The aim of this prospective randomized study is to compare the safety and efficacy of two strategies for sedation of critically ill patients submitted to mechanical ventilation: continuous intravenous infusion of dexmedetomidine versus intravenous bolus of midazolam. The two groups will be compared as to the incidence of of delirium, evaluated with the Confusion Assessment Method for ICU (CAM-ICU) and the percentage of time within the target sedation using the Richmond Agitation-Sedation Scale (RAAS).Eligible patients will be 18 years or older intubated and mechanically ventilated for less than 48 hours prior to start of study drug and anticipated ventilation duration of at least 48 hours. Calculated sample size is 146 patients (73 patients in each group)

Inclusion Criteria:

• Adult mechanically ventilated, critically ill patients

Exclusion Criteria:

• pregnancy or lactation, severe liver disease, hearth rate less than 50, hypotension despite volemic repletion and vasoactive drug