In the MLS subgroup (RASS 0/-3), sedation and analgesia will be drived by propofol and opiates. Once the goals of sedation achieved with usual hypnotic, dexmedetomidine will be initiated at a dose of 0.5 μg/kg/h, and then gradually increased or decreased by increments of 0.1 μ g / kg / h all hours to a maximum dose of 1.4 μg/kg/h, depending on the target sedation achievement. Propofol will be discontinued at study drug initiation. Dexmedetomidine infusion will bemaintained at least during 48 hours, evaluation of the efficacy and safety will be continuous during this period.