Antinociceptive effects of magnesium sulfate for monitored anesthesia care during hysteroscopy: a randomized controlled study

BACKGROUND: Opioids are the most effective antinociceptive agents, they have undesirable side effects such as respiratory depressant and postoperative nausea and vomiting. The purpose of the study was to evaluate the antinociceptive efficacy of adjuvant magnesium sulphate to reduce intraoperative and postoperative opioids requirements and their related side effects during hysteroscopy. METHODS: Seventy patients scheduled for hysteroscopy were randomly divided into 2 groups. Patients in the magnesium group (Group M) received intravenous magnesium sulfate 50 mg/kg in 100 ml of isotonic saline over 15 min before anesthesia induction and then 15 mg/kg per hour by continuous intravenous infusion. Patients in the control group (Group C) received an equal volume of isotonic saline as placebo. All patients were anesthetized under a BIS guided monitored anesthesia care with propofol and fentanyl. Intraoperative hemodynamic variables were recorded and postoperative pain scores were assessed with verbal numerical rating scale (VNRS) 1 min, 15 min, 30 min, 1 h, and 4 h after recovery of consciousness. The primary outcome of our study was total amount of intraoperative and postoperative analgesics administered. RESULTS: Postoperative serum magnesium concentrations in Group C were significantly decreased than preoperative levels (0.86 ± 0.06 to 0.80 ± 0.08 mmol/L, P = 0.001) while there was no statistical change in Group M (0.86 ± 0.07 to 0.89 ± 0.07 mmol/L, P = 0.129). Bradycardia did not occur in either group and the incidence of hypotension was comparable between the two groups. Total dose of fentanyl given to patients in Group M was less than the one administered to Group C [100 (75–150) vs 145 (75–175) μg, median (range); P < 0.001]. In addition, patients receiving magnesium displayed lower VNRS scores at 15 min, 30 min, 1 h, and 4 h postoperatively. CONCLUSIONS: In hysteroscopy, adjuvant magnesium administration is beneficial to reduce intraoperative fentanyl requirement and postoperative pain without cardiovascular side effects. Our study indicates that if surgical patients have risk factors for hypomagnesemia, assessing and correcting magnesium level will be necessary. TRIAL REGISTRATION: ChiCTR1900024596. date of registration: July 18th 2019.