Introduction

The marriage of aviation and medicine has expanded the reach of the critical care unit and other specialized units beyond an individual hospital. The incorporation of monitoring, ventilators, oxygen and suction, infusion pumps, etc., allows full critical care medicine in the air ₁.With increased availability over the past decade we have seen a rise in the number of critically ill or injured patients transported by aeromedical means for definitive care at regional centers _{2 3}. Practitioners of all specialties are likely to send or receive patients by aeromedical transport (AMT).

Unfortunately, many advertised “air-ambulance” services are nothing more than business aircraft staffed by a moonlighting paramedic or nurse obtained on a catch-as-can basis by a charter aircraft company 2.There may be no medical direction at all and thus no practice standards, appropriate education of personnel, quality assurance, or medical control.

The proper and safe use of aeromedical transport requires a basic understanding of the medical aspects of flight and the capabilities and constraints of the aeromedical environment. It is the purpose of this paper to review this data and provide the non-aeromedical practitioner with guidelines for the use of AMT.

History of AMT

The history of AMT, like that of most other innovations, is marked by enthusiasm, skepticism, conservatism, and concern.

As early as 1784, after the balloon flight demonstrations of the Montgolfier Brothers, physicians began to consider the benefits their patients could gain from flight. Jean-Francois Picot theorized that not only could patients tolerate balloon flight, but that they would in fact benefit form purer air encountered at altitude ₄. Although many believed the advent of AMT occurred during the seize of Paris, the romantic notion that seriously ill or injured patients were moved by balloon from that city unfortunately are incorrect 4.

AMT using heavier-than-air machines was initiated in 1909, when Captain George Gosman, built a plane specifically for this purpose ₅. However, it was not easy to convince the government to approve further development of Gosman’s aircraft following its destruction in a crash and it was never used to transport actual patients. In 1917, the French Dorand AR II, was the first air ambulance that actually carried patients. Over the next several decades the “ambulance airplane” industry grew, mainly in the Military. World War II saw great increases in the use of AMT. It is estimated than more than one million patients were airlifted by the Unites States from all theaters of this conflict with an overall death rate of 4/100,000 4.,₆.

The Korean War brought new challenges and opportunities for AMT. In 1950, the use of the helicopter for the front-line medical evacuation of patients during combat was authorized 4. More than 17,000 patients were transported by Army helicopters alone from January 1951 to 1953. The outstanding medical evacuation system developed during the War in Vietnam owed much to the experience gained during the Korean conflict. The effective use of helicopters for AMT in Vietnam and their appearance almost nightly on domestic television, kindled interest in their use for air evacuation in the civilian community.

At approximately the same time, interest in improving care in the civilian pre-hospital arena arose, and services previously only available in the hospital were delivered in the field by ambulances staffed by health care workers ₇. It was not long before these expanded medical services were married to the helicopter to provide the first United States AMT capability. Since that time AMT in the civilian sector has exploded.

In 1979 there were more than 500 air taxi operators who performed air ambulance missions in the continental United States, and over 200 who provided this service in Alaska alone. In 1990 there were more than 170 aeromedical programs in operation in the United States ₈. The number of AMT has also increased dramatically over the last 2 decades.

Types of AMT

In general AMT can be divided in two broad categories: Fixed wing or airplane AMT and helicopter AMT. These two types of AMT share many characteristics in common. The deciding factor in choosing one over the other generally relates to efficiency.

Fixed wing AMT tends to be a more efficient process for patients more distant than approximately 200-250 miles. For transports shorter than 250 miles, helicopter AMT is routinely used.

Advantages of helicopter AMT

1. Speed: Modern helicopters routinely used in medical missions are capable of sustained speed in excess of 150 mph 9. Combined with the ability to move point to point, patient’s speed advantage may translate in to a time-savings over other forms of patient transport. Interestingly, this attribute has led some investigators to determine “optimal distances” for helicopter use based on transport time 10.

2. Accessibility: Vertical take-off and landing capabilities permit evacuation of patients from areas inaccessible to other transport vehicles. Injuries during mountaineering or excursions into wilderness areas are possible examples.

3. Specialized personnel and technology: Aeromedical services for the most part are based at tertiary care centers and are staffed with highly skilled and trained personnel. They are routinely equipped with sophisticated medical technology and bring their advanced capabilities to patients across a wide geographic area.

These unique attributes of helicopter AMT should be the basis for considering this particular transport mode.

