Preoperative analgesics are being increasingly used to provide analgesia in
the intraoperative and postoperative period. Opioids reduce anaesthetic re
quirements, although the effect varies with the different drug and species.
The aim of this work was to determine whether buprenorphine reduces the mi
nimum 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 ta
il clamp, was applied. The duration and degree of reduction of the MAC of i
soflurane were recorded. Basic cardiovascular and respiratory measurements
were also recorded. Buprenorphine (10, 30 and 100 mu g/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 cardiovas
cular and respiratory depression and had a longer-lasting action than morph
ine. In conclusion, buprenorphine has a dose-related isoflurane sparing eff
ect in the rat similar to that caused by morphine at clinical doses of both
drugs.