Dose–Response of Perineural Magnesium Sulphate in Ultrasound-guided Infraclavicular Brachial Plexus Block: A Randomised Double-blind Study

Background Regional anaesthesia is widely preferred for upper limb surgeries, particularly those performed below the elbow; however, the limited duration of action of local anaesthetics remains a clinical challenge. The addition of adjuvants such as magnesium sulphate (MgSO4) contributes to better block characteristics and sustained analgesic effect. Purpose: This study was designed to investigate whether varying doses of perineural MgSO4 influence the characteristics of ultrasound-guided infraclavicular brachial plexus block performed with bupivacaine. Materials and Methods Following approval from the Institutional Ethics Committee and registration with the Clinical Trials Registry-India, this prospective randomised double-blind single-centre study was undertaken in 90 ASA I-II adults undergoing elective upper limb surgery. Participants were randomly allocated into three groups ( n = 30 each). All groups received 20 mL of 0.5% bupivacaine, with Group A receiving 125 mg MgSO4, Group B 250 mg MgSO4 and Group C 500 mg MgSO4, diluted with normal saline to maintain equal volume. The principal outcome measure was postoperative analgesia duration, with block characteristics and adverse events analysed as secondary outcomes. Continuous variables were analysed using one-way analysis of variance, and categorical variables using the Chi-square test. Results The duration of analgesia increased significantly in a dose-dependent manner, with Group C demonstrating the longest duration (711 ± 38.98 min), followed by Group B (582.17 ± 29.64 min) and Group A (466.33 ± 32 min) ( p < .001). Sensory and motor blockade developed more rapidly and persisted longer in Group C than in the other two groups. None of the participants experienced block failure or adverse effects such as hypotension, bradycardia, nausea or vomiting. Conclusion Perineural MgSO4 significantly enhances the duration of analgesia and improves block characteristics in ultrasound-guided infraclavicular brachial plexus block in a dose-dependent manner. Among the doses studied, 500 mg MgSO4 provided the most prolonged analgesia without increasing adverse effects, supporting its safe and effective use as an adjuvant to bupivacaine in upper limb surgeries.