Differences in alveolo-capillary equilibration in healthy subjects on facing O 2 demand.

Oxygen diffusion across the air-blood barrier in the lung is commensurate with metabolic needs and ideally allows full equilibration between alveolar and blood partial oxygen pressures. We estimated the alveolo-capillary O ₂ equilibration in 18 healthy subjects at sea level at rest and after exposure to increased O ₂ demand, including work at sea level and on hypobaric hypoxia exposure at 3840 m (P _A  ~ 50 mmHg). For each subject we estimated O ₂ diffusion capacity (DO ₂ ), pulmonary capillary blood volume (Vc) and cardiac output ([Formula: see text]). We derived blood capillary transit time [Formula: see text] and the time constant of the equilibration process ([Formula: see text], β being the slope of the hemoglobin dissociation curve). O ₂ equilibration at the arterial end of the pulmonary capillary was defined as [Formula: see text]. L _eq greately differed among subjects in the most demanding O ₂ condition (work in hypoxia): lack of full equilibration was found to range from 5 to 42% of the alveolo-capillary PO ₂ gradient at the venous end. The present analysis proves to be sensible enough to highlight inter-individual differences in alveolo-capillary equilibration among healthy subjects.