Objective To evaluate the effect of dexmedetomidine as a supplementary to combined lumbar plexus-sciatic nerve block for arthroscopic knee surgery.Methods Fifty patients who were scheduled for arthroscopic knee surgery and ASA class Ⅰ or Ⅱ grade,were divided into control group and dexmedetomidine group by random digits table method with 25 cases each.The patients of dexmedetomidine group were given loading dose dexmedetomidine 0.8 μ g/kg(injection time 10 min) by intravenous infusion,then were given maintenance dose 0.4 μμ g/(kg· h) to the end of operation.The patients of control group were given 0.9％ sodium chloride,methods with the dexmedetomidine group.After 10 min,combined lumbar plexus-sciatic nerve block was performed by a nerve stimulator in both groups.Fentanyl 50 μμ g was infused intravenously to the patient whose effect of block was inadequately.The mean arterial pressure (MAP),heart rate (HR),pulse oxygen saturation (SpO2),respiratory frequency at preanesthesia (T0),beginning of dexmedetomidine infusion (T1),beginning of surgery (T2),10 minutes of surgery (T3) and the end of surgery (T4) were recorded,and intraoperative untoward reaction,using of fentanyl and doctor's satisfaction rate were recorded.Results There were no statistical differences in MAP and HR at T0 between the 2 groups (P＞ 0.05).MAP and HR at T2-4 were significantly higher than those at T0 in control group,furthermore MAP and HR at T2-4 in control group were significantly higher than those in dexmedetomidine group [MAP:(137 ± 18) mm Hg(1 mm Hg =0.133 kPa) vs.(107 ± 14) mm Hg,(132 ± 11) mm Hgvs.(107 ± 11) mm Hg,(131±13) mm Hg vs.(95 ± 12) mm Hg;HR:(99 ± 17) times/rmin vs.(88 ± 14) times/min,(99 ± 14) times/min vs.(81 ± 15) times/min,(97 ± 14) times/rmin vs.(76 ± 11) times/min],there were statistical differences (P ＜ 0.05).There were no statistical differences in respiratory frequency and SpO2 between the 2 groups (P ＞0.05),but 2 cases in dexmedetomidine group,5 cases in control group appeared SpO2 ＜0.90,they quickly returned to normal (＞ 0.97) by holding up the submaxilla.The using rate of fentanyl and doctor's satisfaction rate in dexmedetomidine group were significantly better than those in control group [28％(7/25)vs.68％(17/25),100％(25/25) vs.80％(20/25)],there were statistical differences (P＜ 0.05).There were no statistical differences in the using rate of atropine and efedrina,the rate of respiratory depression between the 2 groups (P ＞ 0.05).Conclusion Dexmedetomidine as a supplementary to combined lumbar plexus-sciatic nerve block for arthroscopic knee surgery has effect confirmed,low untoward reaction,high doctor' s satisfaction rate.