Evaluation of the Efficacy of Magnesium Sulphate as an Adjuvant to Lignocaine for Intravenous Regional Anaesthesia for Upper Limb Surgery

Several additives have been combined with local anaesthetics for intravenous regional anaesthesia to improve block quality, analgesia and to decrease tourniquet pain. Magnesium sulphate is one potential additive. This prospective, randomised, double-blinded study was conducted in 30 ASA physical status I or II patients undergoing upper limb surgery under tourniquet. In group L, patients received intravenous regional anaesthesia with lignocaine alone (9 ml of 2% lignocaine diluted with normal saline to total volume of 36 ml). Patients in group M received intravenous regional anaesthesia with lignocaine plus magnesium sulphate (6 ml of 25% magnesium sulphate plus 9 ml of 2% lignocaine diluted with normal saline to total volume of 36 ml). Assessment was by observing the response to injection of drug; sensory and motor block and tourniquet pain. The mean time of onset of sensory block was 12.40 and 3.47 minutes in groups L and M respectively (P <0.001). The average times of onset of motor block in groups L and M were 17 and six minutes respectively (P <0.001). Of the patients in group M, 66.7% reported moderate to severe pain while the drug was being injected, compared to 20% in group L (P=0.011). There was a statistically significant difference in visual analogue scale for tourniquet pain at 10 and 30 minutes after tourniquet inflation (lower in group M). These findings indicate that magnesium sulphate added as an adjuvant to lignocaine hastens the onset of sensory and motor block and decreases tourniquet pain. However, there is increased incidence of transient pain on injection if magnesium sulphate is added.