Modern aircraft produce a cabin pressure which equates to an altitude of 5000 to 8000 feet despite flying much higher.
At this height a 3% desaturation of arterial blood occurs (1). This has little of no effect in healthy people but in those with cardiac failure, myocardial ischaemia, severe anaemia, respiratory disease and cerebral insufficiency hypoxia may produce problems.
Most airlines will provide supplementary oxygen which will overcome such problems if forewarned.
Hypoxia
The 'cabin altitude' in commercial aircraft is typically between 5,000 and 8000 feet
- results in a concomitant decrease in the partial pressure of alveolar oxygen (PAO2) - at the upper limit of cabin altitude of 8,000 feet, the cabin pressure is approximately 565 mm Hg and P AO2 is approximately 75mm Hg
- however, due to the shape of the oxy-haemoglobin dissociation curve, this only results in a fall of arterial oxygen saturation to around 90% and is well tolerated in healthy travellers
- passengers with medical conditions associated with hypoxia or reduced oxygen-carrying capacity in the blood, such as respiratory and cardiac conditions or severe anaemia, may not tolerate the reduction in barometric pressure without additional support.
Barometric pressure
The decrease in ambient pressure in the cabin as the aircraft climbs to its cruising altitude will cause any gas to increase in volume by approximately 30%
- as the aircraft descends to land, the increasing cabin pressure will lead to a corresponding reduction in volume
- gas within body cavities may cause problems if it is trapped and unable to expand freely or if there is obstruction to the free flow of air preventing equalisation of air pressure
- effects of this are most commonly seen in passengers suffering from upper respiratory tract infections, who may suffer ear or sinus pain and bleeding (otic/sinus barotraumas). Similar and potentially more serious issues may occur following surgery, if gas is introduced to the abdominal cavity or the eye, or in people with lung bullae or an non diagnosed pneumothorax
There is also a potential for interference with the function of medical devices, such as insulin pumps, as a result of formation/expansion of air bubbles.
Humidity and hydration
The ambient air at typical aircraft cruising altitudes has very low levels of water vapour and as a result the humidity levels in the cabin are typically in the range 10 to 20% compared to that in buildings, which is in the order of 40 to 50%
- contrary to a widely held belief, the low humidity in the aircraft cabin does not result in dehydration
- research has shown that the additional insensible fluid loss amounts to approximately 150ml over an 8 hour flight, with no evidence of any change in plasma osmolality
- increased fluid loss would readily be compensated by the normal homeostatic mechanisms
- low humidity may result in drying of the mucous membranes of the lips and tongue, leading to a sensation of thirst, and can also cause problems for contact lens wearers due to corneal drying
Jet lag or circadian dysrhythmia
- is a consequence of transmeridian travel and, as well as being an annoyance for travellers may be of medical significance for passengers who require regular medication
- consideration should be given to the timing of medication, e.g. in patients with diabetes who are treated with insulin - Eastward travel will shorten the day, and generally mean a temporary reduction in insulin doses (whereas westward travel will extend the day, and possibly increase insulin requirements however, adjustments to insulin doses are rarely necessary if patients are crossing fewer than five time zones (1))
This section of the system represents guidelines only and doctors should bear in mind that each airline has its own medical standards and regulations.
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