Perioperative magnesium infusion and postoperative pain

Background: 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. Methods: 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 assessments of pain and side-effect scores. Results: 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. Conclusions: 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.