Effect of gas density and ventilatory pattern of steady-state co uptake by the lung

The steady-state uptake of carbon monoxide was measured in five normal subjects to study stratified inhomogeneity of gases in the pulmonary airways. An attempt was made to assess the effect of stratified inhomogeneity by varying the physical properties of the inspired gases and the depth and rate of ventilation by the subjects. Twelve different controlled patterns of breathing were followed by each subject on each of three gas mixtures (20% O2 with N2, He and SF6 plus 0.04% CO in each case). The experimental results show that CO uptake was significantly different when breathing the different density gases but that it was always greatest from the most dense gas SF6 and least from He. If the uptake of CO from the inspired gas across the alveolar membrane were controlled by simple molecular diffusion then as gas density decreases uptake would increase. As inspiratory flow rate was increased there was an almost linear increase in CO uptake for each gas mixture with a difference in uptake between the different-density gases which became most marked at the higher flow rates. Flow rate was increased in two ways: by maintaining tidal volume and altering breathing frequency or by maintaining frequency and altering tidal volume. The results for the two extremes, helium and SF6, indicated a difference between these methods of increasing flow rate. In the case of SF6, doubling the flow rate by increasing breathing frequency gave an uptake only marginally lower than doubling the depth of penetration. With helium, however, it was necessary to treble the frequency to obtain the increment of CO uptake achieved by doubling the depth of penetration. The experimental observations are explained by the use of a model based on Taylor dispersion in the conducting airways. The conclusion is that the results confirm the presence of significant stratified inhomogeneity in the airways of the lung.