Rheological properties of excised rabbit lung stiffened by repeated hyperinflation

Two rheological properties, stress adpatation (S) and hysteresis loop area (H), were studied in normal fresh rabbit lungs and in lungs stiffened by 3 hr repeated inflation at zero end-expiratory pressure. S was measured on the deflation limb at various volume levels ranging from 100 to 25% total lung capacity (TLC). H was obtained in another group of lungs before and after stiffening, for tidal volumes ranging from 5 to 30% TLC, at end-expiratory levels from 25 to 70% TLC. In stiff lung, S after deflation from TLC was biphasic (initial pressure rise, then a fall) whenever the starting pressure exceeded approximately 10 cmH2O. At low lung volumes (25% TLC) stress rose monotonically toward 10 cmH2O. Only monontonic adaptation has previously been reported for normal lung. Tidal H in stiff lungs was increased at all volume levels, occupying roughly 20% of a bounding pressure-volume rectangle, resembling that of normal lungs near TLC but twice that of normal loops at lower volumes. The results suggest that above an equilibrium or transition pressure around 10 cmH2O the alveolar lining in stiff lung may have film properties more resembling those of a viscous liquid, becoming solidlike below this pressure. Similar viscous liquid properties appear to exist on the inflation limb of normal lungs, and near TLC on their deflation limb.