Comparison of intravenous and intrathecal magnesium sulfate as adjuvants to bupivacaine in spinal anesthesia for lower limb surgery: a prospective comparative study

Background and importance: The management of postoperative pain is still a major problem at the conclusion of lower limb orthopedic surgeries. Bupivacaine spinal anesthesia is effective, although it has a short-lasting effect. An N-methyl-D-aspartate receptor antagonist, magnesium sulfate, is investigated as an adjuvant to improve painkillers. This paper is a comparison study comparing the analgesic properties of intravenous (IV) bolus versus intrathecal magnesium sulfate as an adjuvant to hyperbaric bupivacaine when used as a subarachnoid block. Materials and methods: This was a comparative study that was carried out under spinal anesthesia in 135 patients who were undergoing upper limb orthopedic surgeries. The participants were grouped into three, namely, Group A, which was administered with intrathecal magnesium sulfate (100 mg) and bupivacaine, Group B, which was administered with IV magnesium sulfate (50 mg/kg), and Group C, which was provided with bupivacaine and normal saline alone. The main outcomes were the postoperative analgesia duration and pain level on the Numerical Rating Scale (NRS). Sensory and motor block characteristics, hemodynamic stability, and adverse effects were taken as the secondary results. Results: The intrathecal magnesium sulfate greatly extended the analgesia duration (298.70 ± 13.60 min) when compared to IV (213.83 ± 11.47 min) and control (184.76 ± 76.32 min). Intrathecal groups always had lower scores on NRS in the postoperative period. Hemodynamic variables were stable, and mild sedation was noted in group IV. Conclusions: Intrathecal magnesium sulfate is better than IV magnesium sulfate in terms of postoperative analgesia with few adverse events and no changes in hemodynamics. It acts as a good supplement to bupivacaine in spinal anesthesia. Optimization of dosing and safety with extended use should be done in further research.