Interhospital transport of critical patients

9 Interhospital transport of critical patients F. L. R U T T E N

Care of the critically ill or injured patient during transport plays an important role in ultimate patient outcome. Transport may be necessary from the scene of the emergency or accident, between care facilities or within a hospital. Whatever the transport situation, it can cause various physiological changes, depending on factors such as patient profile and logistics (for example, space, power supply, available equipment). The team in charge of patient care must therefore be able to ensure proper operation of the critical care equipment, to monitor the patient's physiological data and to render appropriate clinical intervention when necessary. A main principle in critical care transport (CCT) is that the level of care during transport at least approximates to the level in a fixed intensive care unit. In spite of this fundamental idea, even today many patients are transported without sufficiently skilled personnel and without proper monitoring of patient's vital functions. However, the incentive to build more sophisticated transportation systems for haemodynamically unstable adult medical/surgical patients is developing (Crippen, 1990). Patients receiving critical care in community hospitals are generally too unstable for transfer to tertiary care centres by traditional basic life support ambulance services. Medical evacuation teams consisting of nurses, emergency medical technicians or even a junior level resident may not have sufficient experience in advanced haemodynamic monitoring and life support and, as a result, are ill-equipped to handle such transports expertly. Himmelstein et al (1984) described substandard stabilization of 33 of 103 patients who were at risk for fifethreatening complications in transit. Schiff et al (1986) described substandard stabilization for 89% of 467 patients transferred from emergency departments to surrounding hospitals. Schiff et al (1986) also reported a 40% higher death rate in patients transferred with inadequate stabilization versus non-transferred patients. Most patients in these two series were transferred because of sociofinancial considerations rather than for medical indications. Olson et al (1987) reported on inadequate stabilization of critically ill medical/surgical patients. They also report that a sizeable number of inadequacies in this group were of an extremely basic nature. Mayer (1987), in his review of the literature, found that between 24 and 70% of transferred patients were inadequately stabilized prior to transport. Martin et al (1990) describe a 1-year prospective review of 78 multiply Bailli~re' s Clinical Anaesthesiology--

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injured patients initially treated at local community hospitals and subsequently transported more than 25 miles to a referral trauma centre. In their study, life-threatening deficiencies were identified in four (5%) patients and serious deficiencies in 62 (80%). Therefore, 'scoop and run' protocols used in trauma are evolving to 'stabilize and transport' protocols for critical care medicine (Crippen, 1990).

TRANSPORT OF CRITICAL PATIENTS

Reasons for transporting critical patients If transport of the critical ill or injured can lead to such a worsening of the physiological condition, what reasons do we have for transporting such patients? One of the main reasons is that critically ill or injured patients should be best served by medical care delivery in a facility possessing the necessary academic resources, monitoring technology, round-the-clock skilled personnel, invasive diagnostic capability and comprehensive surgical care. A facility without these necessities attempting the same level of care promotes delay and inefficiency, resulting in significantly increased cost of treatment and necessarily increased mortality and morbidity (Crippen, 1990).

The timing of the transport When there is agreement about the necessity of a transport, the question will be: what will be the best moment to start the transport? As mentioned above, the patient must be stabilized as much as possible before transport commences. In the care of the severely injured patient, one goal is treatment at a dedicated trauma centre with as short a time interval as possible between the occurrence of the trauma and the arrival of the patient at the trauma centre. Regionalization of health care for trauma, premature neonates and burned patients has become commonplace, and the value of a rapid transportation system in the initial care of traumatically injured patients has been demonstrated (Shackford et al, 1986). The sooner a trauma victim arrives at a trauma centre, the lower the mortality and morbidity (Cales and Trunkey, 1985), especially if the patient gets to the trauma centre within the 'golden hour' (Boyd, 1982). This is often difficult to achieve, especially in rural areas, where trauma patients have a higher mortality rate. The increased time required to get trauma patients from rural areas to facilities that provide definitive trauma care may be a contributing factor to the higher rural trauma mortality rate (Baker et al, 1987). Although there is no established standard rule for how soon a referring physician should request, for example, a helicopter to transport a trauma patient under his or her care to a trauma centre, it is of paramount importance that a referring physician in a rural emergency department (ED) organizes adequate transport to the trauma centre as soon as practically possible. Garrison et al (1989) demonstrated in their study a lengthy period between the time of arrival of trauma

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patients at EDs in rural eastern North Carolina and the time the trauma centre was notified that the helicopter was needed. No objective associated factor indicated why patients had different times-to-request. Patients that had predictably worse outcomes, as indicated by the Injury Severity Score (ISS), the need for invasive intervention at the trauma centre and short-term mortality, were not transported sooner. Personal observation suggests that this is often also the case at many other places in well-developed countries. So, the critically ill or injured patient should be transported if necessary as soon as possible, but at the right time, in the right way and to the right place. Transportation puts a heavy burden on the patient, as it produces additional injury to the soft tissues, augmented blood loss and more pain (Rommens et al, 1989). Therefore transport should be limited as much as possible. A critically ill patient receiving intensive care has the same poor condition during transport. The same conditions present in the hospital in the intensive care unit should be present during the transport of critical ill or injured patients. The competence of the crew

