Effect of intravenous magnesium sulfate on post-operative pain after infraumbilical surgery under subarachnoid block: A randomized controlled trial

Background: Anti-nociceptive effect of Mg2+ is due to antagonization of N-methyl D-aspartate receptors. The duration of post-operative analgesia was prolonged when magnesium was given as an adjunct for peripheral nerve blocks. A few clinical trials have examined the effect of adding intrathecal Magnesium sulfate (MgSO4) to local anesthetic agents. Aims and Objectives: The aim of the present study was to evaluate the effect of intravenous (IV) MgSO4 on the duration of sensory block, and the duration of analgesia of hyperbaric bupivacaine in subarachnoid block. Materials and Methods: This double-blinded, randomized control trial was done with 80 adult patients divided into two groups posted for infraumbilical surgery under subarachnoid block. One group received pre-operative MgSO4 bolus at the dose of 50 mg/kg 30 min before spinal anesthesia. During the intraoperative period, a MgSO4 infusion of 10 mg/kg/h was started 10 min after giving a subarachnoid block and was infused over 1 h (MG group). The control normal saline (NS) group received NS in the same method as the MG group. Post-operatively total duration of analgesia and the requirement of post-operative analgesic in 24 h were recorded and compared between the two groups using standard statistical tests. Results: MgSO4 provided significantly faster onset (4.98±0.72 vs. 8.58±1.05 min) and longer duration (156.60±12.30 vs. 107.30±9.30 min) of sensory blockade (P=0.001 and <0.0001, respectively), significantly longer total duration of analgesia (290.83±5.27 min vs. 260.53±6.48 min; P<0.0001) compared to NS (P=0.038 and <0.0001, respectively). A total number of rescue analgesic doses was significantly higher in the NS group compared to the MgSO4 group (2.55±0.50 vs. 1.28 ±0.44; P=0.005). The magnesium group showed a significantly higher Ramsay sedation score. Conclusion: In infraumbilical surgeries under a subarachnoid block, IV infusion of MgSO4 significantly prolonged the duration of sensory block and post-operative analgesia, and decreased the total analgesic consumption without any significant hemodynamic variations and serious adverse effects.