The 4D trial is an investigator-initiated, prospective, multicenter, randomized, double-blinded, two-arm trial, randomizing 300 non-intubated ICU patients with diagnosis of agitated delirium to receive dexmedetomidine or placebo as a cure. The primary outcome measure is a composite of duration of agitation or delirium or the use of intubation with deep sedation and mechanical ventilation. Secondary outcomes include mortalities at 7 and 28 days, ICU length of stay and occurrence of adverse effects. The sample size will allow the detection of a 50% decrease of agitation duration (120 minutes), of an absolute reduction of delirium duration (1 day) and of a 50% relative decrease of intubation and mechanical ventilation, with a type 1 error rate of 1.8 % (error risk inflation due to components of composite) and power of 90 %, assuming a 15 % incidence of intubation and mechanical ventilation requirements, an agitation duration of 240 minutes and a delirium duration of 3 days. One hundred and 10 patients by group will be needed. A intermediate analysis is scheduled and requires the inclusion of 150 patients in each group.