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Background: Delirium is frequent in patients in the ICU, but its association with the outcome of weaning from mechanical ventilation has not been assessed. Circadian rhythm alteration may favor delirium. In the current study, we assessed the impact of delirium during weaning and associated alterations in the circadian rhythm of melatonin excretion.Methods: This was a substudy of 70 participants of the B-type Natriuretic Peptide for the Fluid Management of Weaning trial, comparing two fluid management strategies during weaning. Patients with or without delirium (as assessed using the Confusion Assessment Method for the ICU) were compared in terms of baseline characteristics and outcomes and the circadian rhythm of melatonin excretion using the 24-h excretion of its urinary metabolite 6-sulfatoxymelatonin (aMT6s).Results: Among the 70 patients included, 43 (61.4%) experienced delirium at the initiation of weaning. Delirium at the initiation of weaning was associated with more alcohol consumption, a greater severity of illness, and medication use before weaning (including neuromuscular blockade, antibiotics, sedatives, and narcotics). Delirium at the initiation of weaning was associated with more respiratory and neurologic complications and a reduced probability of successful extubation (Cox multivariate model hazard ratio of successful extubation = 0.54; 95% CI, 0.30-0.95; P = .03). Delirium was also associated with a significant reduction in peak, mean, amplitude, and total values of aMT6s urinary excretion during the first 24 h of weaning (general linear model F statistic = 5.81, P = .019).Conclusions: Delirium is frequent at the initiation of ventilator weaning. It is associated with a prolongation of weaning and an alteration in the circadian rhythm of melatonin excretion.Trial Registry: ClinicalTrials.gov; No.: NCT00473148; URL: www.clinicaltrials.gov. [ABSTRACT FROM AUTHOR]