Direct (pulmonary) and indirect (extrapulmonary) ARDS are distinct syndromes with important pathophysiologic differences. The goal of this study was to determine whether clinical characteristics and predictors of mortality differ between direct or indirect ARDS.
A retrospective observational cohort study of 417 ARDS patients. Each patient was classified as having direct (pneumonia or aspiration, n=250) or indirect ARDS (non-pulmonary sepsis or pancreatitis, n=167).
Patients with direct ARDS had higher lung injury scores (LIS, 3.0 vs. 2.8, p<0.001), lower SAPS II scores (51 vs. 62, p<0.001), lower APACHE II scores (27 vs. 30, p<0.001), and fewer non-pulmonary organ failures (1 vs. 2, p<0.001) compared to patients with indirect ARDS. Hospital mortality was similar (28 vs. 31%). In patients with direct ARDS, age (OR 1.29 per 10 yrs, p=0.01, p for interaction=0.03), LIS (OR 2.29 per point, p=0.001, p for interaction=0.058), and number of non-pulmonary organ failures (OR 1.67, p=0.01) were independent risk factors for increased hospital mortality, while pre-existing DM was an independent risk factor for reduced hospital mortality (OR 0.47, p=0.04, p for interaction=0.02). In indirect ARDS, only number of organ failures was an independent predictor of mortality (OR 2.08, p<0.001).
Despite lower severity of illness and fewer organ failures, patients with direct ARDS had similar mortality to patients with indirect ARDS. Factors previously associated with mortality during ARDS were only associated with mortality in direct ARDS. These findings suggest that direct and indirect ARDS have distinct features that may differentially affect risk prediction and clinical outcomes.