Effects of different doses of dexmedetomidine on median effective concentration of ropivacaine for brachial plexus block

Abstract

Objective To determine the optimum dose of dexmedetomidine administered locally through evaluating the effects of different doses of dexmedetomidine on the median effective concentration(EC50)of ropivacaine for brachial plexus block. Methods American Society of Anesthesiologists physical status Ⅰ or Ⅱ patients of both sexes, aged 19-50 yr, weighing 50-80 kg, scheduled for elective ulna and radius fracture open reduction and internal fixation, requiring ultrasound-guided axillary brachial plexus block, were randomly assigned into 4 groups using a random number table: control group(group C)and dexmedetomidine 0.4, 0.6 and 0.8 μg/kg groups(D1, D2 and D3 groups). Axillary brachial plexus block was performed only with ropivacaine in group C. In D1-3 groups, axillary brachial plexus block was performed with the mixture of ropivacaine and dexmedetomidine 0.4, 0.6 and 0.8 μg/kg, respectively.The effective block was defined as complete loss of pain sensation in the areas innervated by the brachial plexus.The volume of local anesthetics was 40 ml.The concentration of ropivacaine was determined by up-and-down technique.The initial concentration was 0.4% and the ratio between the two successive concentrations was 1.0.If the block was effective, the next patient received a lower dose of ropivacaine; or conversely if ineffective, a higher dose was given in the next patient.At least 7 independent crossover pairs were observed in each group.The EC50 of ropivacaine was the mean of the concentration of ropivacaine of each crossover pair.The occurrence of brachial plexus block-related adverse events, adverse cardiovascular events and over-sedation was recorded. Results In C, D1, D2 and D3 groups, 20, 22, 24 and 19 patients were enrolled, respectively.Compared with group C, the EC50 of ropivacaine was significantly decreased in D2 and D3 groups, and no significant change in the EC50 of ropivacaine was found in group D1.No patients developed adverse events in group D1.The incidence of bradycardia was 17%, but it was transient in group D2.In group D3, the incidence of bradycardia and hypotension was 58% and 32%, respectively, and they required special treatment, and the incidence of over-sedation was 10%. Conclusion The optimum dose of dexmedetomidine is 0.6 μg/kg when mixed with ropivacaine for brachial plexus block. Key words: Dexmedetomidine; Amides; Brachial plexus; Nerve block; Dose-response relation, drug