It seems reasonable to expect such variation in view of the considerable time period that elapsed between the onset of the disease and the first CT, although no formal relationship between this delay and the degree of ground-glass opacification could be established, probably because the radiologic features of ARDS are frequently modified according to the nature of precipitating event and the complications of supportive therapy. This seems unlikely to be the whole explanation, however, as several of our study population still had evidence of such appearances in convalescence (Table 3).
The second stage of ARDS is a direct consequence of the increased pulmonary vascular permeability that characterizes the condition. Ultrastructural changes are evident in both the alveolar epithelium and the capillary endothelium. so
The resultant pulmonary edema and the outpouring of hemorrhagic proteinaceous fluid and hyaline membranes into the air spaces classically results in radiographic consolidation with associated air bronchograms. The latter were visible in five of eight of the study group. The degree of consolidation reflects the type, severity, and duration of lung injury. In the current study, CT revealed the consolidation to be variable in distribution, even when the plain chest radiograph suggested it to be uniform. Second, although previous studies have shown greater consolidation in dependent lung regions, with more normal aeration of the nondependent lung, in the current study, consolidation tended to be concentrated at the periphery. This difference may reflect the timing of our initial scans, which were not always performed immediately following the onset of clinical lung injury, or might be consequence of the methods of ventilatory therapy used. In particular, the trend toward pressure-controlled, in-verse-ratio places the emphasis on active alveolar recruitment, and we speculate that this might obviate the well-recognized tendency toward dependent consolidation.