Composition of the medical (or flight) crew is one of the most controversial aspects of CCT. Grande (1990) states that the level of care during transport should approximate to that in a fixed intensive care unit. This has led to the question of who should provide that care. A review of current literature shows numerous examples of transport situations where physician presence is desirable for the purpose of making relevant clinical decisions for critically ill patients. Gore et al (1983) reported establishing a physician-staffed transport system in central New England specifically for the stabilization and transport of unstable cardiac patients. They characterized 78% of these patients as 'unstable' (i.e. in a critical condition, including cardiogenic shock, comatose, electrically unstable, including ventricular tachycardia and ventricular fibrillation, and recurrent chest pain). Approximately onethird of the transferred patients had life-threatening complications during transfer, of which the majority could not have been predicted. The presence of a physician trained in critical care resulted in successful management of all but one of these life-threatening conditions. Kaplan et al (1987) also found that patients in cardiogenic shock had an increased number of complications and a greater need for physician intervention. From their experience they did not believe that it was possible to predict by preflight screening and consultation which patients would require physician attendance during transport. Crippen (1990) stated that CCT teams must be constituted to provide the expertise necessary for safe initial stabilization of critically ill patients. They must have the capability of not only rapid mobilization to outlying patients, but also the expertise in dealing with sophisticated technical life support systems. Moecke (1990) argued that a main factor in critically ill patient safety is the medical qualifications of the attending team. The consultative council of the German Air Rescue (Deutsche Rettungsflugwacht e.V.) recommend the following minimal experience for an attending critical care physician during transport:

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F.L. RUTrEN More than 3 years' clinical experience in a specialty closely related with critical care medicine, plus Minimum of 6 months of full-time employment on an intensive care unit, plus Certification as an outhospital emergency physician.

Because in Germany many critical care patients are transported by helicopter, there are also a number of qualifications needed in relation to aeromedical transport:" 1. 2. 3. 4.

Knowledge of aviation physiology and pathophysiology. Knowledge of medical instrumentation in the aviation environment. Flight safety. Basic knowledge of juridical and tactical aspects of flying (abroad).

If there is agreement about the fact that a critical care physician should be a member of the CCT team, accompanied by a paramedic or intensive care nurse, the next question is: should the referring physician bring the patient to a tertiary care facility or should a tertiary care physician fetch the patient from the referring clinic to the centre? Theoretically the latter, fetching the patient from the referring clinic to the tertiary facility, seems to be the best way, for many reasons. When a patient needs to be transferred to a higher facility, it is because their condition is particularly serious, and requires a higher level of knowledge and experience. It is therefore a good option to bring a tertiary care physician to the patient, stabilize as much as possible, and start transport under optimal conditions. It seems to be especially important to have a specialized team available for paediatric cases. Macnab (1991) describes the results of a retrospective study of 130 seriously ill or injured children transported to a tertiary level intensive care. They determined the incidence of secondary insults incurred as a function of escort training. Of all the insults incurred, approximately 8% occurred with specialized paediatric transport escorts who were accompanied by a tertiary care physician (group III, n = 52), 20% with specialized paediatric transport escorts alone (group II, n - 44) and 72% with escorts who had not received specialized paediatric transport training (group I, n = 34). Based on their results, they recommend that all coordinators review the qualifications and experience of their transport team members and all potential escorts to assess their ability to provide optimal care for the children they transport, particularly during long journeys, transfer by air, and when serious illness or injury is involved. That the composition of a transport team, especially during air transport, plays an important role was shown by Harris (1987). In a clinical trial studying the delivery of cardiopulmonary resuscitation (CPR) and emergency cardiac care (ECC) in a medical helicopter, 40-45% of physicians, nurses and paramedics certified for advanced cardiac life support (ACLS) performed below standard compared with full-time flight nurses and paramedics. Personnel regularly engaged in aeromedical activities perform far better than those similarly qualified but less familiar with the aviation environment. The ideal crew should include regular partners who train and work together. The temptation to assign personnel to aeromedical

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duties without adequate preparation, testing and commitment should be resisted. In conclusion, for transferring critically ill or injured patients, a critical care physician is to be preferred. However, a good critical care physician is not always a good CCT physician. Special training in transport medicine, especially for aeromedical evacuation, is essential. I personally consider the anaesthetist, involved in emergency and critical care medicine, as a good option.

Equipment Equipment for CCT generally includes those items directly involved in supporting and monitoring the patient, that is, mechanical ventilation, monitoring equipment and intravenous fluid infusion pumps. A critically ill or injured patient in an intensive care unit does not essentially change during transport. But what do we see many times? When such a patient is being transported there often is a quality downgrading of the equipment. When in the intensive care unit the patient is ventilated with a ventilator with many special functions like alarms at high or low airway pressure, but these functions are often absent on ventilators used during transport, when the risk of disconnection or obstruction of the airway tubing is high and is a potential cause of a dangerous situation which may go unnoticed when there is also a lack of sufficient monitoring. On the other hand, it cannot be concluded that the same equipment used in the intensive care unit should be used during transport. Although the same functions should be maintained during transport, the equipment used during transport needs different qualities. The equipment should be compatible with the transport environment in its size, weight, durability, safety, readability of displays and power supply. For air transport the equipment should also be compatible with the aviation environment. The size and the weight should be as small as possible, without loss of essential functions. This often leads to a compromise between transportability and quality of functions. However, the functions needed should always take precedence. Durability is a very important factor in choosing the type of transport equipment. Once a transport is started, there is no back-up equipment with the same performance and no immediate technical support for emergency repair. Especially when the transport will take several hours, technical failure can cause a real emergency. Durability can be very good in a stable situation in the hospital, but that does not always mean the equipment will have the same durability in the transport environment, where vibrations, shocks or pressure changes in aircraft can damage vital elements of the equipment. Safety is an often ignored item. This includes safety not only for the patient but also for the accompanying team. Transport equipment should, for example, be tested for electrical safety and pressure safety if pressurized gases are used, and the outside of the instrument should be checked for sharp edges.

