Emergency Medical Transportation Guidelines for Nurses in Primary
Care
Chapter 2 - Aeromedical Evacuation
The Basics of Aerospace Medicine
Physiological Effects of Flight on the Human Body
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).
Table 2-1: Gaseous Components of the Atmosphere
Gas |
% of Total |
Oxygen |
21 |
Nitrogen |
78 |
Trace gases |
1 |
Total |
100 |
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).
Table 2-2: Barometric Pressure at Various Altitudes
Altitude (ft)* |
Barometric
Pressure:
In mm Hg |
Barometric
Pressure:
In psi |
Sea level |
760 |
14.7 |
1 000 |
733 |
14.2 |
3 000 |
681 |
13.2 |
5 000 |
632 |
11.8 |
7 000 |
586 |
11.3 |
10 000 |
523 |
10.1 |
12 000 |
483 |
9.3 |
14 000 |
447 |
8.6 |
16 000 |
412 |
8.0 |
18 000 |
380 |
7.3 |
20 000 |
350 |
6.8 |
25 000 |
282 |
5.5 |
30 000 |
226 |
4.4 |
35 000 |
179 |
3.5 |
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.
Table 2-3: Effects of Altitude on a Typical Healthy Person
Altitude* |
Oxygen Saturation |
Effects on Vision |
Other Effects |
4000 to 5000 ft ASL |
>93% (no effects of hypoxia) |
Some impairment of night vision |
|
5000 to 8000 ft ASL |
90% to 93% |
Greater impairment of night vision |
|
8 000 to 10 000 ft ASL |
88% to 90% |
Some impairment of day vision |
Reduced ability to perform tasks |
10 000 to 14 000 ft ASL |
83% to 85% |
|
Critical loss of judgment, accompanied by euphoria and fatigue |
14 000 to 20 000 ft ASL |
<83% |
|
Severe loss of judgment, accompanied by belligerence or euphoria |
> 20 000 ft ASL |
Severe hypoxia |
|
Death occurs in a short time |
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).
Table 2-5: Time of Useful Consciousness (TUC) at Various
Altitudes
Altitude
(ft) |
Altitude
(m) |
Approximate TUC*
(minutes) |
18 000 |
5 486 |
20--30 |
25 000 |
7 620 |
3--5 |
30 000 |
9 144 |
1.5 (90 seconds) |
40 000 |
12 192 |
≤0.25 (15 seconds or less) |
*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.
Table 2-6: Volume Expansion of Gases
Altitude (ft) |
Altitude (m) |
Relative Gas Volume |
0 |
0 |
1.0 |
5 000 |
1500 |
1.2 |
10 000 |
3000 |
1.5 |
15 000 |
4500 |
1.9 |
18 000 |
5400 |
2.0 |
20 000 |
6000 |
2.4 |
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
100150 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.
|