Mechanical response of the lung at high frequency

Abstract

For the purposes of learning about pulmonary structure, interpretation of high frequency impedance of branching airway systems may be facilitated by a theoretical prediction of input impedance. We have developed ways of treating special classes of asymmetry in branching structures which simulate a variety of observations unaccounted for in current symmetric models. This self-consistent analysis, which assumes that small sub-units can be joined in an appropriate manner to simulate ever-larger branching structures, also incorporates the geometric and dynamic properties of the airway walls and enclosed gas. This analysis has been successfully applied to models of adult and infant lungs, and mirrors trends found in existing data. The model also suggests quantitative and qualitative dependences of system impedance responses to certain airway system characteristics, such as wall properties, branching asymmetry, and central and peripheral airway obstructions. Modal overlap and clumping are observed. Wall properties and branching asymmetry are both found to be important at low frequencies (≲1 kHz), while the asymmetric nature of the structure predominates at higher frequencies (≳3 kHz). The effect of central airway obstructions is most marked at higher frequencies (>2 kHz) where the number of apparent modes has decreased as modal regularity increases. Peripheral airway obstructions, in contrast, have small effect at high frequency.