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Many transport monitors and ventilators use liquid crystal displays (LCDs) because these save power. A big disadvantage of LCDs is the sometimes bad visibility of values, especially in a sunny environment. Most manufacturers have been trying to improve this negative property, but there still is no optimal result in a strongly illuminated environment. Most equipment used in the hospital setting is powered by 110 V/220 V alternating current (a. c.). Most ambulances don't have this kind of electrical power as standard and other forms of power supply such as 12V direct current (d.c.), pressuri2ed air or oxygen or combinations of these are used during transport. Systems powered only by pressurized air or oxygen, sometimes in combination with electrical power, should be avoided for several reasons, including safety, the lack of interchangeability of connectors between different countries, and the extra weight which needs to be carried. Equipment powered by 12 V d.c. is to be preferred because almost every car in the world has 12 V d.c. power, which can be used through special plugs or even by plugging into the cigarette lighter in the dashboard of the car. So with 12 V d.c. driven equipment it is possible to go over almost the whole world. In addition, an internal battery power supply is obligatory. Not only in the situation of a malfunction of the electrical system, but also during loading and unloading, there is no external power supply available. Internal batteries should give enough back-up for at least 1 hour, but this is an absolute minimum. A battery capacity of more than 3 hours has to be recommended for several reasons: older batteries have a much smaller capacity than new ones and due to loading times of hours a battery may be not maximally charged when transport takes place. Eventually recharging can be forgotten by the team coming back from a mission. Liquid acid batteries can be used in ground ambulances, but there is a danger of leakage of acid when they are not sealed or when there is an accident. In air transport these batteries are prohibited. For these reasons gel or nickel-cadmium batteries are to be preferred.

Monitoring Many medical equipment companies manufacture items that possess CCTspecific characteristics: integrated three- and four-channel monitors to display parameters such as invasive or non-invasive blood pressure, electrocardiogram, central venous pressure, intracranial pressure, pulse oximetry and temperature. In addition, there is increasing interest in the use of capnographs, and even small, lightweight spectrometers are available for CCT. The CCT physician should be aware of how equipment will function in the transport sphere and the degree to which the reliability of its data output will be affected; it may sometimes be safer to have no data than unreliable data. Pulse oximetry should always be available during CCT. Pulse oximetry offers a highly reliable, rapid, non-invasive indication of the arterial oxygen haemoglobin saturation. Many pulse oximeters offered by various manufacturers show an excellent correlation with arterial blood gas determinations in both healthy volunteers and very sick patients over an oxygen

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saturation range of 60 to 100%. During CCT, medical personnel can get the earliest information of a potential hypoxic state by monitoring patients by pulse oximetry. A reliable, portable oximeter should prove to be a valuable asset during patient transport. Compact, portable pulse oximeters are available that could easily accompany patients during ground or aeromedical transport. In many countries, especially in Western Europe, there is an increasing use of capnography in CCT. Carbon dioxide monitoring gives information on both the patient and the ventilator system. Since it gives information on such a wide range of variables, it is seldom sufficient to give a specific diagnosis by itself; analysis by the user is also necessary. It is for this reason that medical personnel using capnography should have learned to interprete the carbon dioxide values measured by the instrument. Some specific benefits of carbon dioxide monitoring in CCT are: 1. 2. 3. 4. 5.

Rapid verification of endotracheal intubation. Quick detection of problems in the airway or malfunctioning of the ventilator or gas supply. Objective, continuous and non-invasive indication of the adequacy of ventilation. Disturbances in gas exchange, circulation and metabolism can be easily recognized. End-tidal carbon dioxide trends approximate to trends in the arterial carbon dioxide tension (Paco2).

In addition, patients who are not mechanically ventilated can be monitored by capnography by using a small cannula inserted via the nose into the pharynx for sampling expiratory gas flow, which can be a rapid detector of respiratory depression if supplementary oxygen is being given (when a pulse oximeter is less accurate in alarming for respiratory depression). For these reasons we use a side-stream capnograph.

Support of vital systems Controlled ventilation. Mechanically ventilated patients frequently require intrahospital transport and are often manually ventilated during transport, resulting in significant variation in minute ventilation and consequently Paco2. A reduction in ventilation in a patient with intracranial hypertension may have disastrous consequences. On the other hand, an increase in ventilation may lead to decreasing cerebral blood flow, resulting in cerebral hypoxia. Patients requiring high airway pressures, positive end-expiratory pressure (PEEP) and a high inspired oxygen fraction (Fio2) to maintain sufficient oxygenation cannot be effectively ventilated manually. The risk of a disturbance in ventilation and/or oxygenation is high. Patients requiring close control of Paco2 and those with a significant abnormality in oxygenation should be transported using a portable transport ventilator. When a patient is transported who is totally dependent on the ventilator, the same modes as on the ventilator in the intensive care unit should be available on the transport ventilator. In particular, the alarms are important

