To evaluate effect of local application of small doses of dexmedetomidine on the medium effective concentration(EC50) of ropivacaine for saphenousnerve block and the mechanism of the analgesic effect induced by dexmedetomidine.

Forty-eight patients of American Society of Anesthesiologists (ASA) stageⅠtoⅡ, who were 60 to 80 years old with body mass index(BMI)<24 kg/m², were scheduled for elective unilateral total knee arthroplasty(TKA) under saphenousnerve block. They were randomly divided into two groups (30 cases each) using a random number table: the ropivacaine group and the combined drug group (ropivacaine mixed with 0.8 μg/kg dexmedetomidine). The volume of local anesthetics was 20 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. Visual analogue scale (VAS) was performed every five minutes to evaluate the block effect. 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. Median effect concentration (EC₅₀) and 95% confidence interval(CI) of ropivacaine were determined using the Dixon-Massey method, and compared by U test. Chi-square test and t test were also used.

The EC₅₀ (95% CI) of ropivacaine was 0.34%(0.32%, 0.36%)in the combined drug group and 0.45%(0.42%, 0.46%) in the ropivacaine group, which was significantly lower in the combined drug group than in the ropivacaine group (Z=-3.219, P<0.05).

Local application of small doses of dexmedetomidine can decrease EC₅₀ of ropivacaine for saphenousnerve block guided by ultrasound, enhance the efficacy of saphenousnerve block with ropivacaine.