Emergence agitation (EA) is a dissociated state of consciousness in which the child is inconsolable, irritable, uncooperative, typically thrashing, crying, moaning, or incoherent. Although usually transient, it is not only an extremely distressing event for children, parents, and staff, but may also result in self-injury or the need for restraint. The prevalence in children appears to be between 10% and 80% depending upon the definition and measurement tools used and is more frequently observed in the pre-school age-group. A clear correlation has been found between EA and negative postoperative behavioral changes, including anxiety, eating and sleeping disorders, enuresis, fear of darkness, that may persist for an extended period of time affecting emotional and cognitive development.Currently, numerous interventions have been studied to manage EA after surgery. Among them, dexmedetomidine (DEX) as a kind of highly selective α2 adrenergic receptor agonist has been done to reduce EA in children. Unfortunately, no studies examined posthospitalization negative behaviour changes.

The high incidence of EA and postoperative behavioural changes has encouraged paediatric anaesthetists and researchers to study methods to improve the perioperative care of children. Dexmedetomidine is a selective alpha-2 receptor agonist with properties that make it attractive to pediatric use. It provides sedation and anxiolysis acting on these receptors in the locus ceruleus of the pons. It also exerts dose-dependent moderate primary analgesic effects through activation of alpha-2 adrenoreceptors in the dorsal spinal horn causing a subsequent decrease in substance P release. The study aims to explore whether a single low-dose dexmedetomidine in the perioperative period has a preventive effect on EA in children, and through short-term and long-term follow-up, to investigate the effect on post-hospitalization behavioural changes.

Inclusion Criteria:

1. Age between 2-7 years old
2. American Society of Anesthesiologists（ASA） score of I or II
3. Selective ear, nose, and throat surgery under general anaesthesia with sevoflurane

Exclusion Criteria:

1. Emergency surgery
2. were intubated before induction of anaesthesia or not planned for extubation after anaesthesia
3. had critical illness with haemodynamic instability, active bleeding, cancer, cardiac diseases including arrhythmias, malignant hyperthermia
4. intellectual disability, or neurological illness with agitation-like symptoms
5. weighed more than 50 kg
6. were allergic to dexmedetomidine
7. The use of sedative or analgesic medications before surgery

Individual Participant Data(IPD) will be available when this trial is finished and the article have been published