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in the transport environment. In a small, bumpy room, accelerating and decelerating, there is a much higher risk of disconnection or occlusion of airway tubing. So, as mentioned before, high and low pressure alarms are obligatory on transport ventilators. Weaning modes are not to be used during transport. The oxygen supply must be sufficient for the whole trip, including loading and unloading and possible delays. The flowmeter most commonly used to measure and regulate the flow of oxygen being delivered to a patient in the hospital is the T h o r p J tube flowmeter, which has a floating ball in a tube. Because these flowmeters must be in a vertical position for an exact reading, which is not always possible during transport, other types of flowmeters should be used. Oxygen-metering devices with a constant flow selector valve are more practical for transport. There are many respirators which work on a minimum oxygen pressure of more than 1.5-3 bar, which are very lightweight and easy to use. Disadvantages include the frequent unavailability of alarm functions, only two modes of inspiratory oxygen fraction ( + 50% oxygen and 100% oxygen), high oxygen consumption and their dependence on a fixed connector type with the oxygen cylinder, which is different in various countries. Therefore we prefer a electrically powered ventilator, with alarm and regulator functions available and with a relatively low oxygen consumption. A humidification system should be used in conjunction with the mechanical ventilator. In addition, the availability of a P E E P valve (external or internal) is, of course, a must in CCT. However, not every ventilator is able to work with P E E P , and many ventilators have different values for tidal volume or inspired oxygen fraction when using (high) P E E P .

Infusions.

Before a CCT can start, it is of paramount importance that there are at least two large intravenous (i.v.) lines in place. When possible, an i.v. line should be placed on each arm to enhance vascular access in the close quarters of the transport vehicle. Only plastic bags should be used, because glass i.v. bottles are heavy, prone to breakage and unadaptable to pressure bags. Replacing a lost line is extremely difficult during transport, so the i.v. line must be well secured. The i.v. lines should be connected to the plastic i.v. bag by a synthetic 'spike' needle instead of a steel needle, giving a much better connection with the i.v. bag. When a steel needle is used there is a high incidence of disconnection from the bag, especially during loading or unloading. An i.v. drip can only be used when there is an infusion pump available, which should have been tested for use in the transport environment. If a pump controlled by a drip counter on the drip chamber is used, there will be malfunction during transport due to shaking of the chamber. When controlled continuous administration of an i.v. medication is needed, syringe pumps are recommended. Many patients on a CCT, particularly trauma-patients, have a need for analgesia. In some countries like the Netherlands or the UK, Entonox, a mixture of equal parts of oxygen and nitrous oxide, is used to provide analgesia. This agent is administered by the ambulance attendant via a

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demand-valve system which is supplied from portable cylinders. Because it is forbidden to use anaesthetic gases or vapours in the cabin of an aircraft, we prefer i.v. analgesics like morphine, fentanyl or ketamine in combination with midazolam. During longer trips i.v. analgesics should be administered by a syringe pump. Before a CCT takes place via ground or air transport, always anticipate all eventualities. Never make preparations on minima. For example, a highway obstruction or a flat tyre could cause a possible delay of hours, so preparations should be made in such a way that an unexpected delay during the trip will not result in a dangerous situation for the patient. AIR TRANSPORT Air transport can be by helicopter, fixed wing aircraft or by commercial airlines. The question of which form of transport to choose, ground or air, is not easily solved by a simple rule, because there are many issues related to air versus ground transport. Many studies has been done to compare ground versus air transport of trauma victims, but in most cases these studies were related to emergency scene flights and not to interhospital transport. Boyd et al (1989) evaluated interhospital air versus ground transport of major trauma patients. They concluded in their prospective study that major trauma patients transported by a helicopter emergency medical service (EMS) had a better outcome than those transported by a ground EMS. The benefit seen with the helicopter EMS was directly related to injury severity and was demonstrated only in the patients with a probability of survival of less than 0.90. They also found that air transport decreased the time from injury until arrival at the trauma centre by 51 min. The mean distance was 53.7 miles for the ground EMS and 56.8 miles for the helicopter EMS. Advantages and disadvantages of air versus ground transport There are a number of parameters of interest in deciding which form of transport to choose. Although there is no unanimity on how to decide, some important factors are considered here. Distance

The greater the distance, the more air transport will come into consideration. For the transport of patients in a critical condition over more than 300 miles, absolute preference to air transport is given. At distances between 25 and 100 miles it depends on the quality of the ground EMS, the road quality and the traffic density, but also on the availability of a helicopter. At distances over 100 miles helicopter transport should be first choice, unless there are absolute contraindications, such as passing over a mountain pass at high altitude with a patient with a closed-loop intestinal obstruction, or financial considerations. Sometimes the road distance is much greater than the celestial latitude because the patient has to come from a place at the other side of a mountain or lake.

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T/me

Time is sometimes of paramount importance. The earlier a trauma patient arrives at a trauma centre, the better the outcome. Non-trauma critical patients should be stabilized as much as possible before transport starts. Weighing up the pros and cons of losing time for further stabilization against 'scoop and run' is very difficult and will be different in very situation, dependent on the patient's diagnosis, initial condition and prognosis.

Comfort Another reason to consider air transport is the aspect of patient comfort. Dependent on the diagnosis, for example cervical fracture, it is sometimes more comfortable and also safer to transport by air.

Weather conditions Aviation is very dependent on weather conditions. A heavy thunderstorm can be a good reason not to fly or to delay the flight. Other conditions like fog or freezing rain can have the same result: no flight. On the other hand, weather conditions can also influence ground transport. For example, when there has been a shower with freezing rain, the roads can be very slippery and dangerous. When the shower is over, the roads often remain slippery for many hours, while the flying conditions are normal. Most pressurized fixed wing aircraft have deicing systems to avoid icing on the wings, engine inlets and propellers. Helicopters, on the other hand, are more vulnerable during icy conditions. Fog can be a reason for not going by air, but ground transport can also be very dangerous in foggy conditions. Fixed wing aircraft used as air ambulances mostly have instrumentation for flying during the night or bad weather conditions. It is particularly important that there is enough visibility to land at the destination, even on instrument. More and more helicopters are now also equipped for flying under instrument conditions.

