More common than no ventilation relative to perfusion in a lung region is an imbalance of ventilation and perfusion, where the ventilation-perfusion ratio falls below the ideal of one but remains greater than zero. These units can be described as if they were the functional equivalent of units with a ventilation-perfusion ratio of zero—this is what is known as shunt physiology. Units with a low ventilation-perfusion ratio, as well as those with a ventilation-perfusion ratio of zero, contribute to the lowering of PaO2 in disease.
When shunt physiology is the basis for hypoxemia, administration of supplemental oxygen fails to completely correct the hypoxemia, because no matter how high the PAO2, pulmonary capillary blood flowing through the shunt vessel fails to come in contact with the alveoli. Consequently, PaO2 does not improve to the degree expected with administration of pure oxygen.
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