Magnesium added to bupivacaine prolongs the duration of analgesia after interscalene nerve block

Purpose Local anesthetic adjuvants have been studied previously in an attempt to prolong the duration of analgesia after peripheral nerve blockade. Magnesium has been shown to have an antinociceptive effect in animal and human pain models. We evaluated the effects of adding magnesium sulphate to long-acting local anesthetics for interscalene nerve block to prolong the duration of analgesia and improve the analgesic quality. Methods We enrolled 66 patients undergoing arthroscopic rotator cuff repair. The interscalene nerve block was performed with 0.5% bupivacaine 20 mL with epinephrine (1:200,000) plus either 10% magnesium sulphate 2 mL (Magnesium Group) or normal saline 2 mL (Saline Group). The following data were recorded for 24 hr after surgery: onset times and durations of sensory and motor blocks, analgesic duration, the pain numeric rating scale (NRS), postoperative fentanyl consumption, and complications. Results The duration of analgesia was longer in the Magnesium Group than in the Saline Group [mean and (standard deviation) 664 (188) min vs 553 (155) min, respectively; P = 0.017]. Patients in the Magnesium Group had significantly reduced pain NRS scores at 12 hr ( P = 0.012), but the cumulative fentanyl consumption was similar in both groups. The onset times and durations of sensory and motor blocks were not significantly different between the two groups. Conclusion The addition of magnesium sulphate to a bupivacaine-epinephrine mixture for interscalene nerve block prolongs the duration of analgesia and reduces postoperative pain.