Perioperative magnesium infusion and postoperative pain

NMDA receptor activation is considered one of the mechanisms involved in postoperative pain and hypersensitivity. Magnesium is the physiological blocker of the NMDA-receptor-complex-associated calcium ionophore. The aim of this study was to determine if a pre-, intra- and postoperative infusion of magnesium would reduce postoperative pain.

In a prospective, randomised, double-blinded and placebo-controlled study, 24 patients undergoing elective hysterectomy in standardised general anaesthesia received a 5 h infusion of either placebo or magnesium laevulinate (initial bolus 8 mmol: then 8 mmol/h) starting with induction of anaesthesia. Postoperative analgesia was by PCA morphine for the first 48 h and patients were followed for 5 d with regular assessment of pain and side-effect scores.

Overall, pain scores were similar with magnesium and placebo infusion, although patients in the magnesium group experienced more episodes of severe or unbearable pain (placebo = 6%, magnesium = 16%, P = 0.02). Median pain scores were higher in the magnesium group only at 3 h postoperatively (P = 0.04): afterwards there were no significant differences. Except for the first postoperative hour (placebo = 12.8 +/- 4.7 mg, magnesium = 9.3 +/- 3.2 mg, P = 0.04), cumulative morphine consumption was similar. Gastrointestinal complication rates and patient satisfaction were similar in both groups.

Perioperative magnesium infusion does not improve postoperative analgesia. At the doses used in this study, the use of magnesium is associated with short-term decreases in postoperative analgesia.