Abstract: Objective To investigate the effects of dexmedetomidine on the balance of Th1/Th2 in pediatric patients under cardiopulmonary bypass. 
Methods Eighty pediatric patients，aged 4-8 years，of American Society of Anesthesiologists(ASA) physical statusⅡ or Ⅲ，scheduled for elective repair of ventricular septal defect under CPB, were randomly divided into 2 groups(n=40 each) using a random number table：control group(group C) and dexmedetomidine groups(group D). Anesthesia was induced with midazolam 0.15 mg/kg, sufentanyl 1 μg/kg, etomidate 0.3 mg/kg and rocuronium 0.6 mg/kg．The patients were endotracheally intubated and mechanically ventilated．In group D， dexmedetomidine 1 μg/kg(initial dose) 〖JP2〗was given over more than 10 min，followed by continuous infusion at 0.5 μg·kg-1·〖JP〗h-1, until the end of operation．The equal volume of normal saline was given instead of dexmedetomidine in group C． Before CPB(T1)， 30 min after begainning of CPB(T2), at the immiadiately of the end of CPB(T3), at 24 and 72 h after the end of CPB(T4, T5)， blood samples were collected for determination of plasma tumor necrosis factor-a(TNF-α)， interleukin-6(IL-6), interferon-γ（IFN-γ） and interleukin-4(IL-4) concentrations. 
Results The plasma IL-4 levels showed ascending and descending trends, and were decreased in group D compared to that in group C at T2-4, there were statistically difference between groups, time points and their interaction(P＜0.05). The plasma IFN-γ and IFN-γ/ IL-4 levels showed descending and ascending trends in two groups, and were increased in group D compared to that in group C at T2-4, there were statistically difference between groups, time points and their interaction(P＜0.05). The plasma TNF-α and IL-6 levels show ascending and descending trends in two groups, and were decreased in group D compared to that in group C at T2-4, there were statistically difference between groups, time points and their interaction(P＜0.05). 
Conclusion Dexmedetomidine can inhibit immunomodulatory effects shifting the Th1/Th2 balance toward Th2 induced by CPB, alleviate perioperative inflammatory response.

Key words: dexmedetomidine, cardiopulmonary bypass, Th1/Th2 balance