Acute Lung Injury/therapy; Continuous Positive Airway Pressure; Lung/physiopathology; Pulmonary Atelectasis/prevention & control; Pulmonary Gas Exchange; Pulmonary Ventilation; Respiration, Artificial/methods; Respiratory Distress Syndrome/therapy; Acute Lung Injury/complications; Acute Lung Injury/diagnostic imaging; Acute Lung Injury/physiopathology; Adult; Aged; Aged, 80 and over; Female; Hemodynamics; Humans; Imaging, Three-Dimensional; Intensive Care Units; Lung/diagnostic imaging; Lung Volume Measurements; Male; Middle Aged; Pulmonary Atelectasis/diagnostic imaging; Pulmonary Atelectasis/etiology; Pulmonary Atelectasis/physiopathology; Radiographic Image Interpretation, Computer-Assisted; Respiratory Distress Syndrome/complications; Respiratory Distress Syndrome/diagnostic imaging; Respiratory Distress Syndrome/physiopathology; Retrospective Studies; Time Factors; Tomography, Spiral Computed; Treatment Outcome