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Figure 3, demonstrating the decrease in barometric pressure with increasing altitude.
Physiological effects of hypoxia at different altitudes are given in Table 2.
ICAO Preliminary Unedited Version — October 2008 II-1-6
1) 2 450 m (8 000 ft): The atmosphere provides a blood oxygen saturation of approximately 93 per cent
in the resting individual who does not suffer from cardiovascular or pulmonary disease.
2) 3 050 m (10 000 ft): The atmosphere provides a blood oxygen saturation of approximately
89 per cent. After a period of time at this level, the more complex cerebral functions such as making
mathematical computations begin to suffer. Flight crew members must use oxygen when the cabin
pressure altitudes exceed this level.
3) 3 650 m (12 000 ft): The blood oxygen saturation falls to approximately 87 per cent and in addition to
some arithmetical computation difficulties, short-term memory begins to be impaired and errors of
omission increase with extended exposure.
4) 4 250 m (14 000 ft): The blood oxygen saturation is approximately 83 per cent and all persons are
impaired to a greater or lesser extent with respect to mental function including intellectual and
emotional changes.
5) 4 550 m (15 000 ft): This altitude gives a blood oxygen saturation of approximately 80 per cent and
all persons are impaired, some seriously.
6) 6 100 m (20 000 ft): The blood oxygen saturation is 65 per cent and all unacclimatized persons lose
useful consciousness within 10 minutes (TUC, the time of useful consciousness, is determined
generally from the time of onset of hypoxia to the time when purposeful activity, such as the ability to
don an oxygen mask, is lost). At 6 100 m (20 000 ft), the TUC is 10 minutes. (It should be mentioned
that a given volume of gas at sea level doubles in volume when the pressure is dropped to that at
approximately 5 500 m (18 000 ft).)
7) 7 600 m (25 000 ft): This altitude, and all those above it, produce a blood oxygen saturation below
60 per cent and a TUC of 2.5 minutes or less. Above this altitude, the occurrence of bends (nitrogen
embolism) begins to be a threat.
8) 9 150 m (30 000 ft): The TUC is approximately 30 seconds.
9) 10 350 m (34 000 ft): The TUC is approximately 22 seconds. Provision of 100 per cent oxygen will
produce a 95 per cent blood oxygen saturation (at 10 050 m (33 000 ft), a given volume of gas at sea
level will have approximately quadrupled).
10) 11 300 m (37 000 ft): The TUC is approximately 18 seconds. Provision of 100 per cent oxygen will
produce an oxygen saturation of approximately 89 per cent. When this altitude is exceeded, oxygen
begins to leave the blood unless positive-pressure oxygen is supplied. (A given volume of gas
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
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