Oxygenation failure

Hypoxaemia is defined by PaO2 <11kPa on FIO2>0.4. May be due to:

• Ventilation-perfusion mismatching (reduced ventilation in, or preferential perfusion of, some lung areas), e.g. pneumonia, pulmonary oedema, pulmonary vascular disease, extremely high cardiac output

• Shunt (normal perfusion but absent ventilation in some lung zones), e.g. pneumonia, pulmonary oedema

• Diffusion limitation (reduced alveolar surface area with normal ventilation), e.g. emphysema; reduced inspired oxygen tension, e.g. altitude, suffocation

• Acute ventilatory insufficiency (as above)

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