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.

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.

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.

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.

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.

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.

1. Restrictive cabin dimensions

The working area inside aircraft is much smaller than in a hospital setting. The low cabin height reduces the pressure head for IV lines and the limited dimensions make handling of large stretcher patients, or those with splints, difficult. There are limitations on patient positioning and minimal space for setting up for procedures or for resuscitation.

2. Mobile environment

Patients, crew and equipment can be exposed to sudden movements, which can cause injury or damage. Mobility within the aircraft is restricted by the need to "belt up". Dislodgment of connections can go unnoticed unless rigorously observed.

3. Equipment access

Drugs and consumables need to be stored in compact containers. Large "fishing tackle" boxes have been found to be inappropriate for stowage and opening in our aircraft. Essential medical equipment has to be kept secured but still within reach. Every crewmember must know exactly what is on board and where it is, so as not to be dependent on other crewmembers in an emergency.

4. Equipment portability and power supplies

Equipment must be able to be operated independently from the aircraft power supply; to be used when the aircraft engines are shut down or taken from the aircraft to a hospital or scene. Transport may vary from ambulance to utility or four-wheel drive.

5. Weight and balance

The maximum take-off weight and centre of gravity must be considered when loading patients. This may compromise the number of patients who can be carried and their relative positioning in the aircraft. High temperatures, short airstrips and the requirement to take additional fuel for long distance flights or due to weather, further reduce payload. For multiple sectors, some aircraft require patients to be loaded in a particular order.

6. Communication

Communication with patients is impaired in the noisy environment. Problems can occur when crew are needing advice or information whilst airborne and aircraft are not in VHF range. HF radio can be patched to any telephone but atmospheric conditions can drastically impair quality compared to "line of sight" wavelengths.

7. Temperature and humidity

Working in hot humid conditions can be unpleasant; performance of procedures and concentration are impaired. Excessive perspiration limits aseptic technique. Aircraft air conditioning systems only work when the engines are running.

8. Lighting

Adequate illumination is required for observations and procedures. This is not always possible away from the aircraft or at some stages of flight. Artificial lighting may impair detection of pallor, cyanosis or jaundice. LCD displays are best backlit and monitor screens suitable for use in both bright sunlight and dim surroundings.

9. Dangerous and corrosive items

Mercurial thermometers pose a risk of corrosion to airframe and control systems if they break in flight. Batteries must be NiCad or sealed also to prevent corrosion. Oxygen is flammable and poses a fire hazard, especially in the presence of fuels and lubricants. Body fluids such as liquor, urine or blood can cause corrosion and large losses may require disassembly of the floor panels to be completely cleaned out. A risk of infection exists for engineers working on aircraft soiled by body fluids.

10. Safety drills

All crew must be familiar with the safety procedures for the aircraft including "wheels-up" landing, fire in flight, rapid depressurization, engine failure, tyre blowout and use of emergency exits. Briefing for passengers is required although this may cause undue anxiety. Most patients pose problems for rapid evacuation.

11. Etiquette of flying

Experience is required in operating hatches, securing seatbelts, moving around the aircraft, using the radio, talking to the pilot and dimming lights at night, during various phases of flight. Selection of appropriate flight profiles, dealing with operational delays and use of equipment such as defibrillators in flight also require an awareness of the pilot and his role.

12. Rules & regulations

In transporting patients by air we must comply with a variety of rules and regulations relating to aircraft operations in addition to those relating to medical and nursing practice. For example transport of psychiatric patients must comply with the state Mental Health Act as well as specific Civil Aviation Safety Authority endorsed operating procedures for transport of mentally disturbed patients. Speedy flight departures are restricted by requirements to obtain a meteorological forecast, submit a flight plan and obtain clearances into controlled airspace.

Oxygen cylinders and regulators fixed outside the pressure hull require a low moisture content to avoid regulators freezing at altitude.

Dr Stephen Langford
Medical Director
RFDS Western Operations

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