Abstract: Objective: To investigate the influence of dexmedetomidine on patient-controlled intravenous analgesia(PCIA)with fentanil after thoracic surgery.Methods: Forty ASAⅠ-Ⅲ patients scheduled for elective thoracic surgery were randomly divided into two groups(n=20,each)according to the PCIA formula.Group C received fentanil 20 μg/kg and group D fentanil 20 μg/kg plus dexmedetomidine 10 μg/kg;and the PCIA was diluted with 100 saline.The initial loading volume was 2 ml,background volume 2 ml/h and bolus dose 0.5 ml;the lockout time was set to 15 min.The number of the pressing attempts,consumption of fentanil within 24 h,the visual analogue scales,and the Ramsay sedation scale at 4,8,24 and 48 h after surgery were recorded;the incidence of vomiting and respiration depression was also recorded.Blood samples were obtained before anesthesia,4,8 and 24 h after surgery for determination of serum cortisol concentration.The patients' satisfactory degree was evaluated 48 h after surgery.Results: Compared to group C,group D had a decreased number of PCIA pressing attempts and consumption of fentanil within 24 h after operation(P<0.01);the Ramsay sedation scale of group D increased at 4,8,24 and 48 h after surgery;the visual analogue scales of group D decreased at 4 and 8 hour after surgery(P<0.01 to P<0.05).The incidence of respiratory inhibition vomiting and serum cortisol concentration after operation had no significant differences between the two groups(P>0.05).The satisfaction degree of the patients in group D was significantly higher than that in group C(P<0.05).Conclusions: The effect of combination use of dexmedetomidine and fentanil is superior to fentanil alone in PCIA after thoracic surgery.