If an alveolus is totally filled with fluid or collapsed, the segment will be perfused but not ventilated at all. Mixed venous blood is shunted across that segment. Increasing the inspired oxygen in the presence of a moderate to severe shunt will not improve PaO2. Intrapulmonary shunting as a cause of hypoxaemia is observed in pneumonia and atelec tasis.
V/Q mismatch and intrapulmonary shunting can often be distinguished by the response of the patient to supplemental oxygen.
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