Air Medical Transport - Clinical Tree

Key Concepts

Air medical transport (AMT) is a critical component of a comprehensive health care system and a vital link for rural communities and critical access hospitals to distant emergency care.
Boyle’s law and Dalton’s law have the greatest impact and explain the development of hypoxia and most common altitude-related symptoms. Other stresses of flight that can affect the patient or crew include temperature fluctuations, dehydration, noise, and vibration.
Although most flight programs do both primary (scene flights) and secondary (interfacility) response, ground ambulance remains the primary means of out-of-hospital and interfacility patient transport.
The helicopter offers several advantages over other transport vehicles, including reduction in travel time by up to 75%, ability to avoid common ground delays (traffic, obstacles, and so on), and ability to fly into locations that may be inaccessible to other modes of travel.
All air medical services require involvement of a medical director responsible for supervising, evaluating, and ensuring the quality of medical care.
As a general rule, helicopters are less useful in urban settings because of the proximity of health care facilities.
Helicopter emergency medical services (HEMS) represents the only modality by which nearly 28% of United States residents have timely access (within 1 hour) to level I or level II trauma centers.
HEMS may benefit in other time-critical situations, including ST-elevation myocardial infarction (STEMI) patients going to a catheterization laboratory and acute stroke patients going to regional stroke centers.
Safety is the predominant concern of air medical operations. Helicopter shopping , the practice of a requesting EMS agency or hospital calling numerous HEMS programs after other programs have declined the flight because of bad weather, must be avoided.

Foundations

Background and Importance

Air medical transport (AMT) is a critical component of a comprehensive health care system, EMS system, and a vital link for rural communities and critical access hospitals to distant emergency care. It is an essential tool to support the regionalization of specialty care for trauma, stroke, cardiac emergencies, burns, pediatric critical care, and more. Many emergency physicians routinely rely upon medical helicopters to transport critically ill or injured patients from smaller rural or community emergency departments (ED) to tertiary care centers.

AMT dates back more than a century to World War I. As early as 1915, the French evacuated soldiers from Serbia using airplanes as ambulances. In 1918, the United States military converted an airplane for the first recorded United States (US) air ambulance 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. Utilizing the Bell 47 helicopter, wounded soldiers were strapped to litters outside the helicopter and transported to Mobile Army Surgical Hospitals (MASH units). During the Vietnam War, Operation Dustoff transported nearly 1 million wounded from the front lines in larger helicopters staffed with medics to initiate care en route.

AMT had a significant impact on the wounded soldier when considering transport times and mortality. World War I saw battlefield transport times between 12 and 18 hours. Of those surviving transport, mortality was 20%. 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 a 2.4% mortality rate. In Vietnam, no soldier was more than 35 minutes from definitive care, and overall mortality was 2.6%.

Encouraged by the military experience, United States civilian AMT began in 1969 with a hospital-sponsored fixed-wing air medical program. The first US civilian helicopter emergency medical services (HEMS) program was established in 1972.

Aviation Physiology

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

Gas Laws

There are four gas laws important to aviation physiology: Boyle’s law, Charles’ law, Dalton’s law, and Henry’s law ( Tables e13.1 and e13.2 ). The cornerstone of aviation physiology begins with Boyle’s law, which describes the behavior of gases in an enclosed space. Boyle’s law is also a contributing factor to hypoxia along with Dalton’s law. No one is exempt from the effects of hypoxia at altitude, and the most threatening feature is its insidious onset and the knowledge that the onset and severity of symptoms may vary by individual.

TABLE e13.1

The Gas Laws

Gas Law	Principle	Clinical Implication
Boyle’s law	The volume of a unit of gas is inversely proportional to its pressure. As altitude increases and atmospheric pressure decreases, the molecules of gas grow apart, and the volume of gas expands. With descent (increasing atmospheric pressure), gas volumes contract. The result is the expansion and contraction of gases within the closed spaces of the body.	Squeeze injuries from contraction of air and associated soft tissues can occur on descent, resulting in barotitis, barosinusitis, and toothache. Reverse squeeze injuries occur on ascent, leading to an increased volume of the air trapped within the space. Examples include the conversion of a simple pneumothorax into a tension pneumothorax and rupture of a hollow viscus. Medical equipment containing closed air spaces, such as intravenous (IV) tubing and pumps, air splints, ventilators, and endotracheal tube and laryngeal airway cuffs, may also be affected by altitude. Responsible in part for hypoxia at altitude due to fewer molecules of oxygen present per volume of inhaled gas.
Charles’ law	As the volume of a unit of gas rises, the temperature of that volume falls.	Explains why the ambient temperature decreases with increased altitude.
Dalton’s law	The total barometric pressure at any given altitude equals the sum of the partial pressures of gases in the mixture. Oxygen still constitutes 21% of the atmospheric pressure at altitude.	A decrease in arterial oxygen tension with increasing altitude, resulting in hypoxia. Initial physiologic responses to hypoxia include tachypnea and tachycardia. With prolonged exposure, cerebral hypoxia causes headache, nausea, drowsiness, fatigue, unconsciousness, and death.
Henry’s law	The mass of gas absorbed by a liquid is directly proportional to the partial pressure of the gas above the liquid.	Sudden decompression at altitude may result in dysbaric injuries. In scuba diving, rapid ascent can result in gas coming out of solution within the bloodstream, resulting in decompression sickness.

