Emergency Medical Transportation Guidelines for Nurses in Primary Care

Chapter 2 - Aeromedical Evacuation

The Basics of Aerospace Medicine

• The Atmosphere
• Gas Laws
• Types of Aircraft

Physiological Effects of Flight on the Human Body

• Altitude and Oxygen Delivery
• Hypoxia
• Altitude and Expansion of Trapped Gases
• Head and Neck (Face, Eyes, Ears, Nose and Throat)
• Respiratory System
• Cardiovascular System
• Gastrointestinal System
• Musculoskeletal System
• Neurological System (CNS)
• The Skin (Integumentary System)
• Barobariatrauma in Obese Clients

The basics of aerospace medicine

The atmosphere

The atmosphere is the envelope of gases surrounding the Earth. It is divided into four stratified layers: the troposphere, the stratosphere, the ionosphere and the exosphere. The troposphere is the layer closest to the earth, extending upward for 50000 to 60000 ft (15240 to 18290 m) at the equator and to 25000 to 30000 ft (7620 to 9145 m) at the poles.

All unpressurized fixed-wing aircraft and most pressurized fixed-wing aircraft used for medevac fly in the troposphere. Unpressurized fixed-wing aircraft usually fly within the first 10 000 ft (about 3000 m) above the ground. As an aircraft ascends in the troposphere, the following factors come into play:

• the temperature drops (by 2°C for every 1000 ft [300 m] ascended)
• atmospheric pressure falls
• water vapor is reduced
• weather problems and turbulence occur

Composition

The atmosphere is composed primarily of oxygen and nitrogen (Table 2-1).

Barometric (Atmospheric) Pressure

Barometric (atmospheric) pressure is the pressure exerted against an object or a person by the atmosphere. The pressure is usually measured in millimeters of mercury (mm Hg) for medical purposes. As an aircraft ascends, barometric pressure falls (see Table 2-2).

Note: psi = pounds per square inch.
*1000 ft = 304.8 m.

Gas Laws

Changes in atmospheric pressure affect the human body according to the following laws governing atmospheric gases.

Dalton's Law

The total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases present.

Physiological significance: Oxygen transfer from the air to the vital organs of a human being is a direct result of atmospheric pressure. Increasing altitude results in a drop in atmospheric pressure. As this occurs, the pressure of the individual gaseous components in the atmosphere also decreases. Therefore, the availability of oxygen declines as altitude increases, which results in oxygen deficiency (hypoxia). Evenhealthy people will suffer hypoxia during flight, and the impact on a seriously ill or injured person is greater than that on a healthy person. (See "Hypoxia," below, this chapter)

Boyle's Law

The volume of a gas is inversely proportional to its pressure when temperature remains constant.

Physiological significance: As altitude increases, atmospheric pressure drops, and gases (including gases trapped in any body cavity) expand. The expansion of gases causes an increase in the pressure on surrounding tissues and may result in tissue damage. This expansion of gases explains the effects of changes in atmospheric pressure on ears, sinuses, teeth and the gastrointestinal tract. Gas in the middle ear or the sinuses that expands under these conditions may not be vented adequately, which can result in pain, inflammation and, in the case of the middle ear, the possibility of rupture of the ear drum.

Henry's Law

The amount of gas that will dissolve in a solution and remain in solution is directly related to the partial pressure of the gas over the solution.

Physiological significance: Henry's law explains the phenomenon of decompression sickness. As decompression occurs, nitrogen may evolve out of solution in the body's tissues, causing localized irritation, localized manifestations such as the "bends" and skin manifestations, and systemic responses such as neurological effects or shock.

Types of Aircraft

Pressurized Aircraft

Pressurized aircraft can fly at higher altitudes, including those that are dangerous to human life, while maintaining physiologically compatible conditions inside the cabin. The aviation benefits of using a pressurized aircraft and flying at higher altitudes are the ability to fly over bad weather and improvements in gas mileage. In medical terms, use of a pressurized aircraft allows control of the atmospheric pressure within the aircraft ("cabin altitude") to meet the client's needs. Pilots and air medical escorts can work together to provide the optimal cabin altitude for the client, according to clinical needs and aviation safety.

See Appendix 4-1, "Suggested Cabin Altitude Restrictions," in chapter 4, "Primary Care during Transport," for optimum cabin altitudes for specific clinical problems.