Aviation Medicine

Aviation remains a competitive industry and the air ambulance component is no exception. Unfortunately, most aircraft used in civilian AMT were not specifically designed with this purpose in mind. Therefore, some compromises are required to provide care in the aircraft environment. In addition, the aviation environment poses new or increased stresses on the patient, caregiver, and medical equipment 9. These factors tend to be maximal in fixed-wing operations and of lesser concern to helicopters.

Oxygen: Hypoxemia is the single greatest threat to anyone that flies. Physiologic effects of hypoxemia can be detected in healthy individuals at altitudes less than 10,000 feet. This hypoxemia occurs as a result of a falling ambient pressure and its magnitude is emphasized in figure 2. Cabin pressurization minimizes this problem in many airplanes, but patients with impaired pulmonary function may be at risk for hypoxemia at commonly achieved cabin altitudes. Adjusting the fraction of inspired oxygen to maintain the inspired partial pressure of oxygen constant throughout the flight profile is a clinically useful technique for preventing hypoxemia 9. The widespread availability of pulseoximeters have lessen the incidence of hypoxemia in AMT at altitude by permitting its early recognition.

Acceleration/Deceleration: The occupants of an airplane as it accelerated or decelerated down the runway experience a change in velocity. Acceleration or deceleration is a vector quantity, having both magnitude and direction. For this reason, proper positioning of the patient to limit stresses induced by sustained acceleration should be accomplished 9.

The acceleration forces experienced in helicopters during routine operations tend to be of low magnitude and so much of those observed in ground transport vehicles.

Gas volumes: Ambient pressure decreases with increasing altitude. Changes in pressure with changing altitude may affect a number of medical devices as well as the patient. Contrary to common belief, cabin pressurization does not eliminate this concern. Pressurization does permit comfortable manned flight at altitudes that could not be attained without it, but generally does not result in a sea level cabin altitude, and thus the equipment and patient will be exposed to some pressure change. Any gas filled structure therefore becomes of concern. Air trapped in the sinuses for example, may expand and cause discomfort and appliances using air-filled cuffs may malfunction or injure the patient with changing altitude.

Humidity: Humidity is a particular concern in fixed-wing operations because cabin air is taken from the ambient atmosphere, even in pressurized aircrafts, when warmed it may contain very little humidity. This may lead to the drying of the patient’s secretions and discomfort during flight 9.

Noise: Modern aircraft produce a substantial amount of noise. The cabins of most airplanes are quiet enough for conversation and patient evaluation but cabins of helicopters are so loud as to preclude auscultation of the lung sounds. Protective headphones and intercom systems are required.

Vibration: Vibration is a repeating, alternating form of motion. The two major sources of vibration during AMT are the powerplant and turbulence from air in which the aircraft is traveling. In addition to causing fatigue and discomfort, vibrations are transmitted to medical equipment in flight and may be the source of monitoring errors and malfunctions 9.

AMT Team

A variety of flight crew attends patients during AMT. The minority of these transport teams include physicians. The vast majority of helicopter transport teams include a registered nurse. There is considerable controversy whether the presence of physicians during AMT improves patient outcome. For example, physician intervention on AMT has not been proven to improve mortality after a traumatic cardiac arrest ₁₁. Snow et. al. addressed the need for physician presence during 295 physician-manned helicopter flights retrospectively and determined that in only 25% of these flights, was a physician actually necessary ₁₂. The oldest and biggest air rescue company in the world, the “Swiss Air Rescue” provides physicians on almost every aeromedical transport 3.

The needs of patients differ, and a flight crew appropriate to needs of the particular patient being transported should be selected to attend the patient.

Safety of AMT

Aviation aspects: Aeromedical rotary wing aircraft have shown an alarming tendency to crash, with resultant loss of life as well as non-fatal injury ₁₃. In 1986, 14 major EMS helicopter accidents occurred, destroying or substantially damaging 9% of the aeromedical helicopter fleet 2. The National Transportation Safety Board (NTSB) undertook a safety study of helicopter air ambulance operations and concluded that poor weather posses the greatest single hazard to EMS helicopter operations ₁₄.

After publication of the NTSB study, an improvement in the helicopter air ambulance accident rates has occurred ₁₅. The Association of Air Medical Services (AAMS), which began a decade ago, has encouraged appropriate medical direction through their minimum quality standards and more recently through a pilot program of accreditation and the establishment of an independent accrediting commission.

