HOW INHOMOGENEITIES AND AIRWAY WALLS AFFECT FREQUENCY-DEPENDENCE AND SEPARATION OF AIRWAY AND TISSUE PROPERTIES

It has been proposed that during mild-to-moderate bronchoconstriction one can partition airway and tissue properties on the basis of input i mpedance (Zin) acquired from 0.1 to 5 Hz (K. R. Lutchen, B. Suki, Q. Z hang, F. Petak, B. Daroczy, and Z. Hantos. J. Appl. Physiol. 77: 373-3 85, 1994). The approach is to apply a homogeneous lung model that cont ains airway resistance and viscoelastic tissue damping and elastance p arameters. The tissue parameters account for the frequency dependence in lung resistance (RL) and elastance (EL). We present an anatomically consistent asymmetrically branching airway model to address two key q uestions: 1) How will lung inhomogeneities, airway wall shunting, and tissue viscoelasticity contribute to increased frequency dependence an d levels of RL and EL during lung constriction? and 2) How much can lu ng inhomogeneities and airway wall shunting contribute to our assessme nt of airway, tissue, and overall lung properties derived from Zin? Th e model incorporates nonrigid airway walls and allows for explicit con trol over the type and degree of inhomogeneous airway constriction or tissue changes. Our results indicate that, from 0.1 to 5 Hz, airway wa ll shunting does not become important unless the entire lung periphery experiences significant constriction. Mild-to-moderate inhomogeneous peripheral airway constriction produces a relatively minor additional frequency dependence in RL and EL beyond that due to the tissues alone . With more extreme constriction, however, there is a marked frequency -dependent increase in EL. This phenomenon may render it impossible to distinguish from a single frequency measurement whether an increase i n EL during bronchoconstriction is a consequence of a true increase in tissue stiffening or simply a consequence of airway phenomena. Finall y, Zin from 0.1 to 5 Hz can be used to provide a reasonable separation of airway and tissue properties for mild-to-moderate homogeneous or i nhomogeneous lung constriction. However, during more severe disease, i nhomogeneities and/or wall shunting will produce substantial overestim ation of tissue damping and hysteretic properties. In fact, the only r eliable indicator of a real change in the tissues may be a change in t he estimate of tissue elastance that is based on data extending to a s ufficiently low frequency.