Types of Pressurized Aircraft

• Citation 1
• King Air Lear Jet
• Electra
• Boeing 737
• Hawker Siddeley 748

Unpressurized Aircraft

Unpressurized aircraft are useful for transporting clients with non-emergency conditions. However, the role of unpressurized aircraft in the transport of acutely ill clients is severely limited by the altitude restrictions indicated for various medical conditions (see Appendix 4-1, "Suggested Cabin Altitude Restrictions," in chapter 4, "Primary Care during Transport").

These restrictions force the unpressurized aircraft to fly at altitudes much lower than usual. This factor has several important implications for the transport of acutely ill clients:

• Lower-altitude trips may take longer because of inclement weather, which delays the arrival and subsequent treatment of the client at the receiving hospital
• Greater turbulence at lower altitudes may result in:
  • more hemorrhage in a client with injuries to an organ (e.g., liver or spleen)
  • more pain, especially in clients with musculoskeletal trauma
  • greater anxiety leading to higher oxygen demands and resultant cardiovascular or pulmonary deterioration
  • greater risk of vomiting and possible pulmonary aspiration
• Greater turbulence at lower altitudes will also directly affect medical care by:
  • making invasive procedures such as initiating an IV line more difficult
  • adversely affecting the performance of medical personnel because of air sickness (i.e., nausea, vomiting, faintness, anxiety)

Aviation considerations may prevent flight at the altitude that offers the best cabin altitude for the client. In such situations, overall safety must be the major consideration.

Types of Unpressurized Aircraft

• Cessna 185
• Twin Otter
• Beechcraft 1900
• Beaver
• Chinook helicopter

Physiological Effects of Flight on the Human Body

Altitude and Oxygen Delivery

The availability of oxygen declines with increasing altitude because of a drop in barometric pressure (according to Dalton's law; see above). The higher the cabin altitude, the lower the atmospheric pressure inside the aircraft and the more significant the effect on tissue oxygenation (Table 2-3). These changes are most pronounced in unpressurized aircraft, where cabin altitude is essentially the same as true altitude. See "Hypoxia," below, this chapter.

Note: ASL = above sea level.
*1000 ft = 304.8 m.

Hypoxia

Hypoxia is a state of oxygen deficiency causing impairment of bodily functions. Onset may be gradual or rapid. Intellectual impairment may be manifested by slow thinking, faulty memory, delayed reaction time, poor judgment and other features. There are both subjective and objective manifestations of hypoxia (Table 2-4).

Table 2-4: Clinical Manifestations of Hypoxia

Subjective Signs

• Insidious onset
• Visual signs
  • Night vision reduced at 4000 ft (1219 m)
  • Day vision reduced at 15 000 ft (4572 m)
  • Blurred vision
  • Tunnel vision
• Air hunger
• Apprehension
• Fatigue
• Nausea
• Headache
• Dizziness
• Confusion
• Euphoria, belligerence, overconfidence
• Insomnia
• Hot or cold flashes
• Numbness
• Tingling

Objective Signs

• Dyspnea
• Hyperventilation
• Cyanosis (late sign)
• Tremors, muscle incoordination
• Decreased level of consciousness (confusion, stupor, unconsciousness)
• Restlessness
• Euphoria, belligerence
• Clamminess
• Tachycardia or bradycardia
• Tachypnea
• Hypertension (initially)
• Hypotension (late sign)
• Seizures
• Arrhythmia

Types of Hypoxia

• Hypoxic hypoxia occurs as a result of interference in the movement of oxygen from the alveoli of the lungs into the bloodstream (e.g., in severe asthma, hyperventilation, pneumonia, emphysema or pneumothorax). This is the type of hypoxia that occurs with altitude.
• Hypemic (anemic) hypoxia occurs when there is a reduction in the oxygen-carrying capacity of the blood. (e.g., in anemias, carbon monoxide poisoning, hemorrhage or circulatory malfunction).
• Stagnant hypoxia occurs when there is a decrease in blood flow to the tissue cells (e.g., in shock, prolonged cold or congestive heart failure).
• Histotoxic (toxic) hypoxia occurs when the tissues are poisoned by some toxic substance, such that the cells are unable to utilize the oxygen (e.g., with drugs, cigarette smoke, carbon monoxide or alcohol).

