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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
aircraft, the cabin pressure at cruising level corresponds to an ambient altitude of 1 500 to 2 450 m (5 000
to 8 000 feet).
PROTECTIVE SYSTEMS
Cabin pressurization
Cabin pressurization is one of the examples of technological solutions to a physiological problem in
relation to aviation. In most modern commercial aircraft the problems of hypoxia and decompression
symptoms are overcome by pressurizing the aircraft cabin to maintain a pressure that is compatible with
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ICAO Preliminary Unedited Version — October 2008 II-1-9
normal physiological needs.
It would seem ideal to maintain sea-level pressure in an aircraft cabin at all times. This solution is
usually impractical due to weight penalties and technical considerations. For these reasons, aircraft cabins
are designed with pressure differentials which represent the compromise between the physiological ideal
and optimal technological design. The pressurization characteristics of different commercial aircraft types
are similar, with minor variations. In general, while the aircraft rate of climb might be in the order of
1000-3 000 ft/min (5-15 m/s) at lower altitudes, cabin altitude increases at a rate of about 500 ft/min (2.5
m/s) which represents an acceptable physiological compromise to equilibrate pressures within the body
and the surrounding environment with a minimum of discomfort. On descent, the usual rate is no more
than 300 ft/min (1.5 m/s).
The normal method of achieving cabin pressurization is by obtaining compressed air from the engine
compressor, cooling it and leading it into the cabin. The pressure level is then set by controlling the rate
of escape of the compressed air from the cabin by means of a barometrically operated relief valve.
Figure 4 indicates a typical pressure differential between the ambient altitude and cabin altitude for a
commercial aircraft.
Figure 4.— Aircraft and cabin altitudes for a commercial aircraft during a typical flight1
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