Lung tissue resistance and hysteretic moduli of lung parenchyma

Lung tissue resistance (Rti) represents a large and labile component of total pulmonary resistance, but the mechanism is unknown. One hypothesis that has received some support in the literature is that on exposure to contractile agonists airway smooth muscle shortens and then, by the agency of elastic interdependence, induces distortion in surrounding parenchyma. Parenchymal distortion induced in the vicinity of a constricted airway is a pure shear deformation, but currently there are no data available for shear hysteresivity. Guided by a microstructural model, we have assigned stiffness and hysteresivity to microstructural elements and then computed how those properties are expressed at the macroscale in bulk hysteresivities for both shear and volumetric expansion. Hysteresivity for volumetric expansion is shown to be a stiffness-weighted average of hysteresivities of all microstructural components. But as the hysteresivity of microstructural elements increases, that for shear deformation increases to some degree but eventually attains a plateau. Blunted hysteretic response in shear seems to be an intrinsic property of pressure-supported structures, like the lung, that require an inflating pressure to ensure mechanical stability. The analysis indicates that that part of Rti attributable to parenchymal distortion can be at most a small fraction of that attributable to volumetric expansion. These results are purely theoretical in nature, and this suggests that caution is necessary in their interpretation. However, the mechanical basis of the results is sufficiently general to conclude that the hypothesis that parenchymal distortion secondary to bronchoconstriction can account for Rti and its changes seems to be implausible.