Abstract: AIM:To observe the effects of ketamine on preventing the increase of postoperative morphine requirement induced by fentanyl.METHODS: Sixty women undergoing total abdominal hysterectomy by spinal anaesthesia were assigned to 4 groups consisting of placebo (normal saline, C), fentanyl (3 bolus of 1 μg/kg, at 15 min intervals, F), ketamine (infusion of 15 μg·kg^-1 ·min^-1 ketamine from the skin incision until 20 min before the end of the surgery, K), ketamine and fentanyl (infusion of 15 μg·kg^-1·min^-1 ketamine from the skin incision until 20 min before the end of the surgery plus 3 bolus of 1 μg/kg fentanyl, at 15-min intervals, FK). The cumulative morphine consumption, pain score, and adverse effects (nausea, vomiting, hallucination, dizziness and itching) were recorded at 1, 3, 6, 12, 24, 48 h postoperatively. RESULTS:There were no significant differences in age, weight, duration of surgery and the post-operative sensory block time. The cumulative morphine consumption in group F was significantly higher than those in group C at 3, 6, 12 h postoperatively (P <0. 05). The postoperative cumulative morphine consumption was similar in group C, K and FK. No differences in posterative pain score were observed among the group. More patients in K and FK group had hallucination during and or after the surgery than those in group C (P <0. 05). CONCLUSION: These results suggested that intraoperative administration of fentanyl increased postoperative morphine consumption, which could be prevented by coadministration of ketamine.

Key words: acute opioid tolerance, hyperalgesia, postoperative pain, ketamine