Effect of dexmedetomidine administered locally on median effective concentration of ropivacaine for paravertebralnerve block

Abstract

Objective To evaluate the effect of dexmedetomidine administered locally on the median effective concentration (EC50) of ropivacaine for paravertebral nerve block (PVNB) . Methods Forty-eight ASA physical status Ⅰ or Ⅱ female patients, aged 20-64 yr, with body mass index<24 kg/m2, scheduled for elective unilateral segmental mastectomy under PVNB, were randomly divided into 2 groups (n=24 each) using a random number table: ropivacaine group (group R) and ropivacaine mixed with dexmedetomidine group (group RD) . PVNB was performed at T4 on the operated side guided by ultrasound and nerve stimulator.Ropivacaine 20 ml and a mixture of ropivacaine and 20 μg dexmedetomidine 20 ml were injected locally in group R and group RD, respectively.The concentration of ropivacaine was determined by up-and-down sequential allocation.The initial ropivacaine concentration was set at 0.35%, and the ratio between the two successive concentrations was 1.2.The EC50 and 95% confidence interval of ropivacaine were calculated using Dixon-Massey method. Results The EC50 (95% confidence interval) of ropivacaine was 0.27% (0.23%-0.30%) and 0.22% (0.18%-0.25%) in group R and group RD, respectively.Compared with group R, the EC50 of ropivacaine was significantly decreased by 19% in group RD. Conclusion Small dose of dexmedetomidine administered locally can significantly enhance the efficacy of PVNB with ropivacaine. Key words: Dexmedetomidine; Amides; Never block; Dose-response relationship, drug