Effect of surface forces on oscillatory behavior of lungs

The effect of alveolar surface tension on lung dynamic behavior was investigated by measuring total lung and tissue impedances in excised rabbit lungs at breathing frequencies of 0.2-0.8 Hz and tidal volumes of 10, 20, and 30 ml before and after lavage with 3-dimethyl siloxane, which provided a constant surface tension of 16 dyn/cm. The lungs were oscillated around the mean deflation pressures of 5 (control) and 8 cmH2O (lavaged), i.e., lung volume of 60% of total lung capacity. The total lung impedance was calculated from measurements of pressure and airflow at the trachea, and tissue impedance was measured by the alveolar capsule technique. The airway contribution was obtained as the difference between total lung and tissue impedances. In the lavaged lungs, dynamic elastance (Edyn) decreased and tissue resistance (Rti) increased relative to the control values over the entire frequency range. Airway resistance increased at the higher flow rates only. The decrease in Edyn could be attributed to the absence of surface film elastance in the lavaged lungs. The increase in airway resistance could be attributed to accentuated flow dependence due to changes in airway geometry and residual lavage liquid. However, the most intriguing result was the increase in Rti in the lavaged lungs. It could be attributed to altered mechanics at the alveolar duct level after lavage. It is concluded that dissipative properties of lung tissue are major determinants of Rti, whereas elastic properties of both tissue and surface film are important determinants of Edyn.