Delirium during intensive care unit (ICU) stay is frequent and associated with significant morbidity, mortality and healthcare related costs. International guidelines suggest its prevention. However, curative treatment remains unclearly established. Despite a non-unequivocal literature, haloperidol is the first line recommended neuroleptic. Dexmedetomidine, an alpha2-adrenergic receptors agonist has shown its efficiency in the treatment of delirium in intubated patients but also in its prevention. Dexmedetomidine represents a widely used alternative to haloperidol. Only few studies have compared the efficacy of dexmedetomidine in non-intubated ICU patients as a first line curative treatment of delirium. Main objective of 4D trial is to demonstrate that dexmedetomidine decreases delirium duration compared to placebo.