Effects of Isoflurane on Vascular Tone and Circulatory Autoregulation in the Feline Small Intestine

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Abstract
The vascular response in autoperfused small intestine was studied in ten cats during basal chloralose anaesthesia and controlled ventilation with either air, nitrous oxide/oxygen (70/30) or 0.7% end‐tidal concentration of isoflurane + nitrous oxide/oxygen (70/30). Intestinal blood flow was measured by the optical drop recording technique, and intestinal perfusion pressure was kept constant at either 100, 75 or 50 mmHg (13.30, 9.98 or 6.65 kPa). At perfusion pressures of 100 and 75 mmHg (13.30 and 9.98 kPa), intestinal blood flow was significantly increased and intestinal vascular resistance decreased during isoflurane + nitrous oxide/oxygen anaesthesia, compared with nitrous oxide/oxygen or air. According to the equation of closed loop gain (Gf), autoregulation was active in the pressure range 100‐75 mmHg (13.30‐9.98 kPa). In the pressure range 75‐50 mmHg (9.98‐6.65 kPa), the autoregulatory capacity was attenuated during air or nitrous oxide/oxygen and absent during isoflurane + nitrous oxide/oxygen. The vasodilator responses and the autoregulatory pattern were not changed by post‐ganglionic intestinal denervation. The intestinal vasodilator effect of isoflurane was further investigated in the denervated intestine, perfused at systemic arterial pressure by local intra‐arterial administration of isoflurane dissolved in a fat emulsion. A dosedependent vasodilator response was hereby observed.