Effect of local application of dexmedetomidine on the medium effective concentration of ropivacaine for brachial plexus block

Abstract

Objective To investigate effect of local application of dexmedetomidine on the medium effective concentration (EC50) of ropivacaine for brachial plexus block and the mechanism of the analgesic effect induced by dexmedetomidine.Methods Fifty-two ASA Ⅰ or Ⅱ patients of both sexes,aged 18-50 yr,weighing 50-80 kg,scheduled for elective forearm or hand surgery,requiring ultrasound-guided axillary brachial plexus block,were randomly assigned into 2 groups (n =26 each):control group (group C) and dexmedetomidine group (group D).Axillary brachial plexus block was performed using only ropivacaine in group C.In group D the mixture of ropivacaine and 0.8 μg/kg dexmedetomidine was injected for the block.The effective block was defined as complete loss of pain sensation in the brachial plexus distribution.The volume of local anesthetics was 30 ml.The concentration of ropivacaine was determined by up-and-down technique.The initial concentration was 0.5 ％ and the ratio between the two successive concentrations was 1.2.If the block was effective,the next patient received a lower dose of ropivacaine,or if ineffective,a higher dose was given in the next patient.The EC50 and 95 ％ confidence interval of ropivacaine were determined using the Dixon-Massey method.Results The EC50 (95 ％ confidence interval) of ropivacaine was 0.32％ (0.30％-0.33％) in group D and 0.44％ (0.42％-0.46％) in group C.The EC50 of ropivacaine was significantly lower in group D than in group C (P ＜ 0.05).Conclusion Local application of dexmedetomidine can decrease the EC50 of ropivacaine for brachial plexus block guided by ultrasound,indicating that the mechanism of the analgesic effect induced by dexmedetomidine is related to the local anesthetic-like effect. Key words: Dexmedetomidine ; Amides ; Brachial plexus ; Nerve block ; Dose-response relationship, drug