Objective To investigate the effects of different doses of dexmedetomidine on the anesthetic efficacy of propofol and remifentanil in patients undergoing plastic surgery.Methods Sixty ASA Ⅰ or Ⅱ patients of both sexes,aged 18-64 yr,weighing 45-75 kg,scheduled for elective plastic surgery,were randomly assigned into 3 groups (n =20 each):control group (C group),low loading dose dexmedetomidine group (D1 group) and high loading dose dexmedetomidine group (D2 group).Dexmedetomidine was infused at a rate of 0.4 μg· kg-1 · h-1 until 30 min before the end of surgery after a loading dose of dexmedetomidine 0.6 (group D1) or 1.0 μg/kg (group D2) was infused at 10 min before induction of anesthesia in groups D1 and D2.Anesthesia was induced with target-controlled infusion of propofol (target plasma concentraiton 4.0 μg/ml) and remifentanil (target effect-site concentraiton 2.0 ng/ml).After the patients lost consciousness,rocuronium 0.6 mg/kg was injected intravenously.The patients were mechanically ventilated after tracheal intubation.Anesthesia was maintained with target-controlled infusion of propofol (target plasma concentraiton 2.0-3.5 μg/ml) and remifentanil (target effectsite concentraiton 1.5-2.5 ng/ml).Narcotrend index was maintained at Class D.Narcotrend index,systolic pressure (SP),diastolic pressure (DP) and HR were recorded before anesthesia (baseline),at 10 min of dexmedetomidine infusion,at 0,1 and 5 min after tracheal intubation,at the end of dexmedetomidine infusion,and at 0,1and 5 min after removal of the endotracheal tube.The induction time,consumption of propofol and remifentanil,time for recovery of spontaneous breathing and orientation,extubation time,and Richmond Agitation Sedation Scale score at 10 min after removal of the endotracheal tube were recorded.The development of sinus bradycardia during operation and complications during recovery from anesthesia were also recorded.Results Compared with group C,the total consumption of propofol and remifentanil,Richmond Agitation Sedation Scale score at 10 min after removal of the endotracheal tube,and incidences of nausea,bucking and agitation during recovery from anesthesia,and Narcotrend index and HR at 10 min of dexmedetomidine infusion were significantly decreased in groups D1 and D2,and induction time was shortened in D2 group (P ＜ 0.05 or 0.01).The incidence of sinus bradycardia during operation was significantly higher in group D2 than in groups C and D1 (P ＜ 0.05).There was no significant difference in the time for recovery of spontaneous breathing and orientation,and extubation time between the three groups (P ＞ 0.05).There was no significant difference in SP,DP and HR before and after tracheal intubation and before and after removal of the endotracheal tube between groups D1 and D2 (P ＞ 0.05).Conclusion Infusion of dexmedetomidine at a rate of 0.4μg·kg-1 · h-1 after a loading dose of 0.6 μg/kg infused before induction of anesthesia can shorten the induction time,reduce the consumption of propofol and remifentanil,effectively inhibit the stress responses to tracheal intubation or removal of the endotracheal tube,and decrease the occurrence of side effects in patients undergoing plastic surgery.