Aeromedical transport exposes patients and their medical attendants to an altered physical environment, as well as an altered working environment. The former will have significant physiological effects on both patients and crew whilst the latter may restrict the ongoing management of the patients' clinical problems.

Physiological considerations:

1. Altitude hypoxia

A normal individual will have their arterial pO2 fall from about 100 mmHg at sea level to around 60 mmHg at 10,000 ft. Patients whose cardiorespiratory function is already impaired will be compromised to a far greater degree. Those patients with ischaemic heart disease, any form of respiratory embarrassment and the unborn fetus, are particularly at risk.

2. Dysbarism

Problems related to expansion of gases at altitude (in accordance with Boyle's Law) will occur in any gas containing body cavity or piece of equipment. One litre of gas will increase in volume by 50% (500 mL) from sea level to 10,000 ft. This may have substantial effects on the patient and any medical equipment in use.

3. Vibration & turbulence

Vibration is periodic, high frequency, low amplitude motion induced by engines and aerodynamics of the airframe whilst turbulence generally refers to aperiodic, high amplitude motion related to weather. These both have consequences for the human body including fatigue and motion sickness. Importantly multiple trauma, eye injuries, premature labour and neonates are compromised.

4. Pressurization & depressurization

One method used to overcome problems of hypoxia, dysbarism and turbulence due to weather, is to use pressurized aircraft. There are a number of considerations as to the pressurization profile required. Also important are the consequences of emergency depressurization.

5. Noise

Communication and auscultation in flight are impaired and valuable auditory cues such as the sound of ventilators, monitors and alarms are masked. Hearing protection in medical attendants has to be considered.

6. Thermal stress

Patients are exposed to a wide range of environmental conditions in contrast to the comfortable interior of a hospital. Apart from natural weather phenomenon such as rain and wind, which can interfere with loading and unloading, patients and crews are exposed to a broad range of temperatures. These extend from very hot (above 50 Celsius inside aircraft) to very cold (around 0 Celsius on airstrips at night in winter). These extremes of temperature can have an adverse on drugs and need to be taken into account when nursing patients - maintaining thermal balance and adjusting fluid requirements appropriately.

7. Acceleration & deceleration

Long duration accelerations ("G forces are of minor significance in aeromedical transport. However short duration acceleration or "crash dynamics" must be considered in the interior design and securing of patients and equipment in aircraft. Restraint systems need to account for the variety of sizes, shapes and clinical conditions of patients plus the range of equipment to which they are attached.

8. Vestibular disturbances

Certain flight procedures and types of acceleration can cause disorientation in pilots, a safety consideration when operating in adverse conditions out of remote areas. Stimulation of the vestibular apparatus also causes motion sickness in patients and crew. Crew should be conditioned through regular flying and prophylaxis and treatment provided for patients when required. Avoidance of vomiting is particularly important in certain cases.

9. Vision

Many modifications of the visual processes occur in aviation but
of most importance to aeromedical evacuation is the impairment of a pilots vision by hypoxia and the requirement to dark adapt at night. This influences internal lighting and operations at night.

10. Fatigue

Specifically a problem for medical crew and a consequence of many factors including hypoxia, noise, vibration, thermal stress, glare, motion sickness, missed meals and shift work.