A chronic, third phase of ARDS occurs in survivors. The early histopathologic features of this stage may be detectable as early as 3 to 7 days postinjury; these are characterized by hyperplasia of type 2 pneumo-cytes, fibroblastic infiltration, and deposition of connective tissue. At this stage, the high permeability pulmonary edema begins to regress. Continuing physiologic impairment is probably the consequence of decreased tissue compliance, V/Q imbalance, diffusion impairment, and destruction of the mi-crovascular bed.
Radiographically, consolidation becomes less confluent and areas of lucency and interstitial or ground-glass opacification supervene as was seen on CT in our own patient group. The pathologic process responsible for the pattern of ground-glass opacification is often complex and may be due to interstitial thickening (by edema, inflammatory cells or fibrosis), partial filling of the air spaces, or a combination of the two. Areas of lucency in the lung periphery can develop at the sites of previous lung consolidation and the transformation of lung into contiguous cystic air spaces referred to as “adult bronchopulmonary dysplasia” has been described, but it is unusual. In the current study, the residuum of interstitial fibrosis was seen as a reticular pattern and linear band-like opacifications, causing so-called traction bronchiectasis of the subsegmental bronchi. read more
Data concerning sequential pathologic changes in the lungs of survivors is limited, but increased collagen content has been observed as early as 2 weeks after the initial injury. The proliferative phase of ARDS is dynamic and reports have shown histologic resolution of initial abnormalities suggesting reparative processes and active remodeling. As far as we are aware, the CT data presented in the current study represent the first attempt to describe follow-up morphologic changes in survivors with ARDS. We chose the LIS of Murray et al to add functional data at each time point to quantify as accurately as possible the degree of physiologic abnormality consequent on the structural changes. Although the LIS incorporates estimations of compliance and the degree to which PEEP is used, the hypoxemia and chest radiography scores enable its application to patients who are self-ventilating, as in convalescence. The correlation between the overall percentage abnormality of the CT scans and LIS was statistically significant. This might be thought to be due in part to mathematical coupling, as a radiographic score is incorporated into the LIS.