Comparative Efficacy of Two Different Dosages of Intrathecal Magnesium Sulphate Supplementation in Subarachnoid Block

Background: Spinal anaesthesia is the primary anaesthetic technique for many types of surgeries. Adjuncts to the local anaesthetics (LA) used in spinal anaesthesia can exhibit undesirable side-effects like respiratory depression, urinary retention, pruritis, haemodynamic instability and nausea and vomiting, limiting their use. Magnesium when used in therapeutic doses avoids all of these side-effects. Materials and Methods: We conducted a randomized double blind study on 90 patients, 30 in each group, scheduled for orthopaedic lower limb surgery under subarachnoid block. Group I: received bupivacaine (0.5%), 12.5 mg + 0.5 ml of preservative free 0.9% normal saline, Group II received bupivacaine (0.5%), 12.5 mg + 0.2 ml (50 mg) of preservative free 25 % magnesium sulphate + 0.3 ml of preservative free 0.9% normal saline Group III: received bupivacaine (0.5%) 12.5 mg + 0.3 ml (75 mg) of 25 % magnesium sulphate + 0.2 ml of preservative free 0.9% normal saline for subarachnoid block. The onset and duration of sensory block, the highest dermatomal level of sensory block, motor block, time to complete motor block recovery and duration of spinal anaesthesia were recorded. Statistical Analysis: ANOVA was applied to determine the significance of difference between different groups. If p-value was significant then Turkey's Post Hoc Multicomparison test was applied. Values of p< 0.05 were considered to be statistically significant. Results: The time of maximum sensory block, time of onset of motor block, duration of sensory block, duration of motor block and time of analgesia request were prolonged in patients given magnesium 50mg and 75mg along with local anaesthetic intrathecally. Conclusion: N-methyl-D-aspartate (NMDA) receptor antagonist, magnesium when administered intrathecally along with local anaesthetics prolongs the duration of spinal analgesia without adverse effects.