Effect of magnesium sulfate infusion on sevoflurane consumption, hemodynamics, and perioperative opioid consumption in lumbar disc surgery

Background: Magnesium sulphate administration was initially reported to reduce postoperative analgesic requirements. Subsequent reports, though, were inconsistent. Specifically, we tested the hypotheses that intraoperative magnesium sulfate administration reduces postoperative requirement for opioids, secondary outcomes were intraoperative muscle relaxant, sevoflurane consumptions and postoperative pain. Methods: Fifty American society of anesthesiology status (ASA) I-II patients who gave a written informed consent were scheduled for elective lumbar disc surgery. The Institutional Review Board approved the study. Patients were randomly assigned to the placebo group (n = 25) or magnesium group (n = 25). Patients assigned to the magnesium group were given an initial infusion of 30 mg/kg (over 10 minutes) starting immediately after anesthesia and completed before intubation. The infusion was then continued at 10 mg/kg/hr throughout surgery. General anesthesia was induced with propofol, 2.5 mg/kg, and maintained with sevoflurane 2 percent in a 50 percent O2/50 percent N2O mixture. The sevoflurane concentration was adjusted to keep bispectral index (BIS) values between 45 and 60. Both groups were given atracurium, 0.6 mg/kg, and a remifentanil infusion at an initial dose of 0.1 mg/kg/h. The remifentanil infusion was adjusted to maintain heart rate and mean arterial pressure (MAP) within 20 percent of baseline values. Atracurium administration was repeated when the train of four (TOF) ratio exceeded 0.30. Intubation conditions were scored. At the end of surgery, the authors assessed recovery using early recovery criteria and Aldrete recovery scores. Postoperative analgesia was maintained with morphine via patient-controlled analgesia. Results: Heart rate, MAP, and pulse oximetry (SPO2) values were similar in the groups at all times. Intubating conditions were similar except that the increase in MAP was greater in the placebo group. Neither BIS values nor sevoflurane consumption differed between the groups; however, significantly less atracurium [95% CI = 4.1 (2.8, 5.5)] and remifentanil [95% CI = 0.14 (0.07, 0.20)] was used in the magnesium group. Side effects, Aldrete scores, and early recovery parameters were all similar in the groups. In the first 24 hours, visual analogue scale (VAS) values for pain were greater in the placebo than in the magnesium group. The magnesium group consumed significantly less morphine [95% CI = 11 (6, 16)]. Conclusion: Intraoperative magnesium administration significantly reduced muscle relaxant and opioid requirements; more importantly, it also reduced postoperative pain and opioid use.