THE ABSORPTION OF CARBON MONOXIDE BY THE LUNGS DURING BREATHHOLDING 1

Abstract

The expired alveolar CO concentration (concn.) was measured in normal subjects after different times of breathholding following a single inspiration of a gas mixture containing a low percentage of CO (0.1-0.3%) as well as 10% He. Since He is relatively insoluble in blood and tissue, its dilution in an expired alveolar gas sample (collected after dead space clearance) is the same as the dilution of the inspired CO by the gases in the lung before any CO had been absorbed by the blood. Therefore the initial CO in the collected alveolar sample equals inspired CO concentration x expired alveolar He concentration/inspired He concentration. The fraction of the initial alveolar CO concentration re[long dash]maining after breathholding did not fall exponentially with time as has been generally assumed. A plot of the logarithm of expired alveolar CO concentration against time curved to the right The most probable cause of this failure to fall exponentially is that diffusing capacity/alveolar volume is different throughout the lung. A practical consequence of this fact is that if one uses the breathholding CO technique for measureing total pulmonary diffusing capacity, the longer the breath is held, the smaller the apparent diffusing capacity. This is not a result of changes in diffusing capacity during the period of breathholding.