EFFECTS OF PULMONARY FIBROSIS ON THE DISTRIBUTION OF EDEMA - COMPUTEDTOMOGRAPHIC SCANNING AND MORPHOLOGY

The pulmonary interstitium acts as an important safety factor against alveolar flooding. To test the hypothesis that in advanced fibrosis, e dema is redistributed away from a less compliant interstitium to flood alveoli, we induced severe left lung fibrosis in six dogs with radiat ion and intratracheal bleomycin. Twenty-four months later, edema was i nduced by infusing 20% body weight lactated Ringer's solution over 30 min, preceded and followed by computed tomography (CT) scanning. Lower robes were frozen, and samples were taken for extravascular lung wate r measurements (QwI/dQI), regional blood volume, and light microscopic grading of interstitial and alveolar edema. The total volumes of the control and fibrotic lungs were 800 +/- 63 and 45 +/- 10 ml(SE), respe ctively, indicative of severe fibrosis. Before edema, the fibrotic car inal and basal slices had CT densities 3.5 and 2.2 times greater than respective control slices. After edema, the densities of all control l ung slices rose 2.5 times and that of fibrotic carinal and basal slice s rose 1.5 times. Edema significantly accentuated the small gravity-de pendent gradient in CT density of control lungs, but it had minimal ef fect on this gradient in fibrotic lungs. The QwI/dQI for control and f ibrotic lower lobes were 8.7 +/- 0.8 and 6.8 +/- 0.7 g H2O/g dry lung, respectively, but the amounts of water per lung volume were similar, and there was no gravity-dependent gradient in QwI/dQI or in regional blood contents. By light microscopy, we found significantly less inter stitial and more alveolar edema in the fibrotic robes. We conclude tha t in severe pulmonary fibrosis, similar amounts of water accumulate pe r lung volume as in controls, and that there is predominant alveolar f looding over interstitial edema. We also conclude that the gravity-dep endent gradients in CT densities postedema in the control lungs are no t accounted for by edema fluid or congestion, but probably by atelecta sis.