Flying and hypoxaemia at altitude: Cabin altitude should have been recorded

EDITOR, - P J Oades and colleagues describe using hypoxic challenge to determine which children with cystic fibrosis may be at risk from flying in commercial aircraft.1 They also assessed hypoxia in the children in an aircraft flying at an altitude of 11 000-12 000m. It would have been useful to record the ambient pressure in the aircraft cabin (cabin altitude) at the time of each measurement of oxygen saturation. The correlation between the aircraft's altitude and the cabin altitude depends on the pressure differential across the cabin wall. The maximum differential varies among types of aircraft (maximum typically 62 kPa for most pressurised passenger aircraft). The cabin altitude varies during the flight according to the schedule of pressurisation in the cabin.

Although Oades and colleagues' recommendations may be valid for many aircraft and flights, additional factors need to be considered. In aircraft that have an emergency oxygen supply for passengers that can be deployed automatically this is activated at cabin altitudes of 3048-4267 m² when the ambient oxygen percentage is equivalent to 14.4-12.3% of that at sea level.2 Furthermore, some passenger aircraft are unpressurised. These may fly at altitudes of …