Partitioning of the alveolar-arterial O2 pressure difference under normal, hypoxic and hyperoxic conditions

The alveolar-arterial O2 pressure difference (AaDO2) is composed of three parts which depend on inhomogeneities of the ventilation-perfusion ratio (AaD(distr.) 1), on size and distribution of the diffusing capacity-perfusion ratio (AaD(distr.) 2), and on the effect of the shunt perfusion (AaD(sh)). These three parts can be calculated for normal, hypoxic and hyperoxic breathing conditions if the inhomogeneities of the function parameters and the size of the shunt perfusion are known. The calculation based on experimental data in 28 healthy subjects shows the following results: (1) Under hypoxic breathing conditions the AaD(distr.) 2 due to diffusion dominates. However, even at alveolar O2 pressures below 45 mm Hg the AaD(distr.) 1 must not be ignored. (2) Under normal breathing conditions AaD(distr.) 2 may be ignored and will under pathological conditions become relevant only if the diffusing capacity-perfusion ratio is below 3.10(-3) mm Hg(-1). (3) Under hyperoxic breathing conditions the AaD(sh) is predominant. However, even with the inhalation of pure oxygen, the AaD(distr.) 1 contributes 10% of the total AaDO2. (4) When evaluating the methods of measurement of the O2 diffusing capacity and of the shunt perfusion the inhomogeneities of ventilation, perfusion and diffusion must be considered.