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approximately quintuples when the altitude changes from sea level to 11 600 m (38 000 ft).)
11) 13 700 m (45 000 ft): The TUC is approximately 15 seconds and positive-pressure oxygen is of
decreasing practicality due to the increasing inability to exhale against the requisite oxygen pressure.
Table 2.— Effects of hypoxia at different altitudes
ICAO Preliminary Unedited Version — October 2008 II-1-7
Figure 3.— Barometric pressure and altitude
A matter of practical importance is that barotrauma may occur at low altitudes because of the steep slope
of the altitude pressure curve at lower levels. Even normal shifts in pressurized cabins can result in
barotrauma since descent from only 2 000 m (6 500 ft) to sea level entails a pressure differential of
150 mm Hg.
Hypoxia
An important characteristic of biological significance of the flight environment is the decrease in partial
pressure of oxygen with increasing altitude.
Hypoxia can for practical purposes be defined as decreased amounts of oxygen in organs and tissues,
i.e. less than the physiologically “normal” amount.
In aviation medicine it is a subject of particular interest due to the fact that pressurized cabins are not
usually maintained at sea-level values and therefore cabin pressures may add a moderate degree of
hypoxia at altitude. Hypoxia has been the object of many studies, and several attempts have been made to
classify and define its stages and varieties. A classification that has gained wide acceptance defining four
varieties of hypoxia is as follows:
a) Hypoxic hypoxia is the result of a reduction in the oxygen tension in the arterial blood and hence in
the capillary blood. It may be caused by low oxygen tension in the inspired air (hypobaric hypoxia)
and is therefore of special significance when considering flight crew. Other causes are
hypoventilatory states, impairment of gas exchange across the alveolar-capillary membrane, and
ventilation-perfusion mismatches.
b) Anaemic hypoxia is the result of a reduction in the oxygen-carrying capacity of the blood. Decreased
amount of haemoglobin available to carry oxygen may be caused by reduced erythrocyte count,
ICAO Preliminary Unedited Version — October 2008 II-1-8
reduced haemoglobin concentration, and synthesis of abnormal haemoglobin (e.g., sickle cell
anaemia). Anaemia is an important consideration when assessing the advisability of air transportation
for passengers with certain clinical entities.
c) Ischaemic hypoxia is the result of a reduction in blood flow through the tissues. It may be caused by
obstruction of arterial supply by disease or trauma, and by general circulatory failure. Coronary artery
disease is of major concern when assessing applicants for licences.
d) Histotoxic hypoxia is the result of an interference with the ability of the tissues to utilize a normal
oxygen supply for oxidative processes. It may be caused by certain biochemical disorders as well as
poisoning and may be of concern in crash survivability.
Subjective symptoms Objective signs
Breathlessness; dyspnoea
Headache
Dizziness (giddiness)
Nausea
Feeling of warmth about face
Dimness of vision
Blurring of vision
Double vision (diplopia)
Confusion; exhilaration
Sleepiness
Faintness
Weakness
Stupor
Hyperpnoea or hyperventilation
Yawning
Tremor
Sweating
Pallor
Cyanosis
Drawn, anxious facies
Tachycardia
Bradycardia (dangerous)
Poor judgement
Slurred speech
Incoordination
Unconsciousness; convulsions
Table 3.— Signs and symptoms of hypoxia
In aviation, hypobaric hypoxia is by far the most common form of hypoxia. The symptoms produced
in the body by hypoxia are both subjective and objective. Rarely are all the signs and symptoms found in
any one person. Table 3 shows common signs and symptoms which might occur. It is difficult to state
precisely at what altitude a given individual will react (i.e., show symptoms). The threshold of hypoxia is
generally considered to be 1 000 m (3 300 ft) since no demonstrable physiological reaction to decreased
atmospheric pressure has been reported below that altitude. In practice, however, a significant decrement
in performance does not occur as low as that, but as altitude increases above that level the first detectable
symptoms of hypoxia begin to appear and a more realistic threshold would be around 1 500 m (5 000 ft).
Symptoms become more pronounced above 3 000 m (10 000 ft) which sets the limit for flight in
unpressurized aircraft unless oxygen is carried on board. Pressurization systems are commonly designed
to provide a physiologically adequate partial pressure of oxygen in the inspired air. In most passenger
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Manual of Civil Aviation Medicine 1(59)
