The main objective is to assess the feasability of dexmedetomidine infusion in terms of efficacy and safety (especially cardiovascular tolerance) in brain-injured patients admitted to intensive care unit and requiring sedation and mechanical ventilation for a predictable duration greater than or equal to 48 hours. Secondary objectives include the study of hemodynamic parameters evolution, dose-response relationship, blood (+/- cerebrospinal fluid) drug concentration, opioates and co-hypnotic consumption.