Factors Influencing Development of Hypoxia

• Altitude: Tolerance decreases as altitude increases
• Rate of ascent: Tolerance decreases as rate increases
• Time at altitude: Tolerance decreases as time at altitude is prolonged
• Individual tolerance: Individual variation in tolerance may be due to individual metabolic rate, diet and other factors
• Physical fitness: Tolerance increases with physical fitness
• Physical activity at altitude: Tolerance decreases with exercise
• Psychological factors: Oxygen consumption is greater than normal in psychologically disturbed people
• Environmental temperatures: Tolerance decreases with extreme cold or heat
• Medications, toxic substances, smoking: Oxygen utilization is inhibited by some drugs and toxins (e.g., carbon monoxide); tolerance decreases with smoking

Time of Useful Consciousness

Time of useful consciousness (TUC) is the period of time from the interruption of oxygen supply to the time when useful function is lost (when the person may be awake and conscious but incapable of taking proper action).

*Individual tolerance varies.

Management

• Identify people at risk before transport
• Supply oxygen and titrate it to maintain saturations as high as possible (unless contraindicated, for example, in COPD; see below)
• Use pulse oximeter (if available) to monitor oxygen saturation
• Bring adequate amounts of oxygen (see Appendix 3-1, "Administration of Oxygen in Flight," in chapter 3, "General Nursing Care Considerations in Aeromedical Evacuations")
• Because the aircraft environment is very dry, humidify the oxygen, unless flight time is expected to be less than 1 hour
• Treat underlying causes of hypoxia (e.g., administer blood for severe anemia or acute and significant blood loss)
• Reduce cabin altitude to minimize the hypoxia associated with flight

Clients with COPD may have chronic hypercapnia, and their respiratory drive may be based on their hypoxic state. In such clients, excessive elevation of oxygen saturation through administration of oxygen may result in respiratory depression. Despite this possibility, it is much more dangerous to withhold oxygen. Supervised oxygen delivery is indicated. If pulse oximetry is available, oxygen should be delivered so as to maintain saturations between 90% and 93%. (See Appendix 3-1, "Administration of Oxygen in Flight," in chapter 3, "General Nursing Care Considerations in Aeromedical Evacuations," for details of oxygen delivery)

Oxygen Paradox

When oxygen is supplied to a person who is hypoxic, the symptoms may initially appear to worsen. This condition is referred to as the oxygen paradox and will disappear after about 15 seconds. The oxygen supply should not be stopped.

The explanation for this phenomenon is not well understood. It is believed that oxygen delivery results in depression of the hypoxic drive. The brief interruption in breathing that results when the hypoxic drive is suppressed causes a short period of cerebral hypoxia and clinical deterioration. The situation resolves as the client's ventilatory drive is depressed and the partial pressure of carbon dioxide (Pco₂) rises, stimulating respirations.

Altitude and Expansion of Trapped Gases

With increasing altitude, gases within body cavities expand (according to Boyle's law; see above and Table 2-6). Such expansion does not result in any difficulty if the concomitant pressure can be relieved. However, if the gases are "trapped" in an organ with inelastic walls and the gases continue to expand within the walls of the organ, some degree of pain and other clinical symptoms and signs may be experienced.

Head and Neck (Face, Eyes, Ears, Nose and Throat)

Aviation factors affecting disorders of the HEENT:

• Reduced partial pressure of oxygen (hypoxemia)
• Reduced atmospheric pressure (gas expansion)
• Decreased presence of water vapor (dehydration)
• Gravitational forces
• Motion sickness
• Vibration

Eye

The following eye injuries may necessitate air medevac:

• penetrating trauma
• chemical or thermal injury
• problems such as acute glaucoma

After penetrating trauma or recent surgery, gas may be trapped in the eyes. Flight at even low cabin altitudes may cause sutures to rupture. Gas can expand, exerting pressure on the blood vessels and the optic nerve. Intraocular contents may be lost.

Hypoxia caused by increasing altitude (ascent) complicates the picture. The retina requires more oxygen per cell than any other tissue. Hypoxia may result in retinal vasodilatation (which may cause rebleeding), increased intraocular pressure and aggravation of pre-existing eye diseases.

Middle Ear: Barotitis Media

Barotitis media is inflammation of the middle ear resulting from an increase or decrease in pressure in the middle ear (according to Boyle's law; see above). It occurs when the pressure differential between the middle ear and the external atmosphere exceeds 100–150 mm Hg. Barotitis media occurs more commonly at lower altitudes, where changes in barometric pressure are greatest. Severity is greater at low temperatures. If the pressure on the tympanic membrane is not relieved, it may cause tissue damage, rupture of the ear drum or see page of serous fluid into the middle ear cavity.

