Analysis of relative contributions to the alveolar-arterial oxygen gradient

A fundamental analysis has been undertaken of O₂ transmission across a static inert gas with simultaneous diffusion of CO₂ in the reverse direction. The overall alveolar-arterial, (A-a) DO₂, gradient has then been derived as the simple sum of four terms representing shunt, ventilation/perfusion inequalities, membrane diffusion and airway diffusion with due allowance for any variation of each process throughout the lung. The expression provides a mathematical framework from which to isolate the net contribution of each process to the overall gradient—each value being a mean weighted according to the ventilation distribution of the gas exchange units. Airway diffusion resistance can be isolated with change of pressure and, from such data, has been estimated as about 6–7% of total (A-a) DO₂ in a healthy man breathing air at normal pressure. The same O₂ data has enabled the effective diameter of the functional gas exchange unit to be estimated as 14.2 mm.—well within the morphological limits for the secondary lobule.