TABLE e13.2

Effects of Altitude on Oxygenation

Altitude (ft)	Barometric Pressure (mm Hg)	P o ₂ (mm Hg)	P ao ₂ (mm Hg)	Pa co ₂ (mm Hg)	Oxygen Saturation (%)
Sea level	760	159.2	103.0	40	98
8000	565	118.4	68.9	36	93
10,000	523	109.6	61.2	35	87
15,000	429	89.9	45.0	32	84
18,000	380	79.6	37.8	30.4	72
20,000	349	73.1	34.3	29.4	66
22,000	321	67.2	32.8	28.4	60

P ao ₂ , Partial pressure of alveolar oxygen; Pa co ₂ , partial pressure of arterial carbon dioxide; PO ₂ , partial pressure of oxygen.

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 altitude increases and temperatures drop, the amount of moisture in the air decreases significantly.

To prevent dehydration during long-distance fixed-wing AMT, fluid intake (oral or intravenous) must be monitored carefully, and all patients should receive humidified medical oxygen. Noise and vibration may represent the most ubiquitous stresses encountered in AMT, and both may interfere with patient care or the function of medical equipment. Hearing protection should always be worn during aircraft operations by patient and crew. Prolonged exposure to environmental extremes may result in fatigue, motion sickness, disorientation, ear damage, and deterioration in task performance.

Specific Issues in Air Medical Transport

Administrative Structure of Air Transport Systems

Air medical services in the United States may be structured in different ways. There has been tremendous growth of medical helicopters in the United States over the last two decades. Data extracted from the ADAMS Database estimates that in 2019 there were 228 HEMS programs flying more than 927 dedicated aircraft throughout the nation. Since the early 2000s, the number of dedicated HEMS aircraft has doubled. During that time, what had been the most common HEMS business model in the United States has changed. The original and “traditional” nonprofit hospital-based operation, sponsored by a single hospital or a consortium of institutions, is now less common than the for-profit community-based programs or hybrid programs. Nearly 70% of the helicopters are for-profit ventures, operated by privately owned (private equity firms) or publicly traded companies. 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 United States Armed Forces provides additional HEMS resources to civilians, but in recent years their role for civilian support has been generally limited to Hawaii and Alaska. Together, the public service and MAST helicopters supply more than 120 additional aircraft for patient transport in the United States. There is no accurate accounting of the number of fixed-wing air ambulance companies or airplanes. Although some hospitals sponsor fixed-wing AMT, it is more common for these programs to be private fee-for-service operations.

Types of Transports

Air medical transports may involve primary or secondary response. Primary responses (“scene flights”) are when the aircraft responds directly to the scene of an accident or illness and transports the patient directly to an appropriate receiving facility. Aircraft involved in secondary responses (interfacility transport) move patients from outlying hospitals to facilities offering higher levels of care. AMT flights may also be classified according to the level of care provided. This may be critical care transport, specialty care transport, or advanced or basic life support.

Air Medical Aircraft

Although ground ambulance remains the primary means of prehospital and interfacility patient transport, AMT has a definite role in the health care delivery system. However, no one aircraft is ideal for the needs of all air medical programs or patients. In addition, certain flight conditions and situations may also influence aircraft and transport consideration.

Helicopters (Rotor-Wing Aircraft)

An estimated 320,000 patients are flown each year by dedicated HEMS operations in the United States. 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, but average transports can be significantly fewer miles. Rotor-wing aircraft have the ability to avoid common traffic delays and ground obstacles and can access rural and remote 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 airport requirements.

Disadvantages to HEMS can include noise, vibration, thermal variances, and other stressors on patients and crew. Weather considerations may at times significantly limit the availability of helicopter transport. Although some medical helicopters can transport two patients, most are configured to transport only one patient. In smaller helicopters, confined spaces and weight restrictions may limit the number of transport personnel or medical equipment that can be carried and may make it more challenging to perform procedures. ( Fig. e13.1 ).

Fig. e13.1, Confined spaces and weight restrictions in many helicopters may limit the number of transport personnel or medical equipment that can be carried and may make it more challenging to perform procedures.

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

Airplanes (Fixed-Wing Aircraft)

Fixed-wing aircraft provide increased range, greater speed, and often more patient, crew, and equipment capacity than most rotor-wing vehicles. Decreased cabin noise and turbulence creates fewer patient management problems and cabin pressurization combats the physiologic impact of altitude and the gas laws. Fixed-wing operations are limited, however, to areas that have airports with runways of appropriate length and refueling facilities. During fixed-wing transports, patient transfers require multiple vehicles for each leg of the transport (i.e., hospital to ground ambulance to airplane).

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.

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