Prospects of endogenous carbon monoxide (CO) measurements in exhaled air

Abstract

Summary. Unlike СО2 at successive breath-holdings, with increasing time, CO concentration aspires to some equilibrium value (to equilibrium concentration) depending on carboxyhaemoglobin content in blood. It is shown that lung diffusion capacity, measured by rate of CO concentration increasing in alveolar volume, approximately twice exceeds total lung diffusion capacity measured by known single breath method, i.e. corresponds to diffusion capacity of alveolar-capillary membrane. On base of mathematical modeling of CO gas exchange process in lung of man, the formula was derived for calculation of integral measured CO concentration in exhaled air, taking into account influence on measuring results of dead (anatomic and device) volume. Influence of dead volume was considered in assumption that in it there is no gas exchange, but in exhaled air volume from it at first passes CO containing in inhaled air, and after CO from exhaled part of alveolar air. Taken into account, that at start of inspiration, dead volume filled by final portion of alveolar air entering to it as a result of previous expiration. Two models were considered: the linear model (LM) when CO gas exchange in changing alveolar volume takes place at constant transfer coefficient and the model of elastic membrane (MEM) when transfer coefficient at breathing is changed in proportion to membrane surface area and in inverse proportion to its thickness. As a result of data analysis it is shown that calculations in MEM approach describe experimental data more adequately and can be used for the solution of the reverse task, i.e. calculation of alveolar-capillary membrane diffusion capacity, lung alveolar volume and equilibrium CO concentration, using three values of measured CO concentration at various breathing regimes. Thus, proposed method, unlike single breath, allows to determine indicated impotent physiological parameters without using of test gas mixtures.