Postoperative pain following tonsillectomy or adeno-tonsillectomy in children is severe and sometimes last for days. Sufentanil is used by epidural route for postoperative analgesia in children. There are few reports about its use for postoperative analgesia by continuous intravenous infusion. Its complications include , hypoxia, sedation, pruritus ,nausea and/or vomiting.

The investigators design this protocol to test efficacy of analgesia of three different rates of continuous intravenous sufentanil infusion for postoperative pain management following coblation tonsillectomy or adeno-tonsillectomy in children. The investigators also compare incidence rate of complications for three groups.

Inclusion Criteria:

• American Society of Anesthesiologists grade I or II children aged 5-8 years undergoing coblation tonsillectomy or adeno-tonsillectomy as in-patient

Exclusion Criteria:

• Children with a history of adverse reaction to opioids, long-term use of opioid medications, and history of chronic pain.
• Psychiatric illness, kidney failure, hypotension, atrioventricular block, myasthenia gravis etc.