Intrapulmonary gas transport of He and SF6 in healthy subjects and patients with chronic obstructive lung disease.

Intrapulmonary mixing of inert gases of different diffusivity, He and SF6, was studied in 5 healthy subjects before and after bronchoconstriction induced by acetylcholine and in 4 patients with chronic obstructive lung disease by analyzing alveolar plateaus, determined at various breath-holding times, tBH. The following results were obtained: (a) In healthy subjects in patients the slope of the alveolar plateau-e.e. the partial pressure difference between 750 and 1,250 ml of expirate-decreased for both gases with increasing 5BH.(b) In healthy subjects complete mixing within the alveolar space occurred faster for He than for SF6. In the patients the change of the alveolar slope was similar for both gases and no horizontal plateau was attained within the experimental range of tBH.(c) In healthy subjects after bronchoconstriction the behavior of the alveolar plateaus resembled that of the patients. However, the differences between He and SF6 were smaller. From these results we suggest that in healthy subjects efficient mixing of inspired gas with lung gas is brought about by diffusion and convection. After acetylcholine these mechanisms are less effective to overcome the inequalities produced by parallel inhomogeneities. In the patients there is a considerable decrease in overall mixing efficiency, probably due to large diffusional resistance within peripheral lung units, so that convection here is the main mechanism contributing to a flattening of the alveolar plateau.