Which is the better adjuvant to ropivacaine in brachial plexus block: dexmedetomidine or morphine? A prospective, randomized, double-blinded, comparative study

Objectives: Dexmedetomidine has been utilized as an adjuvant to prolong single local anaesthetic injection technique in brachial plexus block despite of adverse effects and cost-effectiveness. Morphine, a classic opioid, is also used as an adjuvant in neuraxial and perineural nerve blocks. In this study, we compared the effects of these two different adjuvants to ropivacaine in interscalene brachial plexus blockade. The onset and duration of motor and sensory blocks are mainly focused in the study. Methods: A total of 92 patients scheduled for distal arm or forearm surgeries were divided into 3 groups in a randomized, double-blind mode. Under the direction of a nerve stimulator, an interscalene brachial plexus block was performed by a single injection of one of the following local anesthetics: 24 mL (120 mg) of 0.5% ropivacaine plus 1 mL saline in Group R; 24 mL (120 mg) of 0.5% ropivacaine plus 1 mL dexmedetomidine 0.75 μg/kg in Group RD; and 24 mL (120 mg) of 0.5% ropivacaine plus 1 mL (2 mg) morphine in Group RM. Onset time and durations of sensory and motor blocks, cardiovascular parameters, peripheral oxygen saturation (SpO2), and adverse reactions were recorded and compared. Results: The onset time in the Group RD was significantly less than that in the Group RM (P<0.05), while the duration in the Group RD was longer than that in the Group RM (P<0.05). Both the onset time and the duration of blocks were not critically different in the Group RM and the Group R. For the side effects, more patients in the Group RD suffered from bradycardia (P<0.05). However, more patients in the Group RM had nausea and vomiting (P<0.05). Conclusions: Compared with morphine, dexmedetomidine can provide superior analgesia for interscalene brachial plexus block in adjunct to ropivacaine at 0.5%. As an adjuvant, morphine may have few significant benefits in peripheral nerve block.