The initial cohort included 1,673 mechanically ventilated adults with ALI. However, 185 records (11.1%) did not include enough information to calculate a BMI. Hospital mortality was not significantly different between the records with a calculable BMI and those without (37.0% vs. 35.9%; p = .764). The remaining 1,488 subjects were included in the analysis ().
Included and excluded subjects in study cohort (ALI, acute lung injury; ARDS, adult respiratory distress syndrome; BMI, body mass index).
shows the unadjusted differences between patients surviving to hospital discharge and those dying during hospitalization. Nonsurvivors had a significantly lower mean BMI than survivors. Hospital survivors and nonsurvivors were similar with respect to race, ethnicity, prior ICU admissions, and medical system characteristics, including type of hospital, presence of a critical care training program, ICU medical team model, and number of hospital beds (data not shown).
Differences in unadjusted analysis between patients surviving to hospital discharge and those dying
Unadjusted associations between BMI categories and outcomes are shown in . Patients with the lowest BMIs had the highest rates of hospital and ICU mortality. There were no differences in the number of ICU complications, lengths of stay, or discharge location. illustrates the significant differences in severity of illness, demographic data, and preexisting diagnoses between the BMI groups.
Differences in unadjusted outcomes among BMI categories
Differences in potential confounders among BMI categories
In a simple logistic regression, the transformed BMI modeled as a continuous variable was associated with hospital mortality (p < .0001). This association remained after adjustment for multiple covariates (p < .0001). shows the shape of the association between BMI and mortality. The odds of mortality are highest at the lowest BMI. These odds then decrease to a nadir between 35 and 40 kg/m2. After this nadir, the odds then begin to increase but remain lower than those seen among the underweight subjects. We refit the model with imputed data for SAPS II scores and found a similar association (data not shown).
Figure 2 Risk-adjusted odds ratio for continuous body mass index (BMI) model and hospital mortality. The solid line indicates the point estimates for the adjusted odds ratios for the BMI shown on the x-axis. The gray area represents the 95% confidence interval (more ...)
The unadjusted analysis using the five BMI categories demonstrated a similar association between BMI and hospital mortality (p < .0001). After adjustment for confounders, there remains an independent association between BMI and hospital mortality (p = .0055, ). As with the model using a continuous BMI variable, the odds of mortality are higher at lower BMIs and lower at higher BMIs. Inclusion of records with imputed SAPS II data did not appreciably change the results (data not shown). Addition of diabetes to the model had minimal effect on the coefficients for the underweight (increasing by 0.5%) and overweight BMI categories (decreasing by 3.3%). For the obese and severely obese categories, the effect was larger (decreasing by 6.5% and by 8.3%, respectively) but failed to alter the coefficients for BMI at the a priori defined level for inclusion.
Risk-adjusted association between body mass index category and hospital mortality
illustrates unadjusted differences in selected processes of care among the BMI categories. For several processes of care (e.g., central venous catheters, pulmonary artery catheters, hemodialysis), there were no significant differences in the unadjusted rates (data not shown). Patients with higher BMIs more commonly received heparin prophylaxis for thromboembolic disease. The inclusion of this covariate in the categorical BMI model increased the odds of mortality for the severely obese by 10%, suggesting mediation of the “benefit” for the severely obese. There was no significant effect modification of heparin prophylaxis based upon BMI category (data not shown). Tracheostomy did not significantly change the association between hospital mortality and BMI in either the continuous or the categorical analyses, and there was no significant effect modification.
Unadjusted differences in selected processes of care among body mass index categories