Furthermore, only the radiographic and hypoxemia scores were calculated at the LIS at the second study point, as all the patients were self-ventilating. This influence on the statistical relationship, however, is unlikely to be important as the cross-sectional nature of CT provides a different and more precise depiction of the extent of pulmonary abnormalities compared with two-dimensional chest radiographs.
As ARDS evolves in survivors, functional recovery is most rapid in the first 6 months and reaches a plateau 1 year after the onset. A reduced carbon monoxide diffusing capacity is the most common abnormality of lung function, being reported in 7 of 16 nonsmokers at 1 year. Reductions of carbon monoxide diffusing capacity are often still apparent even when corrected for alveolar volume, suggesting that the most important defect is a loss of pulmonary capillary surface area. Reading here
Severe exercise-induced desaturation is uncommon 6 or more months after ARDS, in contrast with marked decreases in oxygenation observed on exertion in patients with established idiopathic pulmonary fibrosis. In our own study, CT evidence of interstitial fibrosis was generally limited and unlikely to account for the entire functional deficit. Few studies have addressed the question of whether poor post-ARDS lung function can be predicted by features of the patient or the patient’s illness. The patient’s age and smoking history have not proven to be accurate predictors of subsequent lung function but ample experimental evidence suggests that even brief exposure to high concentrations of oxygen causes lung injury and can contribute to the development of fibrosis. Some studies have correlated prolonged exposure to an FIO2 above 0.6 with irreversible lung injury, but others have failed to find an association. We were unable to find any obvious association between the degree of residual abnormality on СТ/LIS in convalescence and the extent to which a high FIO2 was required to support the patient in the acute phase of the syndrome. The high percentage of abnormalities detected at follow-up in our study group may reflect the relatively short time interval from diagnosis to the follow-up scan (mean=96 days; range, 17 to 187 days) and the CT study of ARDS survivors over a longer period is warranted.
We have documented the changes in CT appearances observed in survivors following severe ARDS and correlated them with a severity score incorporating physiologic variables. The implications of these observations for management and how modifications in therapy affect outcome in patients with ARDS remain to be determined, but with the more aggressive treatment and imaging of critically ill patients, it seems appropriate to become acquainted with the natural history at clinical, imaging, and pathologic levels. The increasing use of CT in the evaluation of patients with ARDS will allow further insights into the natural history of the disease and its long-term effects on the lung.