Remifentanil is a useful and relatively safe opioid, but acute tolerance to it frequently develops, as patients who have received remifentanil based anesthesia often suffer postoperative hyperalgesia. This study investigated whether a small dose of ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, can attenuate postoperative pain suffered with remifentanil anesthesia.

32 patients who were scheduled to undergo laparoscopic cholecystectomy were randomly assigned to two groups: a ketamine group (group K) and a control group (group C). All patients were given remifentanil by target controlled infusion (TCI) to the effect site (concentrations: 4.0-6.0 ng/ml). Just before incision patients in group K received 0.15 mg/kg ketamine intravenously and patients in group C received the same volume, but only of normal saline. Pain scores measured by Numerical Rating Scale (NRS) and additional use of analgesics were immediately recorded by a blinded investigator at the post-anesthesia care unit (PACU) and general ward up to 24 hours after surgery.

The average NRS scores upon arrival to PACU, 5, 10, 15 and 30 min in PACU, and 5 hours after discharge from PACU were significantly lower in group K than group C. The frequency of additional analgesics use was also significantly lower in group K.

It is suggested that a small dose of ketamine attenuates the postoperative pain felt with remifentanil anesthesia. This finding also suggests that ketamine may decrease the possible acute tolerance developed with remifentanil anesthesia.