The Varying Impact of Frailty With Increasing Body Mass Index on Survival Up To 3 Years After ICU Admission: A Retrospective Registry-Based Study.

Objectives: Frailty is associated with poorer outcomes in critical illness, but it is unclear if this relationship is consistent across different body mass index (BMI) levels.

Design: A retrospective multicentric registry-based observational study using the Australia New Zealand Intensive Care Society Adult Patient Database.

Setting: Criticallly ill patients admitted to 1170 ICUs between January 1, 2018, and March 31, 2022.

Patients: All adults aged 16 years and older with a documented Clinical Frailty Scale (CFS) and BMI.

Interventions: None.

Measurements and main results: The primary outcome was survival up to 3 years following ICU admission. We used Cox proportional hazards models, linear and nonlinear regression models to investigate the association between frailty (defined as CFS, 5-8), in reference to those without frailty, and the mortality risk up to 3 years, and whether this association varied with BMI, after adjusting for key confounders. We included 282,586 patients, of whom 49,070 (17.4%) were frail. Frailty was most prevalent in patients with BMI less than 18.5 kg/m2 (34.8%), became less frequent as BMI increased, and more prevalent again in BMI greater than or equal to 40 kg/m² (18.8%). Overall, frailty was associated with lower 3-year survival (47.5% vs. 82.2%) and increased mortality (hazard ratio, 1.67; 95% CI, 1.62-1.73). However, the association between frailty and survival was not uniform. The concomitant presence of frailty was associated with progressively larger increases in mortality as BMI categories increased beyond the reference group of 18.5-24.9 kg/m2. There was no effect of BMI on the relationship between frailty and mortality for BMI less than 18.5 kg/m2. This relationship was consistent in multiple sensitivity analyses.

Conclusions: The association between frailty and outcomes after critical illness differed across BMI categories with a larger increase in the risk of mortality noted at higher BMI levels. Our findings may have implications for managing concurrent obesity, frailty, and critical illness.