Objective: To determine the association between body mass index (BMI) and hospital mortality for critically ill adults.
Design: Retrospective cohort study.
Setting: One-hundred six intensive care units (ICUs) in 84 hospitals.
Patients: Mechanically ventilated adults (n=1,488) with acute lung injury (ALI) included in the Project IMPACT database between December 1995 and September 2001.
Measurements and main results: Over half of the cohort had a BMI above the normal range. Unadjusted analyses showed that BMI was higher among subjects who survived to hospital discharge vs. those who did not (p<.0001). ICU and hospital mortality rates were lower in higher BMI categories. After risk-adjustment, BMI was independently associated with hospital mortality (p<.0001) when modeled as a continuous variable. The adjusted odds were highest at the lowest BMIs and then declined to a minimum between 35 and 40 kg/m2. Odds increased after the nadir but remained below those seen at low BMIs. With use of a categorical designation, BMI was also independently associated with hospital mortality (p=.0055). The adjusted odds were highest for the underweight BMI group (adjusted odds ratio [OR], 1.94; 95% confidence interval [CI], 1.05-3.60) relative to the normal BMI group. As in the analysis using the continuous BMI variable, the odds of hospital mortality were decreased for the groups with higher BMIs (overweight adjusted OR, 0.72; 95% CI, 0.51-1.02; obese adjusted OR, 0.67; 95% CI, 0.46-0.97; severely obese adjusted OR, 0.78; 95% CI, 0.44-1.38). Differences in the use of heparin prophylaxis mediated some of the protective effect of severe obesity.
Conclusions: BMI was associated with risk-adjusted hospital mortality among mechanically ventilated adults with ALI. Lower BMIs were associated with higher odds of death, whereas overweight and obese BMIs were associated with lower odds.