In the DS subgroup, "deep" sedation (RASS -4/-5) will be achieved by propofol infusion. If sedation level are not achieved with the maximum recommended propofol dose (5mg/kg/h), midazolam will be started to reach target sedation level with a maximum dose of 0.15mg/kg/h. Once the goals of sedation will be achieved a dexmedetomidine infusion will be initiated at a dose of 0.5 μg/kg/h and then gradually titrated by increase of 0.1 μg/kg/h every hour up to the maximum patient-tolerated dose (maximum dose of 1.4 μg/kg/h). Conventional hypnotic doses will be adjusted according to the sedation target. In parallel to dexmedetomidine titration, midazolam first and then propofol will be decreased. The study drug will be maintained at the maximum patient-tolerated dose for a total of 48 hours during which the efficacy and safety will be assessed continuously. After this period, dexmedetomidine will be discontinued. According to the patient's neurological evolution, either sedation objectives will remain identical (RASS -4/-5), or sedation will be decreased. Conventional hypnotic will be used again to achieve these objectives.