Air Medical Transport

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Chapter 191

Air Medical Transport

Perspective

The history of air medical transport (AMT) dates to before World War I, when the French evacuated soldiers from Serbia using airplanes as ambulances as early as 1915. The first recorded use of a U.S. military air ambulance was in 1918 when an airplane was converted to accommodate a litter patient in the rear cockpit. During World War II, more than 1.1 million sick and wounded soldiers were airlifted to the United States during the last 3 years of the war. The Korean War introduced the helicopter to AMT, and more than 20,000 battlefield medical evacuations were flown during the conflict. During the Vietnam War, Operation Dustoff transported nearly 1 million of the injured from the front lines to care.

The impact of AMT on the wounded soldier was clear. During World War II, the average time from injury to definitive care was 6 to 12 hours, with a mortality rate of 5.8%. In Korea, the time was 2 to 4 hours, with 2.4% mortality. In Vietnam, the time was 65 minutes, and mortality was less than 1%.1 Encouraged by the military experience, civilian AMT in the United States was propelled by the 1969 start of the first hospital-sponsored, fixed-wing air medical program. The first civilian helicopter emergency medical services (HEMS) program in the United States was established in 1972.

Aviation Physiology

A working knowledge of aviation physiology is vital to understanding of the effects of AMT on pilots, medical personnel, and patients.2

Boyle’s Law

The cornerstone of aviation physiology is Boyle’s law, which states that the volume of a unit of gas (“unit” defined as a specific number of molecules) is inversely proportional to the pressure on it. In concrete terms, Boyle’s law means that as altitude increases and atmospheric pressure decreases, the molecules of gas grow apart, and the volume of the gas expands. With descent (increasing atmospheric pressure), the molecules are condensed, and gas volumes contract.

Physiologic difficulties from expansion and contraction of gases within the closed spaces of the body may occur with altitude change. Squeeze injuries from contraction of air and associated soft tissues within closed cavities occur on descent and are common causes of barotitis, barosinusitis, and toothache. Reverse squeeze injuries occur on ascent, as decreasing barometric pressure leads to an increased volume of the air trapped within the space, exerting pressure on structures. Ascent injuries can include conversion of a simple pneumothorax into a tension pneumothorax or rupture of a hollow viscus by expansion of intestinal gas. The operation of medical equipment containing closed air spaces, such as intravenous tubing and pumps, air splints, ventilators, and endotracheal tube and laryngeal airway cuffs, may also be affected by altitude.37

Boyle’s law is predominantly responsible for the presence of hypoxia at altitude as there are fewer molecules of oxygen present per volume of inhaled gas at altitude. Similarly, dispersion of molecules of water vapor within a gas volume is seen at height, and “dry air” results.

Dalton’s Law

Dalton’s law states that the total barometric pressure at any given altitude equals the sum of the partial pressures of gases in the mixture (Pt = P1 + P2 + P3Pn). Whereas oxygen still constitutes 21% of the atmospheric pressure at altitude, Boyle’s law notes that each breath brings fewer oxygen molecules per breath to the lungs, and hypoxia results (Table 191-1). The clinical effect of Dalton’s law is manifested as a decrease in arterial oxygen tension with increasing altitude.

Initial physiologic responses to hypoxia include tachypnea and tachycardia. With prolonged exposure, oxygen supply to the brain becomes insufficient to support cerebral metabolism. Progressive cerebral hypoxia causes headache, nausea, drowsiness, fatigue, unconsciousness, and death. Although the onset and severity of symptoms may vary with individuals, no one is exempt from the effects of hypoxia.

Additional Stresses of Flight

Other stresses of flight that can affect the patient or crew include temperature fluctuations, dehydration, noise, and vibration. Temperature changes may produce increases in metabolic rate and oxygen consumption. As noted, at altitude the moisture per volume of air falls. Fluid intake should be monitored carefully during transport, and all patients should receive humidified medical oxygen. Noise and vibration represent the most ubiquitous stresses encountered in AMT, and both may interfere with patient care or the function of medical equipment. Hearing protection should be worn at all times during aircraft operations by patient and crew. Exposures to environmental extremes may result in fatigue, motion sickness, disorientation, ear damage, and deterioration in task performance.