Costs There is no doubt about the costs of a helicopter programme; ground transport will always be cheaper than helicopter. Helicopter programmes are very expensive and are often heavily subsidized by their sponsoring hospitals, government or private institutions because collections do not cover the operating costs. The annual operating budget for a helicopter is between $800 000 and $1500 000. The cost will, of course, depend on many variables such as availability, personnel, equipment and the type of helicopter. A very important factor is whether the helicopter can start within a few minutes or whether it has to be changed from a helicopter taxi into an air ambulance. When a helicopter can also be used for other purposes like normal passenger flights, the costs will be much lower. However, its availability for patient transport is of course not guaranteed, and it will also take a

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while before the helicopter is ready for a critical care flight. The best way is to have a full-time ambulance helicopter standing by, but the costs are then a lot higher. Flights of over 300 miles with a fixed wing turboprop aircraft start to be cost effective against ground transport. For CCT the costs are not only related to the equipment, including the ground ambulance or airplane, but also the personnel. When there is a need for a critical care physician to accompany the patient, for example, the costs will be very dependent on the time the physician is away. Going a long distance by ground transport can also mean a stay overnight, which means that the costs for personnel continue. It can also be of importance that the critical care physician comes back as soon as possible because there is nobody to replace him during the trip.

Specific recommendations for air transport CCT by air currently utilizes two types of aircraft--the rotary wing helicopter and the fixed wing aeroplane or jet. Transports can be primary, from the scene to a hospital, or secondary, between hospitals. Secondary transports can be national or international, repatriation flights. Secondary flights can be performed by helicopters, fixed wing aircraft or jets specially equipped for CCT or by commercial airliners. Some commercial airliners can even provide critical care facilities by arrangement (for example, KLM Royal Dutch Airlines, Amsterdam, The Netherlands). Especially for intercontinental, long distance flights this can be a very good and much cheaper alternative to the small business jet, which has more space limitations and needs more frequent stops for refuelling, a factor to be avoided if possible. When there is a call for help from an undeveloped country with a low grade of medical care, repatriation is often indicated. However, because of poor information about the condition of the patient and the real diagnosis, a 'scout' may be sent to clarify the situation and to stabilize the patient as soon as possible, if needed, before transport takes place. It is important to make high demands on the scout. He should not only be one of the best physicians of the team, but he should also be able to work under primitive conditions in a not infrequently hostile environment. In addition, he should also be capable of organizing the repatriation as best as possible in cooperation with his home-base, which can be very difficult due to bad communication facilities. Solving medical problems are only 50% of his work; the other 50% is organizational. When it is decided to send a scout to a patient with an unreliable diagnosis, he should take all the equipment needed for the worst diagnosis with him. When this could result in a longer stay for stabilizing the patient, for example more than 2 days, it is recommended that a scouting (flight) nurse should accompany the scout.

Basic flight physiology Before considering the various physiological effects of a decreased atmospheric pressure and the effects of the changes in pressure associated with

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climb and descent, a knowledge of what these pressures might be is essential. In most helicopters or in aircraft without a pressurized cabin, the pressure in the cabin will be the same as the ambient pressure, which depends on the altitude. At about 18 000 feet the pressure is approximately halved. What the pressure will be at different altitudes in a pressurized cabin will be dependent on the type of aircraft and the flying altitude. The limitations to pressurization depend mainly on the integrity of the cabin to withstand the pressure differential between the ambient and the cabin pressure. Pressurized aircraft are designed to withstand a differential of between 52 and 60 kPa. Excess pressure is vented to the outside for safety reasons (McNeil, 1983). Some high-performance aircraft can maintain sea level pressure over 20 000 feet. Because it is economically very unfavourable to fly at lower altitudes (higher fuel consumption resulting in more refuelling stops) and because of a greater dependence on weather conditions at lower altitudes, there should be a very good reason to maintain cabin pressure at sea level. Normally there will be a cabin pressure equivalent to an altitude between 4000 and 8000 feet. Most commercial airliners have cabin pressures equivalent to 6000-8000 feet, but our own observation is of pressures greater than 9000 feet in older types of aircraft on scheduled flights. The physiological effects of decreased atmospheric pressure can be divided into those related to the expansion of gases and those related to the reduction in available oxygen. The main physical law governing the volume of gases influencing the changes that occur with altitude is Boyle's law. Boyle's law states that, at constant temperature, the volume of a given mass of gas is inversely proportional to the pressure on it. When expanding gases are free to maintain equilibrium with the pressure in the cabin as it is changing, there is no danger of excessive positive or negative pressure in an open cavity such as the open mouth or unobstructed nasopharynx. When a cavity is closed or semi-closed, positive or negative pressures can be harmful and/or painful if equilibrium cannot be achieved with the cabin pressure. Effects on patients

The facial sinuses, being in bony structures, have little resilience other than the soft tissue comprising the linings. Positive or negative pressures in these cavities due to lowering and normalizing the cabin pressure during a flight can be extremely painful if the ostia fail to allow equilization of pressure. As with eustachian obstruction, relief may be obtained by using a nasal decongestant in a spray form at least half an hour before take-off and before starting the descent. (In regular commercial flights descent will start at about 20 min before the estimated landing time.) Beside the ear and sinus cavities, there are other situations where collections of gas can give rise to problems with altitude. Gastrointestinal tract. There is normally gas in the gastrointestinal tract, but in healthy individuals, expansion of intestinal gas rarely causes any significant problem because of the resilience of the visceral walls. Gas expansion in a closed-loop intestinal obstruction, as may occur in many critical care

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patients, can cause a serious deterioration of the condition. A diseased bowel such as one weakened by ulceration or diverticulitis may not tolerate an increased pressure and may perforate. The same applies to newly sutured gastrointestinal anastomoses. Especially in children, gaseous distension of the stomach can limit the depression of the diaphragm and compromise respiratory vital capacity.

