Cohort Study Participants

We performed a secondary analysis of data collected from adult intubated patients with ARDS undergoing bronchoscopy for suspected VAP. Bronchoscopy for diagnosis of clinically suspected VAP is standard practice at our institution, and these patients represented a convenience sample of ARDS patients cared for in the intensive care units of our institution in 2008 and 2009. ARDS was defined as: 1) PaO₂/FiO₂ <300 mmHg while receiving ≥5 cmH₂O positive end-expiratory pressure, 2) diffuse parenchymal infiltrates, 3) pulmonary arterial wedge pressure <18 mmHg or lack of clinical evidence of congestive heart failure, and 4) no other obvious diagnosis explaining these findings. ARDS severity was based on the Berlin consensus definition as mild (200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg) or moderate/severe (PaO₂/FiO₂ ≤200 mmHg) [13]. Clinical criteria for suspected VAP, based upon American Thoracic Society/Infectious Disease Society of America guidelines [14], included mechanical ventilation ≥48 hours, new or progressive pulmonary infiltrates on imaging, and ≥1 of the following: fever, leukocytosis or leukopenia, increased purulent endotracheal secretions, and no antibiotic changes for 72 hours. Control samples were obtained from healthy, non-smoking volunteers.

Written informed consent was obtained from participants or legal next of kin. Study procedures were approved by the Institutional Review Board at the University of Washington.