The study was designed to define the interaction of intravenous infusion of dexmedetomidine, an a2-adrenergic agonist, and sevoflurane in children having surgery by using the minimum alveolar concentration which can block adrenergic and cardiovascular responses to incision(MACbar) of sevoflurane as the measure of anesthetic potency.

The investigators observed the effects of different doses of dexmedetomidine on the minimum alveolar concentration of sevoflurane to blockade adrenergic response to surgical incision in 50% of children (MACBAR). And to explore the safety of use of dexmedetomidine in pediatric patients.

Inclusion Criteria:

• aged 2 to 6 yr
• American Society of anesthesiologists I or II
• scheduled to have general anesthesia and to have skin incisions on the abdomen

Exclusion Criteria:

• anemia or abnormal preoperative electrolyte concentrations
• a history of cardiovascular or central nervous system diseases
• a history of pulmonary disease
• known hepatic or renal disease
• a body weight exceeding 50% of the upper limit of ideal body weight for height
• taking drugs with cardiovascular or central nervous system effects
• use of any experimental drug within the past 30 days
• allergy to any drug that may be administered during the study
• exposure to general anesthesia within the previous 7 days
• second- or third-degree atrioventricular block
• taking drugs preoperatively that would interfere with MAC determination(e.g.,opioids or sedatives)