Pneumothorax. Any pneumothorax must be treated prior to medical transfer by air. Even a small pneumothorax will expand to altitude and in doing so may tear healing adhesions, causing bleeding or a tension pneumothorax (McNeil, 1983). Any possibility that there is a pneumothorax should be excluded, for example by considering the history of the accident, and a new ( < 24 h) X-ray of the thorax should be performed and assessed for a pneumothorax before air evacuation can take place in a hypobaric environment. During the flight it is almost impossible to diagnose an expanding pneumothorax until signs of a tension pneumothorax are present, causing an emergency which may have been avoidable. For this reason there should always be equipment for thoracic drainage on board during aviational CCT.

Injured eye. An injured eye is very susceptible to hypoxia, and when there is air trapped in the eye after penetrating trauma, extrusion of the global contents can occur when the atmospheric pressure decreases in the cabin.

Dental cavities. Modern dental techniques usually assure that air is not trapped when a cavity is filled. An apical abscess, for example as a complication after facial tramna, can form gas according to the nature of the infection. Expansion of this gas can cause severe pain.

Air from diagnostic procedures. The introduction of air into cavities for radiological reasons, such as for an air encephalogram, presents a serious hazard for an air transfer patient. Sufficient time (5-10 days) must elapse after such a procedure for the air to be absorbed before a flight under hypobaric conditions can be made.

Trauma. When an open wound has been treated or when an operation has been performed, there may be air trapped in the wound. Irrigation of wounds usually does not lead to the escape of all the trapped air. When a limb is rigidly enclosed in a plaster cast, even a small amount of swelling provoked by the expansion of air can set the stage for circulatory embarrassment. Instruments for cutting the cast to give the limb more room should always be available. Scuba divers. This group of people descend to a hyperbaric environment and are likely to have an increased total amount of nitrogen in their tissues. This excess tissue nitrogen puts these individuals at risk for decompression illness if they fly after diving. After a decompression dive, the diver should not be transported within 24 h unless the cabin pressure is maintained at sea level.

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Oxygenation. The alveolar oxygen tension decreases with altitude in a linear manner up to 8000 feet in an individual not having any abnormality that would limit oxygen uptake or alter carbon dioxide production. The arterial oxygen tension (Pao2) will be approximately 7.3 kPa in healthy individuals at an altitude of 8000 feet. Because many patients will have lower than normal Pao2 at sea level, the decrease in Pao2 at 8000 feet might reach dangerous levels when no adequate measures are taken. Gong et al (1984) found that a person's sea-level Pao2 could be used to predict the Pao2 that would be experienced a't altitude according to the formula: Pao2 = 22.8 - 2.74x + 0.68y where x is the altitude in thousands of feet and y is the sea-level Pao2 in millimetres of mercury. The proper use of this equation will allow the physician to estimate which patients would experience profound hypoxaemia at altitude and to prescribe supplemental oxygen more selectively. Gong et al (1984) emphasized that the equation was developed in a group of normocapnic patients and should therefore not be applied to patients with resting hypercarbia. Our own observations, however, suggest another cause of hypoxaemia in some patients with cardiac or pulmonary diseases during aeromedical transportation. Unless there is a satisfactory oxygen saturation during level flight, in some patients a decrease in oxygen saturation measured by pulse oximetry (ear probe or forehead) is seen during descent and even after landing. This p h e n o m e n o n is almost always seen in premature neonates. Leighton-White (1972) reported a 7-year study of airline pilots who experienced in-flight sudden death. They found that cardiovascular disease was the underlying pathology in all of the 17 fatal collapses, but that eight occurred in the approach and landing phase and five occurred during taxiing after landing. Thus 13 of the 17 deaths did not occur during level flight when the Pao2 would be expected to be lowest. Shesser (1989) suggests that acute stress rather than hypoxia is the predominant immediate precipitating cause of these deaths. But after our own observations in some patients where there was a decrease in oxygen saturation during descent and landing, there is a very strong reason to doubt this conclusion. Hypoxaemia due to pulmonary shunting is a more acceptable suggestion as an explanation for these observations. Another factor seems to be the rate of descend. When there was a rate of 'climb' in cabin pressure of 100 feet/min or less, these former mentioned observations of hypoxaemia were not seen.

Stress. Not every patient likes flying and he will already be stressed at the moment he is told he is being flown to another hospital. In many diseases such as myocardial infarction, excessive stress should be avoided as much as possible, and sedation with a short-acting benzodiazepine like midazolam is recommended before take-off, before descent and landing and during heavy turbulence. The type of aircraft also has some influence on the increase in stress. A turboprop will make a lot more noise than a jet and will fly at a lower level, being more influenced by weather conditions and causing more stress to the patient.

