Serial distribution of airway diameters from input impedance and computed tomography

Indirect measures of airway diameter such as respiratory system input impedance (Zin) have been widely used to infer or quantify bronchoconstriction, or bronchodilation. One such measure, Zin above 100 Hz has been shown to be primarily influenced by airway geometry and airway walls but not by lung and chest wall tissues. We used a recently developed method based on a complex asymmetrically branched network of tubes with nonrigid walls to analyze Zin from 100 to 2,000 Hz in control and bronchoconstricted (histamine injection) dogs. The resulting estimates of airway diameters indicated that peripheral airways were constricted far more (approximately 30% of their control diameters) than central airways (i.e., 0% in the trachea). Separate measurements of changes in airway diameters were made in an excised dog lung using high resolution computed tomography. The observed changes in airway diameter between lung volumes of total lung capacity (TLC) and functional residual capacity (FRC) were quantitatively consistent with those obtained from Zin data in our control dogs at FRC. We conclude that this systems identification method can be used to estimate the distribution of airway diameters from Zin.