Risk Factors

• Intercurrent upper respiratory tract infection
• Otitis media
• Soft-tissue injury secondary to head or facial trauma

Additional Stresses

• During sleep, the swallowing reflex is diminished and the middle ear is not ventilated as often as when the client is awake
• Breathing 100% oxygen on long flights can cause ear discomfort and distress several hours after descent

Signs and Symptoms

• Feeling of fullness in the ear
• Hearing dull
• Pain and tenderness
• Rupture of tympanic membrane (during ascent, the eardrum is pushed outward, whereas during descent, the eardrum is actually drawn inward)
• Bleeding from the ear

Management: Considerations for Transport

• Before ascent and descent, instruct client about the importance of clearing the ears
• Teach maneuvers to equalize pressure:
  •  Yawning
  •   Swallowing
  •   Extension of lower jaw
  •   Valsalva maneuver: close the mouth, pinch the nostrils and blow sharply; several short blows are more effective than one continuous effort
• Have infants feed from a bottle (watch for gastric distension and gas)
• Administer decongestants, including topical vasoconstrictors (e.g., xylometazoline)
• Request pilot to ascend and descend slowly and gradually
• May be necessary to restrict cabin altitude for high-risk clients (e.g., children and anyone with upper respiratory tract infection [URTI])
• Awaken sleeping passengers and clients before descent so they can consciously clear their ears

Sinuses: Barosinusitis

Barosinusitis is an inflammation of the soft tissues in the sinuses due to positive and negative pressure changes that occur as a result of changes in barometric pressure (according to Boyle's law; see above).

Signs and Symptoms

• Dull to sharp pain below one or both eyes or, occasionally, in the cheek bones
• Lacrimation
• Nosebleed

Management: Considerations for Transport

• Administer decongestants or topical vasoconstrictors (e.g., xylometazoline)
• On descent, recommend Valsalva maneuver, but do not allow the client to perform this maneuver during ascent, as it will worsen the situation
• Advise vigorous nose blowing
• Use topical or systemic vasoconstrictors and analgesics in accordance with the severity of the condition
• If possible, avoid transporting the client by air if he or she has a concurrent URTI
• Restriction of cabin altitude or a more gradual descent (or both) will help

Teeth: Dental Pain (Barodontalgia)

Teeth with cavities or recent fillings may be sensitive to gas expansion during ascent (according to Boyle's law; see above). Toothache and pain (barodontalgia) may result. Usually the pain is in a single tooth.

Management: Considerations for Transport

• Descend to a lower altitude
• Take preventive measures (dental care)
• Restrict flying for at least 24 hours after invasive dental work (e.g., root canal)

Respiratory System

Aviation factors affecting lower respiratory tract conditions (e.g., asthma, COPD, bronchiolitis):

• Reduced atmospheric pressure (gas expansion)
• Decreased presence of water vapor (dehydration)
• Gravitational forces
• Reduction in partial pressure of oxygen leading to hypoxia (see "Hypoxia," above, this chapter)

Overpressurization syndrome (relating to Boyle's law; see above) may develop when alveoli spontaneously rupture in association with gas expansion during ascent. This is most common in clients with air-trapping disease such as asthma, bronchiolitis or emphysema. Sudden decompression in the aircraft results in rapid expansion of gases, which could result in pneumothorax, pneumomediastinum or air embolus.

Pre-existing pneumothorax could become a tension pneumothorax if not treated with appropriate decompression (with a chest tube) before the flight. For a client who has had a chest tube, extreme caution should be taken if the client must be transported by air within 7 days after the tube is removed.

Cardiovascular System

Aviation factors affecting cardiovascular conditions:

• Reduced partial pressure of oxygen (hypoxemia)
• Reduced atmospheric pressure (gas expansion)
• Decreased presence of water vapor (dehydration)
• Gravitational forces

Cardiorespiratory stresses associated with flight, such as hypoxia, fatigue and increased catecholamine levels, can be hazardous for anyone with cardiovascular disease.

A client with unstable angina or MI is at risk of more ischemia if not managed appropriately in flight. The potential for arrhythmias must be recognized.

