Right to left intracardiac or intrapulmonary shunt.
Alveolar-capillary diffusion block: alveolar space filling with fluid, or alveolar collapse.
Ventilation-perfusion mismatch, brought about by some perfusion of non-ventilated alveoli (insufficient gas exchange) or by the ventilation of non-perfused alveoli (wasted ventilation). Increasing the ambient oxygen concentration corrects hypoxia due to reduced alveolar ventilation, or due to impaired alveolar-capillary diffusion, but has no effect in the presence of right-to-left shunting.
Normally there is an alveolar-arterial pO2 difference of about 10-15 mm Hg. This is produced, under physiological circumstances, by:
* Venous admixture, produced by the drainage of desaturated blood from the bronchial circulation into the pulmonary veins, and the drainage of coronary venous blood into the left ventricle by the thebesian veins. This represents blood that has bypassed the gas exchange zone.
* Ventilation/perfusion gradients from the bases to the apices of the lungs, leading to the admixture of relatively less oxygenated blood from the bases with better oxygenated blood from the apices.
The alveolar-arterial pO2 difference = PAO2 — PaO2
where, by the alveolar gas equation, PAO2 = FiO2 — (PaCO2/0.8) Rectangular hyperbolas relating alveolar gas concentrations to alveolar ventilation all obey the following general rules: The vertical asymptote is zero alveolar ventilation;
The horizontal asymptote is the inspired concentration of the gas under consideration;
The curve is concave upwards for gases being eliminated from the body and concave downwards for gas being taken up into the body; The curves move away from the intersection of the asymptotes as the volume of the gas being exchanged increases.
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