Principles of Air Medical Transport Systems

Administrative Structure

Air medical services may take several forms. Despite a tremendous growth of operations in the past 20 years, the most common type of HEMS program in the United States remains the hospital-sponsored operation transporting patients from outlying referral centers or accident scenes to tertiary care centers. A single hospital or a consortium of institutions may sponsor these flight programs. In 2011, there were more than 220 dedicated HEMS programs operating more than 750 dedicated aircraft throughout the nation.8 Approximately 54% of the programs were hospital sponsored, and the balance were operated by privately owned or publically traded companies. Whereas these for-profit companies represent 46% of the programs, they operate more than 61% of the dedicated helicopters in the United States.8 Public service agencies may also sponsor air medical services or partner with private companies; vehicles used by these programs are often multifunctional aircraft that serve in medical, search and rescue, fire suppression, and law enforcement roles. The Military Assistance to Safety and Traffic (MAST) program operated by the U.S. Armed Forces provides additional HEMS resources to the community,9 but in recent years their role for civilian support has been generally limited to Hawaii and Alaska.9 Together, the public service and MAST helicopters add more than 120 additional aircraft available for patient transport.8 There is no accurate accounting of the number of fixed-wing air ambulance companies or airplanes. Although some hospitals do sponsor fixed-wing AMT, it is more common for these programs to be private fee-for-service operations.

Air Medical Aircraft

Although the ground ambulance remains the primary means of out-of-hospital and interfacility patient transport, the use of the air ambulance has grown significantly since the 1970s. No one aircraft is ideal for the needs of all air medical programs or patients.

Helicopters (Rotor-Wing Aircraft)

The helicopter offers several advantages over other transport vehicles. Traveling “as the crow flies” at speeds of 120 to 180 mph, helicopter transport time is often 75% less than that for an equivalent distance by ground. The service area of helicopter programs is generally up to 150 to 200 miles from its base of operations. Rotor-wing aircraft have the ability to avoid common traffic delays and ground obstacles and can fly into locations that may be inaccessible to other modes of travel. Helicopter landing zone requirements are a disadvantage compared with ground ambulances but offer an advantage over the airport requirements of airplanes.

Disadvantages to HEMS include noise, vibration, thermal variances, and other stressors on patients and crew exaggerated by rotor-wing flight. Weather considerations may significantly limit the availability of helicopter transport. In smaller aircraft, cramped spaces and weight limitations may limit the number of patients, transport personnel, or equipment that can be carried. This may sometimes compromise optimal patient care (Fig. 191-1).

Many helicopter programs permit flight only under visual flight rules. When the weather conditions (ceiling and visibility) fall below established program minimums, a program may decline to undertake a transport for safety reasons. However, an increasing number of programs are equipping their helicopters and training their pilots for instrument flight rules (IFR) to allow safe travel in less favorable weather conditions. IFR flight may facilitate use of fixed locations, such as hospital helipads, but it does not facilitate travel to the scene of illness or injury.

Airplanes (Fixed-Wing Aircraft)

Although rotor-wing missions attract more media attention, fixed-wing flights constitute a significant portion of AMT operations. Fixed-wing aircraft provide increased range, greater speed, and often more patient, crew, and equipment capacity than do rotor-wing vehicles. The decreased cabin noise and turbulence creates fewer patient management problems, and pressurization combats the physiologic impact of altitude. Fixed-wing operations are limited to areas that have airports with runways of appropriate length or condition and refueling facilities. During fixed-wing transports, patient transfers require multiple vehicles to go from hospital to airport and back.

Various fixed-wing aircraft are available for medical transport. These range from unpressurized light planes with single- or twin-piston engines to pressurized turboprops and jets. The selection of the ideal aircraft depends on the nature of the air medical mission.

Air Medical Flight Crew

Air medical crew members represent the broad spectrum of health care providers. AMT services that provide critical care transport, advanced life support, or specialty care transport must staff the vehicle with a minimum of two medical personnel to provide direct patient care.10,11 The majority of AMT programs in the United States provide critical care transport teams composed of one registered nurse and an additional crew member (paramedic, respiratory therapist, physician, or a second nurse); most common is the nurse-paramedic crew.12 AMT crew configuration may also be mission dependent. A service may at times believe that it is appropriate to use a single medical crew member. For example, it may be appropriate while transporting a stable patient on a routine interfacility transport. Certain flight conditions and situations may also necessitate flying with a single crew member, including heat, humidity, altitude, distance, fuel on board, and weight of the patient.12

Flight nurses generally have extensive experience in intensive care units or emergency departments. They may be specialized within the transport team to care for adult, pediatric, or neonatal patients. Paramedics often make their greatest contribution in the transport of critical patients from the scene of illness or injury. Respiratory therapists bring expertise in airway and ventilator management and oxygen delivery systems. Flight physicians may be residents or attending physicians. Much of the early research in AMT crew composition focused on the need for the flight physician. Although this remains controversial, what seems clear is that the crew used by an AMT program must be explicitly tailored to the needs of the community and the patients it serves.

