Determination of Residual Volume and Residual Volume/Total Capacity Ratio by Single Breath Technics

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Abstract
The evident failure of "single-expiration" analyses to give accurate or consistently reproducible values of residual volume can be attributed primarily to uneven pulmonary ventilation. This in turn limits the value of expired gas as a source of information about gas which remains in the lung. The hope that the approx. linearity of the "alveolar plateau" would permit taking the nonuniformity factor into account does not appear to be justified. The reasons for the tendency for the concn. of N2, used as a relatively insoluble reference gas, to follow a linear relationship with expired alveolar volume are obscure, and apparently even this tendency cannot be regarded as a rule. However, the single breath approach may have some utility. The methods based on uniform ventilation equations, while admittedly incorrect theoretically and greatly lacking in accuracy, are at least extremely simple and rapid and permit an approx. classification as to RV/TC ratio to be made within a few min., using only automatic analysis of the N2 concn. of a single expiration.
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