Influence of inhomogeneities and airway walls on the frequency
dependence and partitioning of airway and tissue properties .
Lutchen, Kenneth R., Joseph L. Greenstein, B[acute]ela Suki.
Department Biomedical Engineering, Boston University, 44 Cummington
Street, Boston, MA 02215
APStracts 3:0021A, 1996.
It has been proposed that during mild-to-moderate bronchoconstriction
one can partition airway and tissue properties based on input
impedance (Zin) acquired from 0.1 - 5 Hz (Lutchen et al. J.A.P.
77(1): 373-385, 1994). The approach is to apply a homogeneous lung
model that contains airway resistance and viscoelastic tissue damping
and elastance parameters. The tissue parameters account for the
frequency dependence in lung resistance (RL) and elastance (EL). In
this study, we present an anatomically consistent, asymmetrically
branching airway model to address two key questions: 1) How will lung
inhomogeneities, airway wall shunting, and tissue viscoelasticity
contribute to increased frequency dependence and levels of RL and EL
during lung constriction?; and 2) How much can lung inhomogeneities
and airway wall shunting contribute to our assessment of airway,
tissue and overall lung properties derived from Zin? The model
incorporates non-rigid airway walls and allows for explicit control
over the type and degree of inhomogeneous airway constriction or
tissue changes. Our results indicate that from 0.1-5 Hz, airway wall
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 in EL during bronchoconstriction is a consequence of a
true increase in tissue stiffening, or simply a consequence of airway
phenomena. Finally, Zin from 0.1-5 Hz can be used to provide a
reasonable separation of airway and tissue properties for mild to
moderate homogeneous or inhomogeneous lung constriction. However,
during more severe disease, inhomogeneities and/or wall shunting will
produce substantial over estimation of tissue damping and hysteretic
properties. In fact, the only reliable indicator of a real change in
the tissues may be a change in the estimate of tissue elastance that
is based on data extending to a sufficiently low frequency.
Received 5 April 1995; accepted in final form 20 December 1995.
APS Manuscript Number A373-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 22 January 96