Effects of extradural morphine on end-tidal isoflurane concentration and physiological variables in pigs undergoing abdominal surgery: a clinical study

OBJECTIVE: To evaluate the effects of preoperative extradural morphine on the end-tidal isoflurane (Fe'ISO) concentration and on physiological variables in pigs undergoing abdominal surgery. STUDY DESIGN: Prospective, randomized, blinded study. ANIMALS: Fourteen healthy pigs (20 +/- 4 kg) undergoing intestinal cannulation. MATERIALS AND METHODS: Anaesthesia was induced with a combination of medetomidine (50 microg kg(-1)) and tiletamine-zolazepam (2.5 mg kg(-1)) injected intramuscularly, and was maintained with isoflurane in air and oxygen (FiO(2) = 50% O(2)). In the first group, morphine (0.1 mg kg(-1)) was administered extradurally before surgery. The second group received an equivalent volume of extradural saline as control. During the experiment, heart and respiratory rates, mean arterial blood pressure, tidal volume and minute ventilation were recorded every 10 minutes. The concentration of Fe'ISO was adjusted, according to the depth of anaesthesia, by an experienced animal nurse. Within treatment groups, time-related changes in Fe'ISO and physiological variables were analysed using a repeated measurement anova. Differences in data between treatment groups were analysed at specific time points using a Mann-Whitney U-test. Results are presented as mean +/- SD; p < 0.05 was considered as significant. RESULTS: After the onset of action of the morphine, the Fe'ISO required to maintain anaesthesia was significantly lower in the extradural morphine group compared with control. During the expected maximal effect of the drug, Fe'ISO was significantly lower in the morphine group (0.6 +/- 0.2%) than in the control group (0.9 +/- 0.2%). The decrease in Fe'ISO indicated that the onset of action of morphine was approximately 30 minutes after injection. No significant differences in other clinical variables were found between the groups. CONCLUSION: Pigs that received extradural morphine before abdominal surgery achieved surgical anaesthetic depth at a lower Fe'ISO concentration. CLINICAL RELEVANCE: Extradural morphine allows abdominal surgery to be performed at a lower Fe'ISO concentrations.