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Positioning. The position of a CCT patient should be in that part of the aircraft where there is least vibration, noise and movement by turbulence; that almost always means somewhere in the middle just at the front side of the wings, in the centre of gravity. The best position to avoid the influences of acceleration or deceleration is a cross-plane orientation. Although this is the most favourable position, in most cases this cannot be executed. In the majority of cases, the head-forward position has to be used because during take-off, level flight and before landing there will almost always be a head-up position of the aircraft. The effect of acceleration on a patient with increased intracranial pressure being transported head-forward can be serious. Acceleration cannot be modified as easily as decleration, and the forces acting on landing and braking need not be as serious as those on take-off. For safety reasons the patient lying on a stretcher in the headforward position should be secured by straps, including the shoulders, to withstand sudden movements in any direction.

Effects on equipment Equipment used in CCT should have special properties. In addition some supplemental requirements are needed in the aviation environment. Some of the most vulnerable parts of the aircraft are the instruments for navigation and communication. When these instruments are not functioning in the right way, a dangerous situation may result. Medical equipment which influences the navigation or communication instruments of the aircraft in a negative way should be withdrawn; one such influence is electrical magnetic interference. Dedrick et al (1989) suggests that defibrillation with current equipment is safe in rotary aircraft currently used for emergency medical transport, despite cramped quarters and sensitive electrical aviation equipment. Defibrillation can be carried out without hesitation whether the aircraft is in flight or on the ground, providing that standard defibrillation precautions are observed. Safety is one of the most important items in aviation and it is of paramount importance that the medical equipment will be of the same level of safety as the aviation equipment. For example, a fire caused by an instrument shortcircuiting the electrical supply or due to an unnoticed change of the plus and minus poles of a 12V plug on some types of instruments is, of course, a disaster in the air. Shanahan and Shanahan (1989) found in 298 US army helicopter crashes between October 1979 and September 1985 that the most commonly reported mechanism of injury was that an individual was struck by or against an object (60.1%). Potentially injurious objects should be removed from locations where an individual may strike them or equipment should be padded or at least should have no sharp edges on the outside and be properly stored. Due to vibrations, turbulence and pressure changes, many types of equipment used in hospital will not function properly, despite being safe for use in aircraft. One of the most important instruments to be carried with an aeromedical CCT is a pulse oximeter. Short et al (1989) found that out of seven pulse oximeters tested, only two were reliable, portable and easy to use.

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When transporting patients who are on a ventilator or have other respiratory insufficiencies, capnography is becoming more and more a standard method of monitoring. We have found the Datex Normocap 200 (Datex Instrumentarium Corp., Helsinki, Finland) functions in a very satisfactory way during aeromedical CCT; even during changes in cabin pressure the values retained their accuracy.

Equipment and air expansion. In general, all equipment containing compartments filled with air will be affected by a hypobaric aviation environment. When the aircraft used has a pressurized cabin, this hypobarism will be gradual and more or less predictable. In spite of the high technology used in aviation with a high level of safety, a rapid decompression of the pressurized cabin can occur. The time in which the pressure in the cabin will become equal to the outside pressure depends on the size of the leak and the air capacity of the cabin. In a small jet aircraft the decompression time will be quicker than in a wide body airliner with the same opening in the cabin wall or other origin of air leaking to the outside. Because this on its own is a real emergency, it is very important there will be no other emergencies following, such as an exploding ventilator or battery. For this reason it is highly recommended that all medical equipment should be tested in a decompression chamber under controlled conditions up to approximately 35 000 feet and with decompression times varying from 3 to 5 s before being used in an aircraft with a pressurized cabin. Orthopaedic air splints and pneumatic antishock suits are very vulnerable to air expansion, which can be very hazardous for the patient. During take-off and descent pressure should be controlled. A high pressure valve has to be present when an intolerable high pressure might occur. Air in i.v. reservoirs will also expand and can cause a high pressure in the container, which can be dangerous if air is pressed into the vein, causing an air embolus. For this reason i.v. bags almost totally filled up with fluid and only containing a small amount of air should be used. Rigid glass bottles are dangerous in the aviation environment for several reasons other than air expansion and should not be used. Balloon cuffs on endotracheal tubes and oesophageal airways in most circumstances contain air, which will of course expand. One of the factors leading to possible damage to the trachea during endotracheal intubation is excessive pressure exerted by a cuffed tube on the tracheal mucosa, resulting in mucosal damage, necrosis of cartilage and even tracheal stenosis. During cuffed intubation, damage to the trachea is least likely when the lateral wall pressure does not exceed the main capillary pressure of the mucosa.During hypobaric conditions up to 8000 feet, the cuff pressure may reach such a high value that severe tracheal damage is likely to occur. Starting with a cuff pressure of 2.4 kPa, the cuff pressure of a standard type endotracheal tube will rise to more than 5.9 kPa at a level of 3000 feet. Because the increase in pressure during the climb is an exponential one, at a normal cruising altitude at a cabin pressure of about 7000 feet the cuff pressure will be far over the safe limit, and will certainly cause tracheal damage. This problem can be avoided using a Mallinckrodt Lanz tube, because this maintains a fixed cuff

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pressure by an automatic regulator valve system even at altitudes up to 12 000 feet (Rutten et al, 1987).

