Safe return of passengers to their country of origin after they become ill overseas is achieved by an understanding of the physics of the civilian flight environment and how it interacts with pathological changes brought about by disease. A passenger travelling in a modern jet aircraft on a scheduled flight was assumed to be comparatively fit by the designers of the life-support systems on board. A perfectly safe environment would be one that could reproduce the barometric pressure and molecular oxygen concentration at sea-level. It would also have a comfortable relative humidity. Aircraft design, however, is a series of compromises between weight, expense, speed, convenience and ease of manufacture. The compressors required to produce a sea level cabin at operating altitudes would be too heavy and too demanding of additional fuel, and the amount of water required to return sea-level relative humidity to the air taken from outside the cabin at -57C would be unfeasible bulky and heavy. Thus, a compromise is reached as to an operating cabin pressure of altitude equivalent to 7,500 feet, and relative humidity of cabin air is around 17%. The barometric pressure is around 80% that of sea-level, which represents a volume increase of about 120-130% to any compressible substance such as trapped air. The fall in molecular oxygen concentration will cause a desaturation of blood oxygen of approximately 1% in the healthy subject.

Hypobaric conditions and disease

The hypobaric conditions described about will cause any disease condition which produces or traps gas to rapidly deteriorate if the patient is exposed to them. Classic absolute contra-indications to flying are recent craniotomy or air encephalogram, recent abdominal surgery, pneumothorax without a thoracic drain, facial injuries with intra-sinusal haemorrhage, otitis media, acute small or large bowel mechanical obstruction and penetrating injury to the globe of the eye. Dental conditions where caries are full of gas produced by the putrefaction of bacteria can give rise to severe odontalgia at altitude, and damage to the tooth. Flight less than forty-eight hours after deep-sea diving below 50 feet can produce the “bends” and death even at modest cabin altitudes. The rate of change of cabin altitude and the direction of the change (barotrauma is worse on descent as the opening of the Eustachian tube is sucked flat by the low pressure in the middle ear, making the immediate equilibration of pressure more difficult) are factors in determining tolerance to pressure effects.

Hypobaric hypoxia and disease

Any disease with an ischaemic component will deteriorate in con- ditions of hypobaric hypoxia, and recent tissue infarctions may extend. Congestive cardiac states which are compensated at sea- level may decompensate at altitude, often in combination with mild exertion, such as walking to the on-board toilet. Organic/Toxic confusional states and alcoholic intoxication are synergistic with hypoxia. The hypoxia gets worse with the time the patient is exposed to it, as the initial hyperpnoea returns to a normal rate. For these reasons, it is recommended that patients should not travel by air for ten days after a myocardial or cerebral infarction in the case of short-haul (same continent) flights and fourteen days in the event of long-haul flights. Patients in uncontrolled cardiac failure should not travel by air until control is achieved, and patients requiring oxygen supplementation at sea-level should be weaned off oxygen before air travel. All acute patients in this group should travel with supplementary oxygen sufficient to provide intermittent oxygen at 2 litres per minute. Such patients may also require a doctor or nurse to escort them on the flight. Portable oximetry has made rational administration of oxygen therapy possible even in flight.

Other physical factors

Traction based methods for providing acceleration, such as the aircraft wheels during takeoff are only able to produce accelerations of 1G, and whilst considerable acceleration is possible with a jet engine line aircraft are exceedingly unlikely to produce accelerations which will compromise patients. However the tilt of take-off and landing may produce severe hydrostatic effects which will effect cardiac patients.

Statutory Requirements

These mainly relate to mobility. A patient, in order to be able to use an airline seat must be able to seat upright during takeoff and landing, and to get out of the seat with the minimum of outside assistance. Patients who cannot do so, or who need to be lying can be accommodated by stretcher on a scheduled flight, with prior arrangement of the airline and with a mandatory attendant. In all matters of fitness to fly, the airline and the doctors designated by the airlines have the final discretion, and the captain can veto the decision of the airline medical department without having to give justification.