Relationship between body mass index and risk of delirium in an intensive care unit

Objective: Delirium in the intensive care unit (ICU) is a common and critical condition that leads to poor prognosis in older patients, but the association between body mass index (BMI) and the incidence of delirium remains unclear. Methods: We retrospectively analyzed 5,622 patients admitted to the ICU of a tertiary referral hospital between 2013 and 2022. We collected sociodemographic data, vital signs, laboratory results, and delirium scale scores. We subdivided the patients into four categories: underweight (<18.5 kg/m²), normal weight (18.5–22.9 kg/m²), overweight (23–24.9 kg/m²), and obese (>25 kg/m²). The primary outcome was the incidence of delirium according to the BMI categories. We performed multivariable logistic regression analysis, adjusted for sex, age, past smoking and alcohol history, benzodiazepine use, and laboratory abnormalities. Results: Among the 5,622 patients in the ICU (mean age, 72.9 years; male, 60.1%; mean BMI, 24.2 kg/m²⁾, the incidence of delirium was 19.0% (1,069 patients). The mean modified incidence of delirium was higher among underweight patients (odds ratio [OR]=1.51, confidence interval [CI]=1.07–2.12, p = 0.02) than among normal-weight patients. Overweight and obese status were not independently associated with delirium (OR=0.90, CI=0.70–1.17, p = 0.43; OR= 0.97; CI=0.77–1.21, p = 0.78, respectively). The area under the receiver-operating characteristic curve of the multivariable logistic regression model was 0.71 (95% CI=0.69–0.73). Conclusions: Underweight status is an independent risk factor for delirium in the ICU. Additional caution is required when evaluating underweight patients for delirium. Obese or overweight status are not associated with delirium, providing evidence for the obesity paradox.