Shunt occurs when venous blood mixes with arterial blood either by bypassing the lungs completely (extra-pulmonary shunt) or by passing through the lungs without adequate oxygenation (intra-pulmonary shunt).
Extra pulmonary (cardiac) shunting is not commonly seen in adult practice. Even when a lesion causes communication between the right and left heart, initially the blood flow will be from left to right. This will cause a reduction in cardiac output and volume overload of the right heart but not shunt. Eventually, compensatory changes may take place, which cause blood to flow from the right to the left heart.
Shunt occurs when blood is transported through the lungs without taking part in gas exchange. The commonest causes are alveolar filling (with pus, oedema, blood or tumour) and atelectasis, fig 3.
Figure 3. Pneumonia. The alveoli are filled with pus preventing gas exchange. Increasing FiO2 will not improve gas exchange as there is no air in the alveolus.
Increasing FiO2 does not normally correct hypoxia caused by pure shunt. This is because the shunted blood in the diseased alveoli does not come in contact with alveolar gas. The deoxygenated blood leaving the diseased alveoli mixes with blood coming from healthy alveoli. In the relatively healthy alveoli, the oxygen saturation will be around 97-99% regardless of the increase in FiO2. The effect of increasing FiO2 on the blood leaving these alveoli will only be an increase in dissolved oxygen, which contributes little to oxygen delivery to tissues.
Despite this, it is almost always worth trying to increase FiO2, either for the small increase in PO2, or to assess the effect on the other areas of lung where different processes might also be taking place.