Rodarte, J. R.

Abstract The sections in this article are: Why is Stress Distribution Nonuniform? Why is it Important to Know Stress Distribution? Simplifications Introduced to Analysis of Stress Distribution Relationship Between Microscopic Structure and Macroscopic Properties Material Properties of Lung Parenchyma Meaning of Other Elastic Constants Implications of Relative Magnitudes of Bulk and Shear Moduli Macroscopic Lung Stress Distribution Local Stress Distribution

Abstract The sections in this article are: Equation of Motion Application of Equation of Motion to a Model Determination of Resistance and Compliance Inertance and Reactance Impedance Time Constant Nonlinear Resistance and Hysteresis Application of Equation of Motion to the Respiratory System Applied Pressures Relationship to Lung Volume Tidal Breathing Opposing Pressures Flow Resistance Maximal Expiratory Flow Gas Compression and Measurement of Flow‐Volume Curves Dynamics of Breathing Breathing at Rest and During Exercise Factors Affecting Respiratory Dynamics

Abstract The sections in this article are: Wave‐Speed Flow‐Limiting Mechanism Viscous Flow Limitation Relation Between Wave‐Speed and Viscous Flow‐Limiting Mechanisms Comments on Modeling

The use of inspiratory capacity (IC) to estimate end-expiratory lung volume (EELV) during exercise has been questioned because of the assumption of constant total lung capacity (TLC). To investigate lung volumes during low-intensity steady-state cycling, we measured EELV by the open-circuit N2 washout method (MR-1, currently Sensormedics 2100) in eight healthy men while at rest and during unloaded and 60-W cycling. TLC was calculated by adding EELV and IC. Measurement variation of TLC was 142 ml at rest, 121 ml during unloaded cycling, and 158 ml during 60-W cycling. TLC did not differ significantly among the three conditions studied. EELV decreased during unloaded (P less than 0.002) and 60-W cycling (P less than 0.001) compared with rest. End-inspiratory lung volume increased only during 60-W cycling (P = 0.03). The decrease in EELV accounted for 100% of the increase in tidal volume during unloaded cycling. Although minute ventilation was similar in the subjects during unloaded cycling, we noted that breathing patterns varied among the subjects. The increase in respiratory frequency was negatively correlated to the change in tidal volume (R2 = 0.54, P = 0.038) and to the change in end-inspiratory lung volume (R2 = 0.68, P = 0.012). We conclude that TLC does not differ significantly during low-intensity steady-state cycling and that use of IC to estimate changes in EELV is appropriate.

The volume displacements of the rib cage and abdomen of relaxed seated subjects were measured as functions of pleural pressure with the chest wall expanded by airway pressure and with the chest wall distorted by an external force applied to the rib cage. From the measured displacements for the two independent loads, the three compliances that describe the mechanical properties of the relaxed chest wall modeled as a linear elastic system with two degrees of freedom were obtained. The cross compliance that describes the coupling between the rib cage and abdomen was found to be small and positive, 0.01-0.02 1/cmH2O. The displacement of the rib cage by the external force was consistent with the displacement predicted by use of standard methods for calculating the mechanical advantage of the force.

Computed tomography scans of isolated dog lung lobes at different lobe volumes were used to determine the variability of parenchymal tissue density and the variability of parenchymal volume changes on the scale of a voxel, a cube 1.5 mm on a side. The variability of tissue density increased with decreasing lobe volume. The variability of tissue density of neighboring voxels was positively correlated; the spatial correlation decreased exponentially with distance with an exponential scale of 0.3 cm. The ratio of the volume of the parenchyma within a voxel to its volume at total lobe capacity was calculated from the tissue density data at two lobe volumes. At a lobe volume of 40% total lobe capacity, the local fractional volumes were 0.42 +/- 0.12. The variability of ventilation that corresponds to this variability of fractional volume is large enough to explain the inefficiency of mixing in the isolated lobe and the slope of the alveolar plateau of nitrogen concentration in the expirate after a breath of oxygen. These results are consistent with data reported earlier on the variability of parenchymal volumes at a scale of 1–10 cm3.

We evaluated the effects of the different patterns of chest wall deformation that occur with different body positions and modes of breathing on regional lung deformation and ventilation. Using the parenchymal marker technique, we determined regional lung behavior during mechanical ventilation and spontaneous breathing in five anesthetized recumbent dogs. Regional lung behavior was related to the patterns of diaphragm motion estimated from X-ray projection images obtained at functional residual capacity (FRC) and end inspiration. Our results indicate that 1) in the prone and supine positions, FRC was larger during mechanical ventilation than during spontaneous breathing; 2) there were significant differences in the patterns of diaphragm motion and regional ventilation between mechanical ventilation and spontaneous breathing in both body positions; 3) in the supine position only, there was a vertical gradient in lung volume at FRC; 4) in both positions and for both modes of breathing, regional ventilation was nonlinearly related to changes in lobar and overall lung volumes; and 5) different patterns of diaphragm motion caused different sliding motions and differential rotations of upper and lower lobes. Our results are inconsistent with the classic model of regional ventilation, and we conclude that the distribution of ventilation is determined by a complex interaction of lung and chest wall shapes and by the motion of the lobes relative to each other, all of which help to minimize distortion of the lung parenchyma.

Five anesthetized supine beagle dogs were scanned using a fast, multislice computed tomographic X-ray technique to determine the orientation of the ribs at total lung capacity (TLC) and functional residual capacity (FRC). A plane was fit to each rib using a coordinate system in which the z-axis was aligned approximately cephalocaudally and the x-z-plane coincided with the sagittal midplane. The orientation of each plane was described by “pump-handle” and “bucket-handle” angles. The ribs rotated downward and inward during a passive deflation of the lungs from TLC to FRC. Rib displacement was not uniform: bucket-handle motion was predominant in the upper ribs, and pump- and bucket-handle motions were equal in the lower ribs. The change in the pump-handle angles between TLC and FRC was approximately 6 degrees for ribs 3–8, and the change in the bucket-handle angles decreased with rib number from 16 degrees for rib 3 to 6 degrees for rib 8. Rib shape was described by fitting an ellipse to the data for each rib; the ribs became larger and more circular with increasing rib number.

A biplane videofluorographic system was used to track the position of metallic markers affixed to the abdominal surface of the left hemidiaphragm in supine anesthetized dogs. Regional shortening was determined from intermarker distances of rows of markers placed along muscle bundles in the ventral, middle, and dorsal regions of the costal diaphragm and of one row on the crural diaphragm. Considerable variability of regional shortening was seen in a given row, which was reproducible on repeat study in individual dogs but which differed between mechanical ventilation and spontaneous breathing. There were no consistent patterns among dogs. Regional shortening obtained from the change in length of rows extending from chest wall to central tendon showed no consistent differences among dogs during spontaneous breathing. At equal tidal volumes, all regions (except the ventral costal diaphragm) shortened more during spontaneous breathing than during mechanical ventilation.