Objective To investigate the effects of different doses of dexmedetomidine on the minimum alveolar concentration (MAC) of sevoflurane for sedation in patients.Methods ASA physical status Ⅰ or Ⅱ patients of both sexes,aged 18-64 yr,undergoing elective lower abdominal surgery performed under general anesthesia,were randomly divided into 4 groups:control group (group C) and different doses of dexmedetomidine groups (D1,D2 and D3 groups).In D1,D2 and D3 groups,the loading dose of dexmedetomidine 0.4,0.6 and 0.8 μg/ kg was intravenously infused over 15 min,respectively,adverse cardiovascular events were then recorded,followed by infusion at 0.4,0.6 and 0.8 μg· kg-1 · h-1 via a pump,respectively,while in group C,the equal volume of 0.9 ％ normal saline was given instead of dexmedetomidine.Sevoflurane administration was begun after completion of infusion of the loading dose.Up-and-down sequential allocation was used to determine the MAC.The initial end-tidal concentration of sevoflurane was set at 0.8％,0.7％,0.6％ and 0.5％ in C,D1,D2 and D3 groups,respectively,and maintained at this level for 15 min.Each time the concentration of sevoflurane increased/decreased in the next patient depending on whether or not the patients correctly followed the verbal command to open his eyes.The ratio between the two consecutive concentrations was 0.9.The middle point between the positive response and negative response served as a crossover pair.After at least 7 independent crossover pairs were observed in each group,the experiment was stopped.The MAC and 95 ％ confidence interval of sevoflurane were calculated.Results The incidence of adverse cardiovascular events was significantly higher in D3 group than in D1 and D2 groups (P ＜ 0.05).In C,D1,D2 and D3 groups,the MAC (95％ confidence interval) of sevoflurane was 0.68％ (0.64％-0.74％),0.50％ (0.47％-0.52％),0.36％ (0.32％-0.41％) and 0.28％(0.26％-0.31％),respectively.The MAC-awake of sevoflurane was significantly lower in D1-3 groups than in group C,in D2 and D3 groups than in group D1,and in D3 group than in group D2 (P ＜ 0.05).Conclusion Dexmedetomidine 0.6μg/kg can significantly decrease the MAC of sevoflurane for sedation,induces no side effects and is the optimum dose in patients.

Objective To evaluate the effects of different doses of dexmedetomidine on the median effective concentration (EC50) of propofol given by target-controlled infusion (TCI) at loss of consciousness (LOC).Methods Eighty ASA Ⅰ or Ⅱ patients of both sexes,aged 18-64 yr,with body mass index ≤25 kg/m2,scheduled for operations under general anesthesia,were randomly allocated to one of four groups(n=20 each): control group (group C) and dexmedetomidine 0.4 μg/kg group (group D1),dexmedetomidine 0.5 μg/kg group (group D2) and dexmedetomidine 0.6 μg/kg group (group D3).Dexmedetomidine 0.4,0.5 and 0.6 μg/kg were infused intravenously over 10 min in groups D1-3,while the equal volume of normal saline was given instead of dexmedetomidine in group C.Propofol was then given by TCI and the EC50 was determined by up-and-down sequential trial.The target plasma concentration was set at 2.0μg/ml in the first patient in each group.The ratio of the target plasma concentration between the two consecutive patients was 1.1.Loss of response to eyelash stimulation and verbal command (2 times) was considered to be signs of LOC.The EC50 and 95％ confidence interval (CI) of propofol causing LOC were calculated.Complications such as bradycardia,hypotension and respiratory depression were recorded.Results The EC50 (95％ CI) of propofol causing LOC was 2.59 (2.51-2.67),2.09 (2.02-2.16),1.82 (1.70-1.95) and 1.60 (1.49-1.72) μg/ml in groups C and D1.3 respectively.The EC50 of propofol causing LOC was significantly lower in groups D1-3 than in group C.Dexmedetomidine significantly decreased the EC50 of propofol required for causing LOC in a dose-dependent manner in groups D1-3 (P ＜ 0.05).The incidences of bradycardia and hypotension were significantly lower in groups D1.3 than in group C (P ＜ 0.05).Compared with group D1,the incidence of bradycardia was increased in groups D2,3 and the incidence of hypotension was increased in group D3 (P ＜ 0.05),There was no significant difference in the incidences of bradycardia and hypotension between groups D2 and D3 (P ＞ 0.05).No patients developed respiratory depression.Conclusion The optimum dose for dexmedetomidine infused intravenously when combined with propofol given by TCI is 0.4 μg/kg and it can decrease the EC50 of propofol administered by TCI at LOC with no adverse reactions.