Practical Considerations in Transporting the Critically Ill by Air


Fixed wing aircraft are used to transport critically ill patients situated beyond the range of rotary- blade aircraft, which in British practice usually means from overseas. Most transfers involve patients who have become ill whilst travelling for pleasure, or while working. Twelve million people travel abroad each year, and about 500 require air ambulance evacuation every year. Air ambulances are either aircraft solely configured for air ambulance work, or are private aircraft converted into air ambulances for the trip. Insurance Companies finance the majority of air ambulances in Britain, usually acting through their agents, the Medical Assistance Companies. As for aircraft types, either turboprop or jet aircraft are used, with the Beechcraft Kingair the most commonly used turboprop, and the Lear 35 series the most commonly used jet. If more than one patient is being transferred, larger jets, such as the HS 125 series (Hawker Raytheon), the Falcon 20 (Dassault), or the Challenger (Bombardier Canadair). All the jets travel at Mach 0.80 - 0.85, have a maximum operating altitude of around 42,000 feet, and can achieve a sea level cabin at around 25,000 feet. Operating ranges vary considerably, from 1,000 to 3,300 nautical miles. Single engine aircraft, such as the Pilatus are prohibited from use as an air ambulance. Patients are loaded into these vehicles from land ambulance stretchers either by hoists, guide rails, ramps, or with narrow-aperture doors, strapped onto an orthopaedic scoop stretcher. On the aircraft, rack-mounted aluminium stretchers running parallel to the long axis of the aircraft take the patients, who are be strapped in, preferably with a four-point harness, for takeoff and landing. Patients are usually placed head forward as the acceleration and tilt of takeoff can cause blood to pool in the head and chest, which is unpleasant and may compromise the haemodynamically unstable patient orientated the opposite way. Intensive care monitoring equipment is either operated by battery or is modified for connection to the ring-main of the aircraft, which may be DC or at AC at 400 Hz. Oxygen is either provided by compressed portable gas cylinders, pallet mounted (commonly 1380 litre “F” sized cylinders at 253 bar, each weighing 10 kilograms), by 6000 litre aircraft mounted bottles, or generated by an oxygen concentrator. LOX is no longer used. Cabin pressurisation can be set at sea-level, although a complex balance must be struck between range, the cost of increased fuel consumption and the percentage inspired oxygen necessary to correct hypobaric hypoxia. Apart from hypoxia, some patients deteriorate in flight because of the expansion of trapped gases (for example, penetrating eye injuries or obstructed bowel), and even a small fall in pressure can be fatal (patients suffering from decompression sickness, for example). Although the absolute cabin altitude is important, so is the rate and direction of the change of cabin pressure. Rapid depressurisation during a steep rate of climb is poorly tolerated, and raising the cabin pressure to that at sea level is more likely to cause barotrauma (for example, to the ears, as the lower part of the Eustachian tube is lax enough to be sucked closed, and this valve-effect prevents pressure equalisation with the outside). Once an air ambulance is airborne, the patient cannot obtain assistance beyond the vehicle and the medical attendants are on their own. Most of the patients transported by air ambulance are acutely ill because of disease or injury to the heart, lungs or central nervous system. The commonest disease process, as in medical practice generally, is arteriosclerosis, leading to ischaemia (reversible oxygen lack in the tissues) or infarction (death of the tissue through prolonged ischaemia). This produces heart attacks and strokes, also other disease entities. Respiratory problems are also common, and are usually due to a loss of lung reserve in the presence of pre-existing disease. Trauma, usually due to vehicular accidents, falling from a height or violence, gives rise to bone fractures, brain and spine injuries and damage to the rib cage. Of the twelve million people who travel outside the United Kingdom each year, 66% stay within a 5-hour journey time by jet. Unfortunately, a significant percentage of patients in urgent need of air ambulance transfer are outside Europe, and transfer times more than 12 hours are not uncommon, and this taxes present air ambulance practice.


No aircraft in air ambulance mode has been specifically designed as an air ambulance; commercial reality dictates that. To be a success, a single design must produce hundreds of sales in the private jet market, but there are not that number of air ambulances of any type in the whole world. Air ambulance configuration is a compromise, as private aircraft are designed to transport six - twelve seated passengers, not stretcher patients. The architecture and layout of the aircraft reflect this limitation; door apertures are narrow, corners are tight, the cabin is often high off the ground, the cabin height may be too low to stand up straight in (Lear 35 is 52” high), and the cabin length may be too short (Lear 35 is 13 feet long). Running water may not be found in the cabin, lighting may be inadequate or wrongly directed for a stretcher and noise insulation insufficient to allow subtle signs of disease to be detected. In many designs, cabin temperature and cockpit temperature cannot be independently controlled. This may lead to mismatch between crew comfort and the requirement of the patient, particularly new-born infants and patients with extensive burns. Cabin pressure differentials assume a working cabin altitude of 6,500 feet, which can severely compromise the critically ill but be unnoticed by the fit. Tilt and acceleration during takeoff, when the patient is laying horizontally, can exacerbate head injury and circulatory instability. Rigid, rack-mounted stretchers cannot compensate for these stresses. Fortunately, parameters of jet performance such as velocity, fuel consumption, range, instrumentation, airfield performance and payload are maximised in a similar way for air ambulance and corporate jet requirements. Increased biomedical instrumentation has been paralleled by advances in avionics, but the possibility of interference between systems has increased, with EFIS (Electronic Flight Instrument System) and FADEC (Full Authority Digital Electronic Control). Sometimes the same manufacturer has supplied components for avionics and biomedical equipment (e.g., claxons with exactly the same warning tone). Long flights will rapidly exhaust batteries and so it is desirable to connect the monitoring equipment to the airframe ring main. This may make the possibility of interference worse. The most serious problem faced by the current air ambulance is that patients are getting older, more ill and further away, and consumption of oxygen over a twelve-hour trip can be beyond the ability of the aircraft to stow the number of bottles required. Ventilated patients (those on mechanical breathing support) can consume 15 litres per minute of oxygen, or one F sized cylinder every one and half hours. Two ventilated patients over 12 hours may need 16 F sized cylinders in total, using worst case assumptions. Patients may go into a “medical stall” if not enough oxygen is carried; attempts to ease the hypobaric hypoxia by reducing the operating altitude lead to increased fuel consumption, reduced range and increased journey time, which then increases the oxygen requirement, etc., etc.


Aircraft Design The new generation of turbofan engines has increased the performance and efficiency of the small jet in ways that benefit the air ambulance patient as well as the corporate passenger. Jets recently introduced, such as the Hawker 125 1000 series and the Lear 60 have longer range, higher maximum operating altitudes, higher cabin differentials and larger cabin volume. Increased cabin volume will ease the air care of patients. Further design improvements will continue to benefit air ambulance transport. However, door apertures and bulkheads need modifying to accommodate stretchers. Access to electrical supply from the generator circuit should be made easier, to allow for the increase in power requirements for incubators and oxygen concentrators. Isolation of this supply from the avionics can be improved to prevent interference. Manufacturers of medical equipment should include FAA/CAA certification for on-board use in the proving phase of all new equipment. Oxygen concentrator technology may be incorporated in future aircraft design to allow lower pressure differentials (i.e., an operating cabin altitude of 10,000 feet but supplemented