Mechanics of edematous lungs

Using the parenchymal marker technique, we measured pressure (P)-volume (P- V) curves of regions with volumes of similar to1 cm(3) in the dependent cau dal lobes of oleic acid-injured dog lungs, during a very slow inflation fro m P = 0 to P = 30 cmH(2)O. The regional P-V curves are strongly sigmoidal. Regional volume, as a fraction of volume at total lung capacity, remains co nstant at 0.4-0.5 for airway P values from 0 to similar to 20 cmH(2)O and t hen increases rapidly, but continuously, to 1 at P = similar to 25 cmH(2)O. A model of parenchymal mechanics was modified to include the effects of el evated surface tension and fluid in the alveolar spaces. P-V curves calcula ted from the model are similar to the measured P-V curves. At lower lung vo lumes, P increases rapidly with lung volume as the air-fluid interface pene trates the mouth of the alveolus. At a value of P = similar to 20 cmH(2)O, the air-fluid interface is inside the alveolus and the lung is compliant, l ike an air-filled lung with constant surface tension. We conclude that the properties of the P-V curve of edematous lungs, particularly the knee in th e P-V curve, are the result of the mechanics of parenchyma with constant su rface tension and partially fluid-filled alveoli, not the result of abrupt opening of airways or atelectatic parenchyma.