Emergence delirium is a common complication in children after anesthesia. The incidence of emergence delirium is reported upto 50%. Prevention of emergence delirium in children is important not only for the patient safety but also for the satisfaction of the parents. Midazolam is the most commonly used medications for prevention of emergence delirium. However, it might lead to delayed awakening from anesthesia and respiratory depression. In this study, the investigators will evaluate whether dexmedetomidine can be effectively and safely administered for prevention of emergence delirium in children compared to midazolam.

In this study, seventy patients scheduled for elective tonsillectomy aged from 24 months to 12 years old Anesthesiologists physical status (ASA) class 1 and 2 will be enrolled. The sample size was calculated based on the former study with a type 1 error estimate of 0.05 at 80% power. The proportion in dexmedetomidine group (group D) was assumed to be 0.4 and the proportion in midazolam group (group M) was 0.1. Considering the drop-out rate of 10%, seventy patients in total were needed.The patients will randomly be allocated into the group D or the group M. The group D will receive dexmedetomidine0.3mcg/kg mixed with 0.9% normal saline, 10mL in total. The group M will receive 0.03mg/kg of midazolam mixed with 0.9% normal saline, 10mL in total. Randomization and drug administration will be done by a single anesthesiologist. Assessment of the patient will be done by another anesthesiologist blinded by the group. Five minutes before the end of the surgery, the study drug will be slowly infused for 5 minutes. In the postanesthesia circuit unit (PACU), presence of emergence delirium will be assessed. The emergence delirium will be assessed by the four-point agitation scale. (1 = calm, 2 = not calm but gould be easily calmed, 3 = not easily calmed, moderately agitated or restless, and 4 = combative, exited or disoriented.) Patients with score 3 and 4 will be diagnosed as emergence delirium and treated with intravenous fentanyl 1mcg/kg. Severity of emergence delirium will be assessed with the pediatric anesthesia emergence delirium scale after 10 mins on arrival of PACU. Pain wil be scored by 5-point scale (0 = no pain, 5 = severe pain ever). Time from discontinuation of anesthetic gas to extubate will be recorded. The data will be compared between the groups by the independent t-test or Mann-Whitney test for the continuous variables. Chi-square test or Fisher's exact test will be used for the categorial variables.

Inclusion Criteria:

• Patients undergoing elective tonsillectomy under general anesthesia
• American society of anesthesiologist physical status 1,2
• aged from 24 months to 12 years old
• obtaining written informed consent

Exclusion Criteria:

• known history of allergy to dexmedetomidine or midazolam
• renal impairment
• hepatic impairment
• long QT syndrome
• developmental disorder
• congenital disorder
• neurologic disorder
• psychogenic disorder