The objective of this randomized clinical trial is to assess the safety and effectiveness of post-adenotonsillectomy analgesia with individualized opioid analgesia regime in children.

Patients will be randomly assigned to individualized opioid analgesia regimen group or conventional opioid analgesia regimen group.

Intravenous 1μg/kg fentanyl is administered as a test in patients with spontaneous breath after sevoflurane induction. A positive result is defined as a decrease of more than 50% in spontaneous respiratory rate (rate after fentanyl test versus rate before fentanyl test). Otherwise it is defined as a negative result.

All the patients are given regular paracetamol and ibuprofen after surgery. The addition of rescue oral morphine is given as needed. The dose of oral morphine is determined according to the results of fentanyl test in the individualized group, while the patients in the conventional group are given the routine dose. The Faces Scale is used to assess analgesic effectiveness by parents from postoperative day 1 to day 10. If the Faces Scale score > 6, the child receive rescue oral morphine analgesia.

At the preoperative appointment, parents are provided with a pulse oximeter including oxygen saturation sensors to take home. The oxygen saturation per night during sleep will be monitored using the pulse oximeter by parents from postoperative Day 1 to Day 10. The research team will instruct parents on how to properly apply the oximeter to the child and repeat these measurements. The primary outcome measure is desaturation events during sleep per night.

Inclusion Criteria:

• Children, ASA physical status I or II, who is scheduled to undergo adenotonsillectomy will be recruited.

Exclusion Criteria:

• Children with craniofacial deformities, neuromuscular diseases, mental retardation, body mass index > 30kg/m2 , or a recent history of opioid use are excluded.