Efficacy of Intrathecal vs Intravenous Route of Magnesium Sulphate as an Adjuvant with 0.75% Hyperbaric Ropivacaine to Augment Quality of Spinal Anaesthesia in Lower Limb Surgeries: A Randomised Clinical Trial

Introduction: Subarachnoid block is commonly used for lower limb surgeries but has a limited duration. Adjuvants like Magnesium Sulphate (MgSO4 ) are added to extend analgesia. Ropivacaine is a highly protein-bound amide local anaesthetic that is structurally similar to bupivacine. The present study compared intravenous versus intrathecal MgSO4 combined with 0.75% hyperbaric ropivacaine to enhance spinal anaesthesia in lower limb surgeries. Aim: To compare the efficacy of MgSO4 as an adjuvant administered via the intrathecal versus intravenous route, in conjunction with 0.75% hyperbaric ropivacaine, to augment the quality of spinal anaesthesia in lower limb surgeries. Materials and Methods: A prospective randomised clinical trial was conducted in the Department of Anaesthesiology, Pradyumna Bal Memorial Hospital, Kalinga Institute of Medical Sciences, Bhubaneswar, Odisha, India, from August 2022 to July 2024. A total of 166 patients undergoing elective lower limb surgery were randomly assigned to two groups. Group IT received 3 mL of 0.75% hyperbaric ropivacaine with 50 mg of intrathecal MgSO4 , while group IV received intravenous MgSO4 (50 mg/kg bolus, 15 mg/kg/h infusion) alongside intrathecal ropivacaine. The time taken for the onset of sensory and motor block was evaluated as the primary objective, whereas the duration of the block, along with haemodynamic parameters and postoperative VAS scores, was assessed as secondary objectives. The statistical analysis was performed using the Mann-Whitney U test and Chi-square test. Results: Of total study participants, there were 59 (73.8%) males and 21 (26.3%) females in group IT, and 54 (67.5%) males and 26 (32.5%) females in group IV. The onset of motor block was faster in group IV (95 seconds) than in group IT (205 seconds), and the onset of sensory block was quicker in group IV (150 seconds) compared to group IT (360 seconds), with both results yielding p-value <0.001. The duration of motor block was longer in group IV (240 minutes) compared to group IT (160 minutes), as was the duration of sensory block (300 minutes versus 240 minutes); p-value <0.001. Conclusion: Intravenous MgSO4 combined with subarachnoid block shortened the onset time of the block, prolonged the duration of sensory block, reduced postoperative pain, and caused minimal haemodynamic instability, making it an effective adjunct for spinal anaesthesia.