Dynamic behavior of lung parenchyma in shear

Dynamic shear properties of excised rabbit lungs were studied by measuring creep deformation after application of a step indentation force to the pleural surfaces by a rigid cylindrical punch. The punch diameter was 9.5 mm, and punch forces were 2, 4, and 6 g. Measurements were made at lung volumes of 40, 60, and 90% of the total lung capacity before and after lavage with 3-dimethyl siloxane, which provided a constant surface tension of 16 dyn/cm at the alveolar surfaces. A power-law model was fitted to creep data and then transformed into the frequency (f) domain by using Laplace transforms. The optimum model parameters were used to calculate shear elastance (E(mu)), shear resistance (R(mu)), and shear hysteresivity (2 pi fR(mu)/E(mu)) between 0.01 and 2.0 Hz. It was found that E(mu) slightly increased and R(mu) decreased nearly hyperbolically with increasing f, both decreased with increasing indentation force, and both increased with increasing mean lung volume. Shear hysteresivity decreased sharply from 0.01 to 0.25 Hz and then assumed a nearly steady value that was an order of magnitude lower than the value reported previously for uniformly oscillated lungs. Changes in E(mu) and R(mu) after lavage were correlated with changes in transpulmonary pressure and not with changes in surface film properties. These results suggest that in the breathing range of frequencies 1) the energy loss of lung parenchyma is a much smaller fraction of the stored elastic energy in shear than in uniformly oscillated lungs and 2) transpulmonary pressure, not dynamic properties of surface film, is the primary determinant of lung dynamic properties in shear.