Upon ICU admission, remimazolam besylate infusion will initiated for light sedation maintenance at a dose range of 0.1 to1.0mg/Kg/h. The target sedation depth is defined as a RASS score of 0 to -2. The RASS score will evaluated and recorded hourly, and the infusion rate of remimazolam besylate will titrated( increased or decreased) accordingly to maintain the patient within the target sedation range. For multimodal analgesia, remifentanil will co-administered continuously at a dose of 0.1 to 0.5ug/Kg/min to ensure a CPOT score of 2 or less.

Upon ICU admission, dexmedetomidine infusion will initiated for light sedation maintenance at a dose range of 0.2 to0.7ug/Kg/h. The target sedation depth is defined as a RASS score of 0 to -2. The RASS score will evaluated and recorded hourly, and the infusion rate of dexmedetomidine will titrated( increased or decreased) accordingly to maintain the patient within the target sedation range. For multimodal analgesia, remifentanil will co-administered continuously at a dose of 0.1 to 0.5ug/Kg/min to ensure a CPOT score of 2 or less.

patients will be sedated with remimazolam via intravenous pump on the day of ICU admission, the dosage will titrate with the depth of sedation evaluated with RASS. If the target sedation depth could not to achieved within the prespecified maximum dose range of remimazolam, a standardized rescue sedation protocol will intiated as follows: an intravenous bolus of propofol at 0.2mg/kg will administered. Sedation depth will evaluated 5 minutes post-injection. The identical dose of propofol bolus will repeated until the desired sedation depth is reached. Continuous propofol infusion will initiated after three consecutive bolus injections of propofol. The continuous propofol infusion rate will titrated within the range of 0.2 to 1.0 mg/kg/h until the satisfactory target sedation depth is established. At the subsequent hourly sedation assessment, continuous propofol infusion will discontinued once the patient successfully maintained a stable RASS score within the 0 to -2 range.

Percentage of time in optimal sedation during sedation in the ICU, sedation depth is assessed using Richmond Agitation Sedation Scale(RASS, -5 to +4), target sedation must has a RASS score -2 to 0.

Ventilation free days(VFDs) at 28 days. VFDs=0 if subject dies within 28 days of mechaincal ventilation; VFDs=28-X if successfully liberated from ventilation X days after initiation，if the patient is reintubated within 28 days, "X" represents the days of the last extubation; VFDs=0 if the subject is mechaincally ventilated for >28 days.