Theoretical Analysis of Gas Exchange in a Lung Model

Abstract

A mathematical model, derived from physiological data, is presented to represent gas exchange in the lung during steady state and, for the first time, unsteady state conditions, The model is used to simulate functional defects that can occur in the diseased lung. From a review of available literature four types of defect are recognised, namely, regional inequalities of ventilation, regional inequalities of blood flow, series inequalities and direct pulmonary blood shunts. The effects of changing minute volume, cardiac output and inspired oxygen concentration upon overall gas exchange are studied for each defect when oxygen uptake and carbon dioxide output are held constant at normal values in the steady state. The unsteady state condition is examined during the uptake and elimination of four hypothetical inert gases having differing blood solubilities, The model is used to assess how well certain pulmonary function tests discriminate between the various defects considered, and from information gained a new protocol for defining lung function is proposed. Utilising a more elaborate version of the model, the influence of nitrous oxide induction and excretion upon overall gas exchange is evaluated, The effect of blood recirculation time, minute volume, cardiac output, lung volume and inspired N,0 concentration upon arterial PO2 and PCO2. are examined during the unsteady state. The change in FRC occasioned when inspired and expired volumes are kept constant, are discussed as an alternative to the general case in which FRC is held constant and expired volume varied, A comparison between the model prediction of expired gas tensions with values obtained from three normal subjects during N,0 induction is also included,