Pulmonary shunt

Pulmonary shunts exist when there is normal perfusion to an alveolus, but ventilation fails to supply the perfused region.

This will lead to a situation where the blood supply leaving a shunted area of the lung will have lower levels of oxygen and higher levels of carbon dioxide (i.e., the normal gas exchange doesn't occur).

A small degree of shunt is normal and may be described as 'physiological shunt'. A physiological shunt fraction of 5% is normal and up to 10% is compatible with normal gas exchange.