The interrelationship between FIO 2 and the gas exchange ratio in the oxygen-carbon dioxide diagram.

The gas exchange ratio (R) represents the ratio of carbon dioxide output to oxygen uptake by the lungs. In the steady state it also represents the ratio of carbon dioxide produced by the tissues to tissue oxygen consumption—the RQ. When this exchange is presented on an oxygen/carbon dioxide diagram (Rahn and Fenn, 1955) it is plotted in terms of fractional concentrations (or tensions) to produce a gas R line which has a slope which is related to but numerically different from R. While it is difficult to measure R in terms of inspired and expired volume, the mathematical slope of the gas R line gives a convenient estimate of gas exchange which can be used in the accurate formulation of respiratory gas equations (Leigh and Tyrrell, 1968). However, the interrelationship between the mathematical slope of the gas R line and R which is implicit in the formal derivation of respiratory gas equations (Riley et al., 1946; Riley and Cournand, 1949; Rahn and Fenn, 1955) is unfortunately never explicit. This paper aims to clarify that situation.