At sea level, the alveolar partial pressure of oxygen ( P A o2 )is 103 mm Hg whereas at 6000 feet it is about 75 mm Hg. This reduces the oxygen content of the arterial blood by 3%. It is not important for healthy passengers but for the unhealthy passengers, the sudden exposure to this mild hypoxia may be the final step over the threshold into significant tissue hypoxia. Conditions such a s cardiac failure, any tissue ischemia, severe anemia and severe respiratory disease are among those that may cause severe adverse reaction to such hypoxia.
The problems of hypoxia may be largely overcome by the use of supplementary oxygen in flight. But it must be remembered that continuous oxygen for a 10-hour journey, even at a low flew rate, may not be practical : it requires equipment that is both heavy and bulky in a situation where weight and space are expensive. Airlines will normally supply additional oxygen on request, but passengers’ own oxygen cylinders are generally unacceptable as they may not comply with safety regulations.
Changes in atmospheric pressure affect many parts of the body. Air trapped in the ears, sinus cavities, gastrointestinal tract, and teeth can cause pain and discomfort as the outside pressure changes. Passengers suffering from cardiorespiratory disorders may be distressed even by the relatively small increase in the volume of gas in the abdomen produced by ascent from ground level to 8000 feet. The maximum cabin altitude and aeromedical evacuation aircraft should, therefore be kept as low as possible, certainly below 8000 feet.
In turbojet aircrafts relative humidity decreases to less than 1% after 2 hours and less than 1% after 4 hours. For a healthy person, low humidity results in nothing more than chapped lips, scratchy or slightly sore throat, and hoarseness. Steps that the medical crew member can take to minimize problems caused by decreased humidity include mouth care, lip palm, and adequate fluid intake.
Patients who receive in-flight oxygen therapy are twice as susceptible to dehydration because oxygen itself is a drying agent. Moisture must be provided by commercial humidification devices for all patients who receive oxygen. Tracheostomy patients in particular require warmed humidification with or without oxygen during air medical transport. Patients who are unconscious or are unable to close their eyelids must be provided with eye care. Administering artificial tears and taping the lids shut prevents corneal drying. Compromised patients predisposed by age, diet, or preexisting medical or surgical complications need special consideration in relation to decreased humidity before air medical transport.