Mode of transport

Hospital design should allow intramural transfer between departments, preferably with orderlies to move beds or trolleys, and nursing and medical staff to maintain care in transit. Manual lifts onto trolleys have the potential of injuring patients and staff, and aids such as sliding devices are advocated. Some protocols recommend using up to five people for major shifts if no physical aids are available. Supporting equipment should be attached to the bed, as trailing devices are hazardous although sometimes unavoidable (e.g. balloon pumps).

Extramural safety requirements include a vehicle with adequate space, electrical power, and oxygen reserve ( TableJ..). Defensive parking at major incidents protects crews against other traffic. Aviation regulations require stretchers to tolerate the deceleration likely in an emergency, and securing patients and equipment by safety harness is easiest in the head-first posture. If the patient is stable, there is little clinical difference between the head-first and the feet-first position, despite the nose-up ascent posture of fixed-wing craft. Transverse positioning produces the least inertial change in body fluids, but few aircraft have the necessary cabin width.

During short trips, and particularly during 'hot' helicopter loading/unloading (i.e. with engines running), it is safer to close gravity-fed intravenous lines and place them on the litter than to elevate them on poles. When flow is restarted, entrained bubbles must be expelled.

Loading aircraft may be difficult because of the weight and size of the stretcher, patient, or equipment, small cabins, narrow aircraft doors and corridors, and the height of the fuselage from the ground.

The reduction of the partial pressure of oxygen in air at altitude affects tissue oxygenation (Henry's law) ( Fig 1). Also, reduction in total ambient pressure may allow entrapped gas in distensible cystic cavities to expand (Boyle's law). Denitrogenation (breathing 100 per cent oxygen) before and during exposure to altitude may help to reduce the amount of entrapped gas (e.g. intraocular air, pneumothorax, pneumoperitoneum, or entrapped intracranial air) ( Hyid§g§aEdiiii§.D.d M.a.d.s.e.D 1994). Nitrous oxide analgesia may be hazardous in patients with decompression sickness or with any entrapped gas, as a counterdiffusion effect may enhance bubble size.

Fig. 1 Boyle's law and Henry's law: the relation between altitude and atmospheric pressure, which may affect oxygenation.

With entrapped intraocular air, a sudden elevation of intraocular pressure (e.g. vomiting, coughing, straining, or hypoxia) may cause loss of vitreous and other ocular contents. Discharge of globe contents at high altitude has occurred. Experimentally, intraocular pressure is increased by about 40 mmHg at a cabin altitude of 8000 ft. These effects may be reduced by eye binding, antiemetics, sitting the patient upright, and breathing 100 per cent oxygen.

Trapped air in an enclosed rigid cavity (e.g. an obstructed nasal sinus) will maintain its pressure, even if the ambient air pressure is decreased. Compression of venous drainage and, later, arterial vascular supply in the cavity may produce pain, bleeding, or ischemic effects.

Endotracheal tube cuffs expand at altitude. If the cuff pressure on the tracheal mucosa becomes excessive, damage may occur. Thus cuff volume may require adjustment on ascent. Conversely, cuffs may shrink on descent and require reinflation. Cuff volume is usually not adjusted for short flights. Inflation with water may have a significant compressive effect on the tracheal mucosa and is not advised. Similarly, the inflation chambers of pneumatic antishock garments or military antishock trousers may expand at altitude. Possible compression of underlying tissues requires scrutiny to avert compartment syndrome or respiratory embarrassment. When intra-aortic balloon pumping is used, the effect of altitude on the balloon gas must be considered.

Similar changes occur in surface transport over mountainous terrain.

Cabin pressures in commercial airliners are usually maintained at about 550 mmHg (75 kPa), or an altitude of approximately 2000 to 2500 m (6000-8000 ft) at an actual altitude of 12 000 m (approximately 40 000 ft). (In aviation, altitude is commonly measured in feet and speed in nautical miles per hour where 1 nautical mile = 1.8 km.) Most patients travel well at conventional cabin pressures, provided that measures are taken to ensure adequate alveolar oxygen ( PAO2).

Air ambulances, executive charter aircraft, and military transport aircraft may maintain sea-level pressure by flying at lower less efficient altitudes where there is a greater likelihood of turbulence. Low-frequency (< 20 Hz) high-energy (large-amplitude) vibration is potentially damaging to various organs and may be encountered in both road and air transport, although in practice it is not usually a problem.

Aircraft cabin temperature may affect the well being of patient and crew and the resilience of any electromedical equipment, particularly when the aircraft is standing and the cabin temperature may range from 50 °C to below zero. Climate control of most aircraft requires the engines to be running, unless an auxiliary power unit is available.

External temperature reduction is approximately 2 °C per 1000 ft; this is sometimes a consideration in helicopter missions since the external temperature at 10 000 ft is about 0 °C.

The choice of fixed-wing aircraft or rotorcraft depends on the equipment carried, size, distance, and availability. Particular needs (e.g. altitude) should be discussed with the pilot before departure. Instrument flight rules (IFR), navigation instrumentation, weather radar, low-frequency radio navigation (e.g. Omega™), and global positioning system (GPS) navigation capability for both types of aircraft enhance safety in unexpected marginal weather. Any mission is at the sole discretion of the pilot and should be aborted if there is significant risk to the aircraft and crew.

Fixed-wing aircraft are faster than helicopters, but they are slower to be mobilized and require intermediary road transport. Helicopters conveniently operate up to a radius of 200 km (111 nautical miles), depending on range and fuel considerations. They can land near a location or deliver resources to inaccessible sites by winching or rappelling techniques. Noise is greater and altitude is lower than for fixed-wing operations. The trend to twin-engine helicopters is principally due to payload and safety. Larger helicopters require a crew member, separate from the medical team, to oversee safety and assist with navigation.

Fixed-wing aircraft are usually turbo-prop twin-engine design. Missions are commonly over 200 km. Other types of fixed-wing aircraft used include executive jets for urgent long-distance transport from an area not well served by commercial airlines, and commercial airliners for long flights (> 1000 km). However, not all carriers permit stretchers. A complex case, with a medical team, a stretcher, and equipment, may occupy 15 seats which is quite costly. Teams need to be self-sufficient in equipment, oxygen, and electrical power. A preflight assessment of the aircraft and utilization of hydraulic food or baggage lifts for loading is prudent. Military medical transport aircraft (e.g. Hercules C 130) are equipped with educated dedicated personel and medical equipment. They may have an intensive care unit configuration for long-distance transport (Gi.l.l.i.g.aD §L §1 1996)

If intercontinental transport is necessary, resolution of a disease or injury should generally be well established before evacuation, but occasionally, because of lack of local resources for example, it may be necessary to move a critically ill patient. Absence of more than 24 h from the home base may be involved, so that enough staff for conventional shifts (up to 12 h) to be worked and a route minimizing airport delays and aircraft changes will be required. Limited availability of supplies en route, language barriers, varied road transport, and uncertain legal status of the team on foreign soil with respect to emergency drugs and blood carried need to be considered.

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