The USAF School of Aerospace Medicine has this to say about the stages of
hypoxia:-
The symptomatology of hypoxia may be divided into stages related to
the approximate pressure, the altitudes, and the oxygen saturations of the
blood.
STAGES OF HYPOXIA
Altitude in Feet
Arterial O2
Stage Breathing Air Breathing
100% Saturation (%) O2
_____________________________________________________________________
Indifferent 0 to 10,000 34,000 to 39,000
95 to 90
Compensatory 10,000 to 15,000 39,000 to 42,500 90 to 80
Disturbance 15,000 to 20,000 42,500 to 44,800 80 to 70
Critical 20,000 to 23,000 44,800 to 45,500 70 to 60
_____________________________________________________________________
As shown in the above table the stages of hypoxia are:
Indifferent Stage. The only adverse effect is on dark adaptation,
which is manifest at altitudes as low as 5,000 feet. It emphasizes the need
for oxygen from the ground up during night flights especially in the case of
fighter pilots. Electrocardiographic changes may occur at altitudes as low
as 5,000 feet; there is also an increase in pulse rate and a slight increase
in alveolar ventilation.
Compensatory Stage. Physiological compensations may provide some
defence against hypoxia so that effects are latent unless the exposure is
prolonged, or unless exercise is undertaken. Respiration may increase in
depth or slightly in rate. Cardiac output increases.
Disturbance Stage. In this stage, the physiological compensations do
not suffice to provide adequate oxygen for the tissues; latent oxygen want
becomes manifest. Subjective symptoms may include fatigue, lassitude,
somnolence, dizziness, headache, breathlessness, and euphoria. Occasionally
there are no subjective sensations up to the time of unconsciousness. The
effect on systems is:
a. Special Senses. Both the peripheral and central vision are
impaired and visual acuity is diminished. Extraocular muscles are weak and
uncoordinated, and the range of accommodation is decreased. Touch and pain
are diminished or lost. Hearing is one of the last senses to be impaired or
lost.
b. Mental Processes. Intellectual impairment is an early sign which
makes it impossible for individuals to comprehend their own disability.
Thinking is slow, and calculations are unreliable. Memory is faulty,
particularly for events in the immediate past. Judgment is also poor.
Reaction time is delayed.
c. Personality Traits. There may be a release of basic personality
traits and emotions as with alcoholic intoxication. There may be euphoria,
elation, pugnaciousness, overconfidence, or moroseness.
d. Psychomotor Functions. Muscular coordination is decreased, and
delicate or fine muscular movements may be impossible resulting in
stammering, illegible handwriting, and poor coordination in aerobatics and
in formation flying.
e. Hyperventilation Symptoms. (See section on hyperventilation).
f. Cyanosis. The colour of the skin becomes bluish from the
reduction of haemoglobin in the capillaries.
Critical Stage. This is the loss of consciousness stage. It may be
the result of circulatory failure ("fainter") or a central nervous system
failure ("nonfainter," unconsciousness with maintenance of blood pressure).
The former is more common with prolonged hypoxia, the latter with acute
hypoxia. With either type there may be convulsions and eventual failure of
the respiratory centre.
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For another set of regulations, New Zealand has the following...
2. Rules on use of oxygen
2.1 Operation of a glider to a height greater than 10,000 ft QNH is
permitted only when oxygen is available for continuous use by each person on
board for any period in excess of 30 minutes that the glider is between
10,000 ft and 13,000 ft QNH.
This allows a pilot to operate above 10,000 ft QNH but below 13,000 ft for
up to 30 mins without being on oxygen. They must however have it available
should they need it. This can allow a glider to achieve a Gold C Height gain
with a climb to 13.000 ft QNH so long as the time above 10,000 ft does not
exceed 30 mins.
2.2 A continuous supply of oxygen must be available and used by each
person for any time they operate in excess of 13,000 ft QNH.
2.3 Crew members of a glider intended to be operated above 10,000 ft QNH
must have satisfactorily completed the training set out in the Hi Altitude
Soaring module of the Advanced Training Syllabus (Appendix 2-C of the MOAP)
A log book entry to this effect satisfies this requirement.
2.4 No glider shall be operated above 45,000 ft QNH without the specific
permission of the Director. Such permission will be subject to such
conditions relating to the provision and use of oxygen as the Director may
prescribe.
2.5 Operations above 25,000 FT QNH are considered by the Operations
Committee to be unnecessarily hazardous given the hostile environment for
both the pilot and the aircraft at that altitude. Pilots are advised to
conduct their operations below 25,000 FT QNH unless engaged in planned high
altitude flight operations.
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