Multibreath washout analysis: modelling the influence of conducting airway asymmetry

The contributions of respiratory and conducting airway asymmetry, gas exchange, and non-uniform ventilation to the sloping alveolar plateau in phase III of the gas washout curve are investigated using mathematical models of the human lung. The models range from fully symmetric, to a detailed asymmetric conducting airway model coupled with 29 445 lumped parameter respiratory airway models. A gas transport equation is solved in the models using a Lagrange-Galerkin method. The alveolar slope is normalised by the mean expired gas concentration to give S_n. The model results confirm that first breath S_n is influenced mainly by respiratory airway asymmetry, and that at near-normal levels of ventilation the conducting airway asymmetry contributes a significant amount to further increases in S_n. Gas exchange moderates the plot of S_n against breath number, such that it approaches a plateau in later breaths. Non-uniform flow also alters S_n, indicating that an accurate description of the pleural pressure gradient will be necessary for more accurate simulations.