Multibreath washout analysis: modelling the influence of conducting airwayasymmetry

The contributions of respiratory and conducting airway asymmetry, gas excha nge, and non-uniform ventilation to the sloping alveolar plateau in phase I II of the gas washout curve are investigated using mathematical models of t he human lung. The models range from fully symmetric, to a detailed asymmet ric conducting airway model coupled with 29445 lumped parameter respiratory airway models. A gas transport equation is solved in the models using a La grange-Galerkin method. The alveolar slope is normalised by the mean expire d gas concentration to give S-n. The model results confirm that first breat h S-n is influenced mainly by respiratory airway asymmetry, and that at nea r-normal levels of ventilation the conducting airway asymmetry contributes a significant amount to further increases in S-n. Gas exchange moderates th e plot of S-n against breath number, such that it approaches a plateau in l ater breaths. Non-uniform flow also alters S-n, indicating that an accurate description of the pleural pressure gradient will be necessary for more ac curate simulations. (C) 2001 Elsevier Science B.V. All rights reserved.