DYNAMIC BEHAVIOR OF LUNG PARENCHYMA IN SHEAR

Dynamic shear properties of excised rabbit lungs were studied by measu ring creep deformation after application of a step indentation force t o the pleural surfaces by a rigid cylindrical punch. The punch diamete r was 9.5 mm, and punch forces were 2, 4, and 6 g. Measurements were m ade at lung volumes of 40, 60, and 90% of the total lung capacity befo re and after lavage with 3-dimethyl siloxane, which provided a constan t surface tension of 16 dyn/cm at the alveolar surfaces. A power-law m odel was fitted to creep data and then transformed into the frequency (f) domain by using Laplace transforms. The optimum model parameters w ere 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 nearl y hyperbolically with increasing f, both decreased with increasing ind entation force, and both increased with increasing mean lung volume. S hear hysteresivity decreased sharply from 0.01 to 0.25 Hz and then ass umed a nearly steady value that was an order of magnitude lower than t he value reported previously for uniformly oscillated lungs. Changes i n E(mu) and R(mu) after lavage were correlated with changes in transpu lmonary pressure and not with changes in surface film properties. Thes e results suggest that in the breathing range of frequencies 1) the en ergy loss of lung parenchyma is a much smaller fraction of the stored elastic energy in shear than in uniformly oscillated lungs and 2) tran spulmonary pressure, not dynamic properties of surface film, is the pr imary determinant of lung dynamic properties in shear.