What's a physiologic shunt?

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 P_aO₂ 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 P_AO₂, pulmonary capillary blood flowing through the shunt vessel fails to come in contact with the alveoli. Consequently, P_aO₂ does not improve to the degree expected with administration of pure oxygen.

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