Effect of intravenous magnesium sulphate on postoperative pain following spinal anesthesia. A randomized double blind controlled study.

BACKGROUND Magnesium sulphate (MgSO4), NMDA receptor antagonist, is known to reduce perioperative requirement of anesthetics and analgesics. However, no studies assessed the effect of MgSO4 on onset and recovery from spinal anesthesia. A prospective, randomised, double blind study was designed to assess the effect of intravenous (IV) MgSO4 on onset and recovery from spinal anesthesia and post operative analgesic requirement following below umbilical surgery. METHODS Sixty patients (ASA class I & II) were selected randomly and divided into two groups. Patients were given either MgSO4 50 mg kg(-1) in 10 mL within 10 min, followed by an infusion of MgSO4 10 mg kg(-1) hr(-1) IV in 4mL (MG group) for 12 hrs or normal saline in same volume and rate for 12 hrs as used in MG group (NS group). After initiating the infusion, spinal anesthesia was given with 0.5% bupivacaine (Hyperbaric) 2.5 mL at L3/4 or L4/5 space. Time taken for sensory block at the level of T-10 and motor block (modified Bromage Score-1) was noted. Postoperatively, time taken for recovery from spinal anesthesia, pain score and requirement of postoperative analgesic in 24 hours were observed and compared between the two groups. RESULTS The first rescue analgesia was required after 334 +/- 202 min in MG group and after 233 +/- 141 min in NS group with significant difference (p < 0.05). The morphine required over 24 hours for analgesia was significantly less in MG group (3.99 +/- 1.25 mg) as compared to NS group (7.13 +/- 2.68 mg) (p < 0.000). CONCLUSION Intravenous MgSO4 improves postoperative analgesia without affecting the onset and recovery from spinal anesthesia.