P10.1 Fitness to fly

Commercial aircraft operate at altitudes of up to 12 500 metres, with the plane’s interior pressurised to 2100–2400 metres. At this “altitude” the alveolar PaO2 for healthy individuals decreases from 103 mmHg (13.7 kPa) to 64 mmHg (8.5 kPa) and oxygen saturation declines from 97% to 93%.

As a general rule, supplemental oxygen is unlikely to be required if the resting oxygen saturation is 95% or higher, and likely to be required if oxygen saturation is 88% or lower. Patients with oxygen saturation values between these levels might require specialist assessment.

Before flying, patients should ideally be clinically stable. Patients recovering from an acute exacerbation are particu­larly at risk. Those already on long-term oxygen therapy need an increase in flow rate of 1–2 L per minute during flight. Careful consideration should be given to any comor­bidity that may impair delivery of oxygen to the tissues (eg, cardiac impairment, anaemia). Exertion during flight will exacerbate hypoxaemia.

The American Thoracic Society currently recommends that PaO2 during air travel should be maintained at more than 50 mmHg (6.7 kPa). At altitude, PaO2 can be estimated from PaO2 at sea level by means of published nomograms. If the PaO2 at sea level is less than 70 mmHg (9.3 kPa), PaO2 at 2300 metres is less than 50 mmHg (6.7 kPa). The natural conclusion is that all patients with a PaO2 less than 70 mmHg (9.3 kPa) at rest at ground level should receive supplemental oxygen (American Thoracic Society 1995, Ahmedzai 2011).

Many lung function laboratories perform high altitude simulation tests (HAST) to assess fitness to fly. These measure arterial blood gas levels or transcutaneous oxygen saturation while breathing a mixture of 15% oxygen and 85% nitrogen, mimicking conditions at 2800 metres.