Airway inhomogeneities contribute to apparent lung tissue mechanics during constriction

Recent studies have suggested that part of the measured increase in lung tissue resistance after bronchoconstriction is an artifact due to increased airway inhomogeneities. To resolve this issue, we measured lung impedance (ZL) in seven open-chest rats with the lungs equilibrated on room air and then on a mixture of neon and oxygen (NeOx). The rats were placed in a body box with the tracheal tube leading through the box wall. A broadband flow signal was delivered to the box. The signal contained seven oscillation frequencies in the 0.234- to 12.07-Hz range, which were combined to produce tidal ventilation. The ZL was measured before and after bronchoconstriction caused by infusion of methacholine (MCh). Partitioning of airway and tissue properties was achieved by fitting ZL with a model including airway resistance (Raw), airway inertance, tissue damping (G), and tissue elastance (H). We hypothesized that if the inhomogeneities were not significant, the apparent tissue properties would be independent of the resident gas, whereas Raw would scale as the ratio of viscosities. Indeed, during control conditions, the NeOx-to-air ratios for G and H were both 1.03 +/- 0.04. Also, there was a small increase in lung elastance (EL) between 0.234 and 4 Hz that was similar on air and NeOx. During MCh infusion, Raw and G increased markedly (45-65%), but the increase in H was relatively small (<13%). The NeOx-to-air Raw and H ratios remained the same. However, the NeOx-to-air G ratio increased to 1.19 +/- 0.07 (P < 0.01) and the increase in EL with frequency was now marked and dependent on the resident gas. These results provide direct evidence that for a healthy rat lung airway inhomogeneities do not significantly influence the lung resistance or EL vs. frequency data. However, during MCh-induced constriction, a large portion of the increase in tissue resistance and the altered frequency dependence of EL are virtual and a consequence of the augmented airway inhomogeneities.