Quantitative Analysis of Gas Transport and Transfer in Human Lungs

Abstract

The aim of this work is to lay foundations for a more extensive use of single and multiple breath washout tests in the diagnosis of lung disorders. It has necessitated an investigation into the processes that control gas transport, transfer and mixing in the air phase and because of the nature of the investigation the work presented here is largely theoretical in context. A hypothesis is developed which suggests that gas stratification and/or its effects may be caused by regional inhomogeneities. To test this hypothesis, a branched lung model is constructed which allows for regional variations in flow within the usual lung structure and includes an allowance for Taylor dispersion. Both diffusion and convection are included in the governing gas transport equation which is solved numerically. The earlier results for this branched model have shown local stratification but simulated single breath oxygen tests indicate only a small slope to the alveolar plateau of the expired nitrogen curve. Results for multiple breath nitrogen washout simulations give an insight into the cause of nitrogen retention during these tests while simulations for oxygen and carbon dioxide transport highlight the contribution that the local stratification may make to (A-a)DO2 and (a-A)DCO1. The earlier results have led to a reappraisal of the boundary conditions for the model and with these modifications it is possible to simulate single breath nitrogen washout curves with realistically sloping alveolar plateaus. Stratification is present. The theoretical results suggest that the slope is due to the natural mixing processes that are shown to be incomplete for a normal breath