Inability to separate airway from tissue properties by use of human respiratory input impedance

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Abstract
Respiratory input impedance (Zrs) from 2.5 to 320 Hz displays a high-frequency resonance, the location of which depends on the density of the resident gas in the lungs (J. Appl. Physiol. 67: 2323-2330, 1989). A previously used six-element model has suggested that the resonance is due to alveolar gas compression (Cg) resonating with tissue inertance (Iti). However, the density dependence of the resonance indicates that is associated with the first airway acoustic resonance. The goal of this study was to determine whether unique properties for tissues and airways can be extracted from Zrs data by use of models that incorporate airway acoustic phenomena. We applied several models incorporating airway acoustics to the 2.5- to 320-Hz data from nine healthy adult humans during room air (RA) and 20% He-80% O2 (HeO2) breathing. A model consisting of a single open-ended rigid tube produced a resonance far sharper than that seen in the data. To dampen the resonance features, we used a model of multiple open-ended rigid tubes in parallel. This model fit the data very well for both RA and HeO2 but required fewer and longer tubes with HeO2. Another way to dampen the resonance was to use a single rigid tube terminated with an alveolar-tissue unit. This model also fit the data well, but the alveolar Cg estimates were far smaller than those expected based on the subject's thoracic gas volume. If Cg was fixed based on the thoracic gas volume, a large number of tubes were again required. These results along with additional simulations show that from input Zrs alone one cannot uniquely identify features indigenous to alveolar Cg or to the respiratory tissues.