Role of the Air-Blood Barrier Phenotype in Lung Oxygen Uptake and Control of Extravascular Water

Abstract

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The air blood barrier phenotype can be reasonably described by the ratio of lung capillary blood volume to the diffusion capacity of the alveolar membrane (Vc/Dm), which can be determined at rest in normoxia. The distribution of the Vc/Dm ratio in the population is normal; Vc/Dm shifts from ∼1, reflecting a higher number of alveoli of smaller radius, providing a high alveolar surface and a limited extension of the capillary network, to just opposite features on increasing Vc/Dm up to ∼6. We studied the kinetics of alveolar-capillary equilibration on exposure to edemagenic conditions (work at ∼60% maximum aerobic power) in hypoxia (HA) (PIO2 90 mmHg), based on an estimate of time constant of equilibration (τ) and blood capillary transit time (Tt). A shunt-like effect was described for subjects having a high Vc/Dm ratio, reflecting a longer τ (>0.5 s) and a shorter Tt (<0.8 s) due to pulmonary vasoconstriction and a larger increase in cardiac output (>3-fold). The tendency to develop lung edema in edemagenic conditions (work in HA) was found to be directly proportional to the value of Vc/Dm as suggested by an estimate of the mechanical properties of the respiratory system with the forced frequency oscillation technique.

Keywords

• alveolar-capillary equilibration
• hypoxia
• exercise
• alveolar diffusion
• alveolar perfusion
• shunt effect