Preoperative analgesics are being increasingly used to provide analgesia in the intraoperative and postoperative period. Opioids reduce anaesthetic requirements, although the effect varies with the different drug and species. The aim of this work was to determine whether buprenorphine reduces the minimum alveolar concentration (MAC) of isoflurane in a dose-related fashion, and whether this effect is similar to morphine when clinical doses of both drugs are used in the rat. Thirty-six male Wistar rats were anaesthetized with isoflurane, and MAC was determined before and after the administration of either buprenorphine or morphine. MAC of isoflurane was determined from alveolar gas samples when a standard noxious stimulus, in the form of a tail clamp, was applied. The duration and degree of reduction of the MAC of isoflurane were recorded. Basic cardiovascular and respiratory measurements were also recorded. Buprenorphine (10, 30 and 100 microg/kg) and morphine (1, 3 and 10 mg/kg) reduced in a dose-dependent fashion the MAC of isoflurane by 15%, 30% and 50%, respectively. Buprenorphine resulted in less cardiovascular and respiratory depression and had a longer-lasting action than morphine. In conclusion, buprenorphine has a dose-related isoflurane sparing effect in the rat similar to that caused by morphine at clinical doses of both drugs.

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