vascular smooth muscle to dilate and then constrict in response to an increase in intraluminal pressure, in an attempt to maintain vessel diameter. Vessel segments taken from the rabbit central ear artery were cannulated, pressurized to 60 mm Hg, and perfused with and suspended in Krebs solution. After exposure to extraluminal l-norepinephrine, vessels contracted to an initial diameter (Di) and were subjected to intraluminal pressure increases to 100 mm Hg. Myogenic reactivity was assessed by measurement of the extent of dilatation after the pressure increase from Di to a maximal diameter (Dm) and then the constriction and recovery (against the pressure increase) to a final (Df) diameter. Myogenicity was further assessed by determining the rate of return of the vessel diameter (angle of recovery) and vessel recovery (defined as Dm - Df/Dm - Di) and expressed as a percentage. Myogenicity was determined before and after exposure to halothane in concentrations of between 1–5%. Halothane significantly attenuated the myogenic response at all concentrations studied. The effect of halothane was maximal at a concentration of 5% where there was virtual abolition of the myogenic response with recovery assessed at 6 ± 2.7% (SEM), compared with control (98 ± 2.5%, P < 0.05). The angle of recovery was likewise attenuated. These data suggest that halothane, in a dose-dependent manner, attenuates myogenicity in the isolated rabbit ear artery preparation.

Implications: Blood pressure is controlled partially by the myogenic response. This refers to the capacity of arteries to dilate and then constrict in response to pressure increase. Using arteries from rabbits, we have shown that administration of halothane reduces or abolishes this response. This observation may be a contributing factor to hypotension caused by halothane.