Opioid tolerance may involve activation of the N-methyl-D-aspartate (NMDA) system. The possible involvement of the NMDA system suggests that one of the NMDA receptor antagonists, magnesium may be a useful adjunct to opioids for the treatment of postoperative pain following remifentanil infusion.

For this study, 70 patients scheduled for major abdominal surgery under remifentanil-based anesthesia were randomly allocated into groups that received either magnesium sulfate (group M) or saline (group C) intravenously. The patients in the group M received 25% magnesium sulfate at a dose of 50 mg/kg in 100 ml of saline, and those in the group C received an equal volume of saline prior to the induction of anesthesia. In addition, patients in both groups received 10 mg/kg/h infusion of either magnesium sulfate (group M) or an equal volume of saline (group C) until the end of surgery. Pain was assessed using a visual analog scale at 30 min, and 6, 12, 24, and 36 hours after operation. The time to the first use of postoperative analgesic and cumulative analgesic consumption in both groups were also evaluated.

The visual analog scale scores for pain and cumulative analgesic consumption were significantly lower in the group M than in the group C. The time to the first use of postoperative analgesic was significantly shorter in group C than in the group M.

Use of the NMDA-receptor antagonist, magnesium sulfate as an adjuvant analgesic reduced postoperative pain in patients undergoing major abdominal surgery under remifentanil-based anesthesia.