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Ann,
It depends on the nature of the air lift as well as the condition of the patient. For instance, transports that involve smaller distances normally use helicopters from one location to the next, whereas longer transports would use a fixed wing aircraft. The fixed wing transports also can vary between smaller single engine props to larger twin engine jets (which is how I travel) with pressurized cabins.
I cannot tell you what the ambient pressures are in the props or helicopters, but the jets that fly at 41000 feet can have cabin "altitudes" between 5000-7000 feet. Low level flights are possible (flying at 25000 feet with the ability to give you a sea level cabin), but they use up fuel and time. When flying with a patient at 7000 feet altitude, you do have to watch the ventilatory status of the patient very carefully. Also, one has to watch for other conditions that may change, such as the possibility of air embolisms that develop as the altitude increases.
The changes that occur and the potential for problems are too numerous to mention here. There are several good resources available in which to explore this avenue further, however.