The goal of this clinical trial is to learn if dexmedetomidine can reduce delirium in critically ill children, needing mechanical ventilation for more than 12 hours. The main question it aims to answer is :

• Does dexmedetomidine reduce the proportion of children presenting at least one episode of delirium during their intensive care unit (ICU) stay ? Researchers will compare dexmetomidine to midazolam, to see if the use of dexmedetomidine reduces the prevalence of delirium.

Participants will be sedated with midazolam or dexmedetomidine, according to randomization arm, and the rest of sedation is determined by the study protocol.

- In the experimental group, participants will receive dexmedetomidine as primary sedative, with opioids for analgesia in order to reach sedation within the target range evaluated with the COMFORT B scale every 4 hours.

The initial dose of dexmedetomidine continuous infusion is 0.7 µg/kg/h with increases of 0.2 µg/kg/h if needed associated to an opioid continuous infusion (sufentanil at 0.2 µg/kg/h or morphine at 40 µg/kg/h). A protocol to adjust level of sedation according to the COMFORT B scale will be followed with maximal doses of 1.4 µg/kg/h for dexmedetomidine, 1.5 µg/kg/h for sufentanil and 300 µg/kg/h for morphine. Other drugs like ketamine up to 2 mg/kg/h, propofol, other opioids, midazolam at bolus or small doses (beginning between 20 µg/kg/h and 0.5 mg/h according to age/weight) are possible and will be reported.

- In the control group, participants with invasive ventilation will receive midazolam as first line therapy with opioids in order to reach sedation within the target sedation range evaluated with the COMFORT B scale every 4 hours.

The initial dose of midazolam is 40 µg/kg/h for children <2 years, 60 µg/kg/h for children after 2 years, and 1 mg/h for children >40kg associated with sufentanil at 0.2 µg/kg/h or intravenous morphine at 40 µg/kg/h. A protocol to adjust level of sedation according to the COMFORT B scale will be followed with maximal doses of 200 µg/kg/h and 10 mg/h if >40kg for midazolam and 1.5 µg/kg/h for sufentanil or 300 mcg/kg/h for morphine. Other drugs like ketamine up to 2 mg/kg/h, propofol, other opioids, are possible and noted. In case of failure, addition of dexmedetomidine will be possible only if midazolam is used at maximal doses.

Inclusion Criteria:

• Children between 1month to 17 years and 6 months
• Sedation and analgesia for mechanical ventilation for more than 12 hours
• Participants covered by or entitled to French social security
• Informed consent, dated and signed, from the participant's legal representative(s). (The emergency inclusion procedure will be used if the legal representative(s) cannot be contacted. When the legal representative(s) and the patient are available and/or able to consent, informed consent from the participant's legal representative(s)
• Ability for participant to comply with the requirements of the study
• Receiving sedation by benzodiazepines for less than 6 hours at the time of inclusion

Exclusion Criteria:

• Patients under guardianship, curatorship or legal protection
• Pregnant or breastfeeding woman (positive pregnancy test for women of childbearing age)
• High-grade heart conduction disorder (Second or third-degree atrioventricular block)
• Status epilepticus
• Intracranial hypertension
• Sedation for more than 6 hours by benzodiazepines at the time of inclusion
• Need for paralytic medication (neuromuscular blocker) at admission to PICU
• Following cardiac surgery
• Palliative care on admission
• Significant bradycardia
• Severe hepatic dysfunction (CHILD score C or worse) due to the hepatic metabolism of dexmedetomidine
• Patient already enrolled in the study previously
• Expected duration of invasive mechanical ventilation inferior to 12 hours
• Hospitalized in a pediatric intensive care unit without computerized prescription software