Serial distribution of airway properties determines in part the response of the lung to high frequency oscillations. We measured the response of excised dog lungs and lobes between 156 and 10,000 Hz and determined the area-distance function of the acoustically equivalent structure having rigid walls, regular branching, and negligible internal losses. The utility of this techique was tested by determining the effects of air trapping, removal of pleura from a dried lung, central airway smooth muscle tone. A strong correlation was found between relative changes in equivalent acoustic area and relative area changes measured radiographically in individual airways at corresponding distances. We conclude that despite departures of the properties of the real lung from the characteristics of the acoustically equivalent structure, changes in the area-distance function computed by this technique provide reasonable estimates of the magnitude and serial distribution of actual changes in airway cross-sectional area.

No abstract

This study offers a basis for evaluating and developing models of stress-strain behavior of the lung in distortion. Tensile forces were applied along three axes to cubes of dog lung parenchyma. With axially symmetrical force-loading, expansion was reasonably symmetrical and pressure-volume relationships were reasonably conventional in range, hysteresis, and time-dependent behavior. When the force load was changed on one axis only, that axis appeared more compliant than it did during symmetrical loading and the other axes changed length in the opposite sign. Similar distortion was apparent at the alveolar level. Data for five specimens over a range of applied loads are filed with the National Auxiliary Publications Service; graphical examples are presented herein. Relationship among the compliances for symmetrical and asymmetrical loadings were consistent with elastic theory. We derived the elastic coefficients, bulk and Young's moduli, and Poisson's ratio from the data. Poison's ratio was about 0.30 in air-filled specimens, but was lower (0.16-0.24) and increases with stress in saline-filled specimens.

We postulated that the variation of maximal voluntary inspiratory pressures (PI,max and Pdi,max) among individuals largely reflects the variation of the structural attributes of the inspiratory muscles, in particular the muscular cross-sectional area of the diaphragm (CSAdi) and its axially projected area (A(thor)). To test this postulate, we measured PI,max in 36 healthy subjects, including 3 children and 15 weight-lifters, and Pdi,max in 11 subjects. Structural measurements by ultrasonography and anthropometric calipers were available as reported in the companion manuscript. We found a high degree of correlation of Pdi,max with diaphragm thickness (tdi), CSAdi, and CSAdi/A(thor) (r2 = 0.89, 0.89, and 0.77, respectively). PI,max was also correlated with diaphragm structural measurements, although less well. The weight-lifters had greater pressures, thicker diaphragms, and greater diaphragm maximal stress (sigma(max)) than adults of similar stature who had not trained with weights. We conclude (1) that both Pdi,max and PI,max reflect in part structural attributes of the respiratory muscles; (2) that the variation of maximal transdiaphragmatic pressures is largely attributable to the normal variation of diaphragm structure; (3) weight lifting increases diaphragm structure and pressures.

How does smooth muscle tone affect the properties of airways during breathing maneuvers? We have studied the dynamic relationships of airway pressure and cross-sectional area in excised dog lobes and isolated tracheal segments during simulated breathing maneuvers. When contracted by carbachol, airways developed complex hysteretic behavior, becoming less compliant in small tidal breaths and rapid deflations, and grossly hysteretic in large tidal maneuvers. For a given sequence of maneuvers the patterns of hysteresis varied with timing. Analogous behavior was seen during length-cycling of dog trachealis muscle strips. The mechanisms suggested by the hysteretic patterns of muscle length-tensional plots are 1) slipping of contractile filaments at threshold forces during lengthening and 2) active shortening of contractile filaments that is slow relative to many respiratory events. The observed behavior indicates that measurements of pulmonary function that depend on airway caliber or stiffness should become increasingly affected by the sequence, amplitude, and timing of breathing maneuvers as smooth muscle tone increases. The stiffness for small amplitude cycling may be favorable for local control of ventilation by parenchymal smooth muscle.