Elucidation of Principles of Gas Exchange by Means of Soluble Tracer Species

Abstract

Because the lung is inaccessible to direct evaluation of events on a fine regional basis in-vivo, its properties are usually inferred from global gas exchange data taken from arterial blood and expired gas. This presentation briefly reviews the many uses of tracer gases of varying physicochemical properties in estimating both global and distributed physiological parameters. In each, the necessary mathematical and physiological underpinnings will be discussed, as will the information content and associated limitations. Approaches to be covered include: (1) Using gases of different molecular weight to infer the importance of rates of diffusion in gas exchange processes that involve both diffusive and convective movement; (2) Using gases of different blood: gas partition coefficients to infer the importance of the convective processes involved in gas exchange, and ventilation/perfusion mismatching, in particular; (3) Using carefully selected gases and procedures to measure variables related to global function (eg, pulmonary blood flow, lung tissue volume, lung gas volume, and lung closing volume); and (4) Using gases with unique chemical properties to determine the importance of chemical rates of reaction between hemoglobin and the gaseous ligands O₂, CO₂, and CO. While all of these techniques have well-known limitations, they have at the same time greatly increased our understanding of how the lung works. When used within the constraints imposed by those limits, they continue to form a group of very useful investigative tools that continue to increase our insights into both normal and abnormal gas exchange processes.