Characteristics of the study population

ARDS, acute respiratory distress syndrome; APACHE, Acute Physiology and Chronic Health Evaluation; a. APACHE III physiology score was calculated with all components on the day of ICU admission; b. Pneumonia, aspiration, pulmonary contusions, or sepsis from lower pulmonary source were categorized as pulmonary injury. Sepsis from an extrapulmonary source, trauma without pulmonary contusions, and multiple transfusions were categorized as extrapulmonary injury. Patients with both pulmonary and extrapulmonary injuries were considered to have pulmonary injury; c. Patient received ≥ 300 mg of prednisone or its equivalent within 21 days or ≥ 15 mg prednisone a day or its equivalent prior to ICU admission.