Magnesium sulfate at two different doses as an adjuvant to bupivacaine in infraumblical (below knee) orthopedic surgeries under spinal anesthesia

Background The use of magnesium sulfate as an adjuvant in neuraxial block has gained popularity, with the aim of improving and enhancing the quality and duration of anesthesia, delaying the onset of postoperative pain, and thus reducing the demand for postoperative rescue analgesics. However, until today, there has been no consensus as regards the ideal dose of magnesium sulfate as an adjuvant in the subarachnoid block. The present study was designed to examine whether the addition of intrathecal magnesium sulfate (50 and 100 mg) would enhance the analgesic efficacy of intrathecal bupivacaine. We hypothesized that the additive effect of magnesium sulfate as an adjuvant to bupivacaine in subarachnoid block is dose dependent. Materials and methods This study was carried out on 90 American Society of Anesthesiology I and II patients of both sexes in the age group of 20–60 years scheduled for below knee surgeries under subarachnoid block. Group 1 (n = 30) patients received intrathecal 0.5% heavy bupivacaine (2.8 ml) +0.2 ml normal saline; group 2 (n = 30) received intrathecal 0.5% heavy bupivacaine (2.8 ml) +50 mg (0.1 ml) magnesium sulfate +0.1 ml normal saline; and group 3 (n = 30) received intrathecal 0.5% heavy bupivacaine (2.8 ml)+100 mg (0.2 ml) magnesium sulfate. The primary outcome measure was the duration of postoperative analgesia, and secondary outcomes included the number of supplemental analgesic requirements, block characteristics, and hemodynamic stability. Results The onset of sensory and motor block was delayed in the magnesium group (100 mg>50 mg). There was a significant prolongation of postoperative analgesia in the magnesium group in a dose-dependent manner, and total dose of rescue analgesic requirement was found to be significantly delayed in the 100 mg group. Patients in all groups remained hemodynamically stable without any adverse effects. Conclusion Magnesium sulfate (100 mg) as an adjuvant to bupivacaine in subarachnoid block prolongs the duration of analgesia and decreases the demand for rescue analgesics compared with the control and the magnesium sulfate 50 mg group.