Variation in alveolar-arterial O2 tension difference at high levels of alveolar O2 tension.

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Abstract
Alveolar-arterial oxygen tension difference ((A -a)D) was measured in 12 normal men, equally divided into the age groups 20-29 and 50-59 years at six levels of alveolar oxygen tension obtained by breathing oxygen mixtures ranging from air to pure oxygen. The (A-a)D increased significantly when mean alveolar oxygen tension increased from 100 to 174 mm Hg, but did not thereafter alter significantly up to a mean alveolar oxygen tension of 526 mm Hg. When alveolar oxygen tension was further increased to 651 mm Hg by breathing pure oxygen, a significant fall in (A-a)D was observed. Calculations of the effect which a small compartment of open but unventilated alveoli (i.e., V/Q = o) would have on the (A-a)D at different levels of inspired O2 tension suggested that the observed variation in (A -a)D might be partly explained on the basis. The possibility was also considered that the anatomical shunt effectively diminishes when pure O2 is breathed.