SUMMARY Due to an increasing tendency in centralizing many special medical functions such as trauma, neurosurgery, neonatology, toxicology, burns and many others, there is also an increasing need for interhospital CCT. Because a patient in a critical condition remains in that critical condition during transport, the essentials needed for such patients in the hospital environment should also be available during CCT as much as possible, which requires high-tech equipment and a well chosen medical team accompanying the patient. The equipment should be especially selected for CCT and the medical team should have extra training in transport medicine before starting a CCT programme. Even this basis is not enough for executing aeromedical CCTs. Equipment should be tested for the aviation environment before use and the medical team should have proper knowledge of aviation medicine. If medical personnel inadequately trained in aviation medicine carry out aeromedical transport using standard inhospital equipment, the patient's safety and even the safety of the crew cannot be guaranteed. REFERENCES Baker SP, Whitfield RA & O'Neill B (1987) Geographic variations in mortality from motor vehicle crashes. New England Journal of Medicine 316: 1384-1387. Boyd DR (1982) Comprehensive regional trauma and emergency medical service delivery systems: A goal to the 1980s. Critical Care Quarterly 5" 1-21. Boyd CR, Corse KM & Campbell RC (1989) Emergency interhospital transport of the major trauma patient: Air versus ground. Journal of Trauma 29: 789-793. Burney RE & Fischer RP (1986) Ground versus air transport of trauma victims: Medical and logistical considerations. Annals of Emergency Medicine 15: 1491-1495. Cales RH & Trunkey DD (1985) Preventable trauma deaths: A review of trauma care systems development. Journal of the American Medical Association 254: 1059-1063. Crippen D (1990) Critical care transportation medicine: New concepts in pretransport stabilization of the critical ill patient. American Journal of Emergency Medicine 8: 551554. Dedrick DK, Darga A, Landis D & Burney RE (1989) Defibrillation safety in emergency helicopter transport. Annals of Emergency Medicine 18: 69-71. Garrison HG, Benson NH & Whitley TW (1989) Helicopter use by rural emergency departments to transfer trauma victims. American Journal of Emergency Medicine 7: 384-386. Gong H, Taskin DP, Lee EY et al (1984) Hypoxic altitude simulation test. American Review of Respiratory Disease 130: 980-986. Gore JM, Haffajee CL & Goldberg RJ (1983) Evaluation of an emergency cardiac transport system. Annals of Emergency Medicine 12: 675-678. Grande CM (1990) Critical care transport: a trauma perspective. Critical Care Clinics 6: 165-183. Harris BM, Schwaitzberg SD & Collett HM (1987) Progress in US Aeromedical Systems. In Hossli G (ed.) Proceedings of the InternationaI Aeromedical Evacuation Congress, Zurich 1985. Zurich: Sweitzerische Rettungsflugwacht (REGA). Himmelstein DU, Woolhandler S, Harnly M e t al (1984) Patient transfers: Medical practice as social triage. American Journal of Public Health 74: 494-497.

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Kaplan L, Walsh D & Burney RE (1987) Emergency aeromedical transport of patients with acute myocardial infarction. Annals of Emergency Medicine 16: 55-57. Leicht M J, Dula D J, Brotman Set al (1986) Rural interhospital helicopter transport of motor vehicle trauma victims: causes for delays and recommendations. Annals of Emergency Medicine 15" 450-453. Leighton-White RL (1972) Airline pilot incapacitation in flight. Aerospace Medicine 43: 661-664. Macnab AJ (1991) Optimal escort for interhospital transport of pediatric emergencies. Journal of Trauma 31: 205-209. Martin GD, Cogbill TH, Landercasper J & Strutt PJ (1990) Prospective analysis of rural interhospital transfer of injured patients to a referral trauma center. Journal of Trauma 30: 1014-1019. Mayer TA (1987) Interhospital transfer of emergency patients. American Journal of Emergency Medicine 5: 86-88. McNeil EL (1983) Airbone Care of the Ill and Injured. New York: Springer-Verlag. Moecke HP (1990) Standards fiir der Interhospital Transport yon Intensivpati~nten. NotfaIlmedezin 16: 773-778. Olson CM, Jastremski MS, Vilogi JP et al (1987) Stabilization of patients prior to interbospital transfer. American Journal of Emergency Medicine 5: 33-39. Rommens PM, Delooz HH & Carlier H (1989) Transport of severely injured patients. In Vincent JL (ed.) Update in Intensive Care and Emergency Medicine, vol. 8, pp 439-444. Heidelberg: Springer-Verlag. Rutten FL, Broek WVD, Verkaaik A & Blecher W (1987) Comparison of cuff pressures of different endotracheal tube types at varying altitudes. In Mossli G (ed.) Proceedings of the International Aeromedical Evacuation Congress, Zurich 1985. Ztirich: Schweizerische Rettungsflugwacht (REGA). Schiff RL, Ansell DA, Schlosser J E e t al (1986) Transfers to a public hospital. A prospective study of 467 patients. New England Journal of Medicine 314: 552-557. Shackford SR, Hollingsworth-Fridlund P, Cooper GF et al (1986) The effect of regionalization upon the quality of trauma care as assessed by concurrent audit before and after institution of a trauma system: a preliminary report. Journal of Trauma 26: 812-820. Shanahan DF & Shanahan MO (1989) Injury in US army helicopter crashes October 1979September 1985. Journal of Trauma 29: 415-422. Shesser R (1989) Medical aspects of commercial air travel. American Journal of Emergency Medicine 7: 216-226. Short L, Hecker RB, Middaugh RE & Menk EJ (1989) A comparison of pulse oximeters during helicopter flight. American Journal of Emergency Medicine 7: 639-643.