Expansion of gases in the abdominal or chest cavity may impair venous return to the heart, reducing cardiac output and compromising tissue perfusion

Gastrointestinal System

Aviation factors affecting GI conditions:

• Reduced atmospheric pressure (gas expansion)
• Reduced water vapor (dehydration)
• Gravitational forces
• Turbulence

Gas pressure in the GI tract is normally equal to the surrounding atmospheric pressure. With increasing altitude (ascent), this gas expands (according to Boyle's law; see above), but pressure can usually be equalized by belching or passing flatus. However, gas expansion in a client with closed-loop intestinal obstruction may cause significant pain, nausea, vomiting, syncope and deterioration; in the worst case, a partial obstruction may be converted into a complete obstruction.

Perforation of a bowel wall may occur if the wall has already been weakened by ulceration, diverticulitis or surgical anastomosis. Gaseous distension of the stomach may limit diaphragmatic excursion, compromising respiratory function. Expansion of gas in an inflamed appendix could result in rupture.

Air sickness is a form of motion sickness resulting from many factors, including movement of the aircraft. It is usually not caused by any single factor. The loss of familiar orientation in flight and apprehension about safety may produce tension, which in turn results in nausea and vomiting. Reassuring clients and making them feel comfortable and at ease can do much to reduce the occurrence of air sickness. For more information about motion sickness, see "Gastrointestinal System," in chapter 4, "Primary Care during Transport"

Fatigue, overindulgence in alcoholic beverages or dietary indiscretion may also predispose a person to air sickness. There will be less danger from these sources if the client has been properly prepared for the flight.

Musculoskeletal System

Aviation factors affecting musculoskeletal conditions:

• Reduced partial pressure of oxygen (hypoxemia)
• Reduced atmospheric pressure (gas expansion and tissue swelling)
• Gravitational forces

Reduced atmospheric pressure during flight causes injured tissue to swell. This can result in neurovascular compromise, particularly if there are casts or restrictive splints on the extremity. Traction splints must not be based on free-hanging weight systems but should be replaced with transport-compatible traction splints

Neurological System (CNS)

Aviation factors affecting neurological disorders:

• Reduced partial pressure of oxygen (hypoxemia)
• Reduced atmospheric pressure (gas expansion and tissue swelling)
• Gravitational forces
• Motion sickness
• Lack of humidity (especially dehydration of the cornea in an unconscious person)
• Increased risk of seizure activity (see below) caused by "flicker vertigo" from strobe lights on aircraft

A decrease in atmospheric pressure increases intracranial pressure, reducing blood flow. The resulting tissue hypoxia compounds the problem, leading to further edema.

Seizure activity may occur as a result of anxiety, hypoxia or hyperventilation. The risk of seizures may be enhanced in any client with head injury or a history of seizures.

Certain skull fractures, particularly basal fractures involving the sinuses, have the potential to introduce air into the brain, a condition called pneumoencephalopathy. Gas trapped in the brain is very dangerous for two reasons:

• Brain tissue is highly sensitive to damage because of its soft, pliable consistency
• The rigidity of the skull means that the brain cannot adapt to the volume expansion

The Skin (Integumentary System)

Aviation factors affecting conditions of the skin:

• Reduced partial pressure of oxygen (hypoxemia)
• Reduced atmospheric pressure (gas expansion)
• Decreased presence of water vapor (dehydration)

Skin Wounds (Lacerations, Surgical Incisions)

Air may be trapped in certain wounds, so there may be increased tension on suture lines or compression of local circulation as the gas expands with ascent.

Bleeding may increase during flight. Therefore, control bleeding before transport.

Burns

The condition of clients with inhalation burns may worsen during flight because of increased swelling of the airways, resulting in hypoxia. Inhalational injury may also result in carbon monoxide poisoning and poisoning by other types of inhaled substances. Such poisoning may result in both hypemic and histotoxic hypoxia. Oxygen saturations may not be accurate in the presence of carbon monoxide poisoning!

Clients with major burns are at risk of hypothermia, as the skin represents a very important means of thermoregulation. Maintenance of a warm cabin temperature is important.

Generalized swelling may be enhanced. Circumferential burns may cause neurovascular compromise and impair respiratory excursion.

Barobariatrauma in Obese Clients

Adipose tissue has greater nitrogen content than other tissue. This nitrogen can be released at high altitudes (according to Henry's law and Boyle's law; see above). The fragility of the cell membrane in fat increases the risk of fat and nitrogen emboli.

Obese clients may be at greater risk during long, high-altitude flights and may experience dyspnea, chest pain, petechiae on the upper body, pallor, tachycardia or tachypnea, or any combination of these.