Optimal Dose of Magnesium Sulfate Infusion in Obese Patients: A Double-Blind Randomized Trial

Background: Magnesium sulfate reduces opioid use and its associated side effects. However, no consensus exists on whether the optimal dosing should be based on actual body weight or adjusted ideal body weight. The primary objective of this study was to compare postoperative analgesia after magnesium sulfate infusion, using doses calculated based on actual body weight versus adjusted ideal body weight. Methods: This prospective, randomized, double-blind, controlled clinical trial included 75 participants who underwent target-controlled intravenous general anesthesia. The participants were divided into three groups: a control group (CG), a group receiving magnesium sulfate calculated by actual body weight (AWG), and a group receiving magnesium sulfate calculated based on the adjusted ideal body weight (IWG). Results: The AWG had significantly lower pain scores than the CG (p < 0.001) and IWG (p=0.017). Opioid use was similar between the AWG and IWG, but significantly higher in the CG (AWG = IWG, p=0.08; CG > AWG, p < 0.001; CG = IWG, p 0.03). The increase in magnesium concentration did not reach clinically relevant levels. Neuromuscular blockade latency decreased in the groups receiving magnesium sulfate (p < 0.001 in both comparisons) compared to the CG. Conclusion: Calculating the dose of magnesium sulfate based on actual body weight enhances postoperative analgesia. The increase in magnesium concentration was not clinically significant and did not interfere with the action of cisatracurium in the groups receiving magnesium sulfate. Trial Registration: ClinicalTrials.gov identifier: NCT04645719.