[Pulmonary diffusing capacity for CO during breathing of inert gas mixtures with differing physical properties...

In order to study the role of diffusion limitation in alveolar space (stratification) for pulmonary gas exchange function, pulmonary diffusing capacity for CO (Dco) was comparatively measured during breathing of room air and of gas mixtures in which N2 of room air was replaced by He or Ar. Measurements in 15 healthy humans using the single breath method yielded the following mean (+/-SE) values: room air, 22.1+/-1.7; He, 25.1+/-1.3; Ar, 17.4+/-1.0 ml-min-1-Torr-1. The clear negative correlation between Dco and the molecular weight of the gases used is most easily explained on the basis of an airway diffusion resistance limiting CO transfer. For room air breathing this resistance is estimated at about 10% of the total resistance to CO uptake (1/Dco). The alveolar exchange of O2 and CO2 is expected to be affected by the diffusional resistance in airways, particularly at high metabolic levels.