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BACKGROUND The effect of dexmedetomidine on the potency of bupivacaine for transversus abdominis plane (TAP) block in pediatric patients has not been investigated. STUDY OBJECTIVE The primary objective of this study was to assess the effectiveness of dexmedetomidine to decrease the concentration of bupivacaine needed for analgesia for ultrasound-guided TAP block in a pediatric patient undergoing hernia repair or hydrocelectomy. DESIGN This is a randomized, double-blind, up-down, dose-finding study. SETTING Operating room. PATIENTS Sixty American Society of Anesthesiologists I and II patients aged 1-4 years scheduled for elective unilateral herniorrhaphy or hydrocelectomy. INTERVENTIONS Patients were randomly assigned to 1 of the 2 groups: group B (0.125% bupivacaine, 1mL/kg) TAP block or group BD (0.125% bupivacaine plus 2μg/kg dexmedetomidine, 1mL/kg) TAP block. MEASUREMENTS The response of each child was observed for 60 seconds after skin incision and evaluated as 'unsuccessful' when skin incision caused a change in hemodynamic parameters (heart rate and mean blood pressure) 20% more than the preincision values. If the response was determined to be unsuccessful, the concentration of bupivacaine administrated to the next patient was increased by 0.02%. If it was successful, the concentration of bupivacaine administrated to the next patient was decreased by 0.02%. RESULTS The minimum local anesthetic concentration of bupivacaine was 0.0839% (0.0137) in the B group and 0.0550% (0.0169) in the BD group. The difference was statistically significant (t=7.165, P=.0001). The total postoperative analgesic dosage of morphine was significantly higher in the B group (0.17±0.04 mg/kg) than the BD group (0.11±0.02 mg/kg, P=.001). CONCLUSIONS The addition of 2μg/kg of dexmedetomidine reduced the minimum local anesthetic concentration of bupivacaine used for a TAP block and improved postoperative analgesia in children undergoing surgery for inguinal hernia repair or hydrocelectomy.