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.3–7

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.

Charles’ Law

Charles’ law follows from the volume effects of Boyle’s law. It notes that as the volume of a unit of gas rises, the temperature of that volume falls. The molecular dispersion seen with increases in gas volume at altitude (Boyle’s law) means there is less chance of molecular collision with resulting generation of heat. Charles’ law explains why the ambient temperature decreases with increased altitude.

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 (P_t = P₁ + P₂ + P₃ … P_n). 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.

Henry’s Law

Henry’s law states that the mass of gas absorbed by a liquid is directly proportional to the partial pressure of the gas above the liquid. Henry’s law has its most familiar applications in diving medicine, in which the increased pressure exerted on gases in the body at depth forces the gases into solution in the bloodstream. Rapid ascent from depth causes the gas to come out of solution within the bloodstream, resulting in decompression sickness. Henry’s law does not carry the same weight in aviation medicine because the degree of change in atmospheric pressure per unit of distance is considerably less than the degree of change in water. However, sudden decompression at altitude may result in dysbarism.

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.

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.

Types of Missions

Air medical missions may involve primary or secondary response. Primary responses (“scene flights”) are responses in which the aircraft serves as the sole means of patient transport to a receiving facility. Aircraft involved in secondary responses—interfacility transport—move patients from outlying hospitals to facilities offering higher levels of care. AMT missions 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 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.

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.^14–19 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.^20–22 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).