Medical aspects: For some entities, it is known that AMT can be accomplished with minimal risk. For example, if persons with acute myocardial infarction are to benefit from emergency thrombolytic therapy, angioplasty, and other interventions, they may require emergency transfer within hours to the 10% of hospitals that provide these services ₁₆. A number of case series of aeromedical transported acute myocardial infarction patients have demonstrated low incidence of complications 16, ₁₇. A current study of transported and non-transported acute myocardial infarction patients receiving thrombolytic therapy demonstrated no increased incidence of bleeding complications, mortality, or other adverse effects attributable to AMT ₁₈. Medical complications secondary to problems intrinsic to flight have been reported. For example, dysfunction of activity-sensing pacemakers has been reported to be caused by the effect of rotor motion and flight vibration (exogenous electromagnetic signals) during AMT ₁₉, ₂₀.

On the other hand, AMT allows the physician to perform several procedures while en route to the hospital. Procedures that can safely be performed in the air include: Intraosseus infusion, central venous access placement, and chest thoracostomy among others ₂₁. Obviously, all these procedures carry their intrinsic placement complications.

When to use AMT?

AMT in general should be reserved for those patients that are critically or seriously ill and require interventions unavailable at the referring hospital ₂₂. A benefit of receiving these interventions of specialized care must be weighted against the risk of transport. In many instances the decision that transport is required is easy as in the case of the patient requiring neurosurgical intervention where no neurosurgeon is available locally. Other times these types of decisions may be quite difficult. General guidelines can be offered for specific illness categories. For traumatic illnesses the American College of Surgeons committee on trauma and advanced trauma life support has promulgated a collection of recommendations for determining the need for inter-hospital transport of critically ill patients to specialized trauma centers. These include:

• Neurological injury of Glasgow Coma Scale of less than 10 or a falling Glasgow Coma Scale.

• Penetrating injuries or depressed skull fractures, or patients with lateralizing neurologic signs.

• Suspected cardiac or intra-thoracic vascular injuries or major chest wall trauma.

• Patients at the extremes of age (less than 5 or greater than 55 years of age) or those with known pre-existing physiological impairments (i.e. cardiorespiratory disease), may be considered for care in specialized centers.

Organized rules covering the spectrum of non-traumatic surgical illness or medical conditions can not be provided. A final analysis and the decision to transfer a critically ill patient rest upon some assessment of the benefits gained from the transfer and the associated risks.

While air transport offers many benefits, liabilities associated to AMT should also be considered in deciding to use air over ground.

Preparing the Patient for Transport

The preparation of the patient for transport must, of course begin with stabilizing the patient’s medical conditions using appropriate medical measures and subsequently contacting the receiving physician and institution. Physician to physician is necessary to ensure that appropriate information is exchanged and optimizing patient care prior and during transport 3. Patients being transported by air should be evaluated with the effects of pressure and other forces in the aviation environment in mind. Closed gas spaces should be decompressed. Nasogastric decompression and urinary catheter insertion should be considered and they may contribute significantly to patient comfort if not previously performed.

A discussion of the patient’s condition and current therapy with the transport team or service will result in additional recommendations to speed the transport process 3.

Reimbursement

For many years, hospital helicopter transport program charges have not reflected true operating costs 2. Patient revenue from other hospital charges have been used to offset operating losses. Charges for helicopter transport continue to rise, with an average 100 mile round-trip mission costing in excess of $2000 in 1990 2 . This figure represents a 40% increase over the average cost of a similar trip reported in 1989.

In many instances, patient’s charges are unpaid. Unfortunately, patient “dumping” in aeromedicine may lead to ethical, legal, professional, and regulatory dilemmas for emergency professionals and health care institutions. It has been suggested that institutional policies for aeromedical transport of severely injured or ill patients should be instituted, irrespective of the patient’s pay or class ₂₃, ₂₄.

Fixed wing operations are generally conducted in a urgent or non-emergent fashion so general practice across the country is to not perform fixed wing transport at a loss, requiring payment at the time of the service.

Conclusions

Emergency aeromedical systems have become an integral part of the practice of critical care medicine. These systems provide specialized care for the severely injured and ill, and thus may be needed for patients of health care practitioners of all types. Understanding the medical aspects of flights and the capabilities of the aeromedical environment will help the non-aeromedical practitioner to use these resources in a safe and a proper manner.