Figure.

Effect of cabin altitude on oxyhaemoglobin saturation

(A) The aircraft passenger cabin is normally pressurised to an altitude of 1524–2438 m. This reduced pressure within the passenger cabin results in lower systemic PaO₂ and decreased oxyhaemoglobin. For most healthy passengers, this results in a decrease in the arterial partial pressure oxygen tension from 95 mm Hg (12·7 kPa) to 65 mm Hg (8·7 kPa) corresponding to an oxyhaemoglobin saturation from 95–100% at sea level (A) to 90% at a cabin altitude of 2438 m (B). (B) Passengers with pre-existing lower sea-level oxyhaemoglobin saturations have greater declines during a flight. In this example, a passenger with mild chronic obstructive pulmonary disease with a sea-level PaO₂ of 70 mm Hg (A) and a FEV₁ of 1·6 L (50% predicted) will have a corresponding reduction of PaO₂ to about 53 mm Hg or oxyhaemoglobin saturation of approximately 84% at a cabin altitude of 2438 m (B). This passenger should be prescribed oxygen for air travel. PaO₂=arterial oxygen partial pressure. FEV₁=forced expiratory effort in 1 second.