The aim of this study is to determine the effective nalbuphine dose in 90% of intubated patients in the ICU, including the continuous infusion dose and a bolus dose during moving to the lateral decubitus position. Pain will assessed using a Behavioural Pain Scale (BPS) requiring a score of 3-4.

The continuous nalbuphine infusion dose starts at 100ug/kg/h, if the BPS scores reaches 3-4, the continuous infusion dose would be decreased by 20 ug/kg/h for next patients. If not, increased by 20 ug/kg/h.

The nalbuphine bolus is injected 5 min before turning the patients and the dose (starting at 0) is increased by 0.05 mg/kg, prior to each subsequent turn to lateral decubitus until a BPS score of 3-4 is obtained.

Inclusion Criteria:

• Mechanical invasive ventilation
• Sedated only with nalbuphine

Exclusion Criteria:

• Pregnant or breast-feeding woman
• Age below 18 or over 80 years
• An indication for deep sedation (e.g.the initial stage of septic shock, acute brain injury or acute respiratory distress syndrome)
• Renal dialysis
• The inability to assess pain by the BPS scale (e.g. paralysis)
• BMI less than 18 or more than 35
• Preadmission use of opioid analgesic for chronic pain
• Severe hepatic failure
• State of consciousness with impossibility to use self-assessment scale; monoamine oxidase inhibitors