There are some limitations to our study that should be mentioned. First, the small sample size of our study. This sample size does not allow discrimination between responders and non-responders nor a categorization of clinical phenotype based on chest radiographs, but this should be the subject of further research. Second, patients were paralyzed throughout the study, thus prohibiting spontaneous breathing and mucus clearance by the patient itself. We choose to do so to eliminate any confounding effect of spontaneous breathing on the Data are expressed as mean ± standard deviations. Aa-DO2 = alveolo-arterial oxygen gradient; OI = oxygenation index; VI = ventilation index; V D /V T = dead-space/tidal volume ratio.