The AMT environment imposes unique considerations on the air medical flight crew that can influence their ability to provide patient care. Human factors work has shown that most medical care procedures are more difficult to perform in an AMT vehicle than in other ground-based settings.13 Auscultation of the heart and lungs, palpation of pulses, performance of cardiopulmonary resuscitation, endotracheal intubation, radio communications while using a respirator or face mask, and recognition of visual alarms may all be impaired while aloft.1419 In addition, fatigue, motion sickness, exposure to engine exhausts, an erratic pattern of work activity, and the high risk involved in AMT operations may affect task performance significantly.2022 Seizures from photic stimuli associated with rotor motion (“flicker illness”) has also been reported.23 High-fidelity simulation of air medical missions can acquaint flight crew to the novel environment, but fiscal and personnel costs may be prohibitive.24

Medical Direction

All air medical services require the active involvement of a physician as medical director responsible for supervising, evaluating, and ensuring the quality of medical care provided by the AMT team.25 Emergency physicians play a significant role, with nearly 50% of all air medical directors having a background in emergency medicine.26 The medical director must have the final authority over all clinical aspects of the air medical service and should ensure that the flight crew have adequate training and qualifications to optimize patient care. Medical care policies and procedures should be established, including specific provisions for on-line and off-line medical control. The Air Medical Physician Association and the National Association of EMS Physicians have established guidelines for the medical director of an air medical service.27,28

Safety

Safety is the predominant concern of air medical operations, and ensuring safe flight is a fundamental part of every flight program.29 Safety must also be an overriding consideration in weighing the risks and benefits of AMT. The role of aircraft pilots and mechanics is essential to the airworthiness of the vehicle, and medical personnel must also be proficient in both routine and emergency operations in and around the aircraft. Checklists may aid in safety practices but alone may not detect significant operational concerns.30 Crew fatigue and other self-imposed stresses that could affect safety, such as the use of prescription or over-the-counter medications, tobacco, and alcohol, must be scrupulously avoided.

Weather requirements (“minimums”) must be strictly enforced. On receipt of a flight request, the pilot must verify the weather conditions and the condition of the aircraft. To ensure impartiality, the pilot should not be told of the patient’s condition or acuity. The pilot maintains the unquestioned right to decline a mission because of aircraft or weather considerations.

The practice of “helicopter shopping” has been a major factor in a number of fatal HEMS events. Helicopter shopping refers to the practice of a requesting EMS agency or hospital calling numerous HEMS operators until one agrees to accept a flight without disclosure to the accepting HEMS operator that other programs have declined the flight because of bad weather or other safety concerns.31,32 The practice has been so common that in 2006, the Federal Aviation Administration issued a letter to all state EMS directors describing helicopter shopping and requesting that they take action to prohibit this practice.33

Landing Zones

Helicopter landing zones are inherently dangerous places. The most obvious risk of injury is impact with rotor blades. This danger is heightened during ground operations because the blades dip lowest to the ground at the slower rotor speeds associated with engine start-up and shutdown. Injuries also may occur as a result of debris being propelled through the air by “rotor wash,” increased noise levels and an inability to hear warnings, and slippery surfaces found on exposed landing sites.

Many hospitals have designated landing areas that are appropriately lit and secured (Fig. 191-2), with fixed coordinates and predesignated liftoff and approach patterns. However, most primary (scene) responses occur at unmarked sites. Ground personnel must be trained to designate and secure a safe landing zone for helicopter operations. AMT programs have an obligation to help train ground staff on proper landing zone setup and conduct (Boxes 191-1 and 191-2).

BOX 191-1

Landing Zone Safety

Vehicles and personnel should be kept at least 100 ft from the landing zone.

Spectators should be kept at least 200 ft from the landing zone.

No smoking or running is permitted within 50 ft of the helicopter.

All items (e.g., intravenous lines, poles) should be kept below shoulder height.

The flight crew opens and closes aircraft doors.

The flight crew directs and supervises the loading and unloading of the patient and equipment.

Ground personnel should use eye and ear protection.

Approach the helicopter only when signaled to do so by the pilot or an onboard crew member.

Approach and depart the helicopter only forward of the rear cabin door and in a crouched position with your head down.

Never approach or depart from the rear of the helicopter.

Stay clear of the tail rotor; it is virtually invisible and extremely dangerous.

If the aircraft is parked on a slope, approach and depart on the downhill side (greatest clearance under the blades).

Keep the landing zone clear of (or hold on to) all loose articles (e.g., hats, scarves, sheets, pillows).

Protect patient from the dust and debris.

Follow the flight crew’s instructions at all times.

In disaster situations and mass casualty incidents, victims, witnesses, and spectators may become hysterical or exhibit signs of an acute situational reaction. These individuals must be kept clear of the landing zone and helicopter at all times. Injured victims who exhibit this behavior should not be triaged for helicopter transport, or they should be transported only with adequate physical or chemical restraints in use.

If you do not know, ask.

Courtesy University of Chicago Aeromedical Network (UCAN), University of Chicago Medical Center and Illinois Association of Air and Critical Care Transport (IAACCT), 2011.

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