Computed tomography was used to determine the vertical gradient of density in the peripheral lung tissue of 12 patients with histologically proved fibrosing alveolitis and 12 patients with chronic bronchitis and evidence of pulmonary emphysema on the computed tomograms. Measurements were made at total lung capacity and at residual volume and compared with similar measurements from 12 normal subjects reported in a previous study. At residual volume the mean peripheral tissue density in the emphysematous lungs was 0.081 kg/l compared with 0.426 kg/l in the fibrotic lungs and 0.323 kg/l in the normal lungs. The observed densities in the three groups were compared with those in a theoretical model predicting the vertical changes of lung density caused by gravitational effects that would be found in lungs with differing compliance. The emphysematous lungs showed a much greater increase of density with descent down the lung than that predicted for normal lungs, and the results were explicable by an increase in compliance. The fibrotic lungs showed considerably less change in density than expected, implying loss of compliance. It is suggested that local changes of compliance are important determinants of vertical density gradients in diseased lungs.
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