T
RAINING
M
ANUAL
–
L
ANCAIR
IV/IVP
mm equals 150 mm of oxygen pressure in the trachea. At sea level on a
standard day in dry air, yours lungs receive oxygen at 150 mm of partial
pressure.
In the lungs, oxygen diffuses through the permeable membranes of the alveoli
into the red corpuscles, flowing through the blood capillaries at a rate roughly
proportional to that partial pressure. Predictably, when oxygen partial pressure
drops sufficiently, blood saturation also falls. And although partial pressure and
saturation do not change at the same rate, the correlation is close enough for the
purposes of this discussion.
Naturally, any increase in altitude above sea level involves a reduction of total
pressure and concomitant reduction in the oxygen partial pressure in the lungs.
When partial pressure in the lungs falls, so does your blood saturation – with very
predictable results.
The problem with hypoxia is its subtlety. Hypoxia is an insidious, virtually
undetectable condition in which you progressively lapse into unconsciousness
while retaining absolute faith in your ability. If you have not experienced hypoxia
in the controlled environment of an altitude chamber, you
must
assume that you
will
NOT
recognize its effects in yourself, even to the point of unconsciousness.
In order to put all this in perspective; consider the hypoxic effects of altitude on a
normal, healthy pilot at particular, benchmark altitudes. And don’t miss the
elementary but important fact that all altitudes are above sea level.
•
5,000 Feet:
Total atmospheric pressure is down to 632.3 mm Hg
(inches of mercury), with an oxygen partial pressure to the lungs of
approximately 122 mm. This reduced partial pressure cannot fully saturate the
blood corpuscles, which, in turn cannot supply all the oxygen the body tissues
would like. Most of those body parts will continue to function normally at this
level of 93 percent blood saturation with one notable exception. The retina of the
eye demands more oxygen than any other organ. At 93 percent saturation, this
little extension of the brain will begin to function somewhat below maximum, so
night vision may be diminished. During night flight at 5,000 feet or above, pay
close attention to instrument readings and maps as well as to ground details,
because your vision may be slightly impaired.
•
10,000 Feet:
Total atmospheric pressure is down to 523 mm H,
about 70 percent of the sea-level pressure. Oxygen partial pressure to the lungs
is about 100 mm, enough to produce only 90 percent saturation of the blood.
Ninety percent saturation is the absolute minimum the brain can tolerate in a
normal, healthy person. This is the highest altitude at which you can trust your
own judgment, even though our discrimination will be somewhat impaired.
Operate at this altitude with care and caution. Short durations of an hour or less
are well tolerated, but longer periods of several hours at or above this altitude
can produce significant effects, especially at night.
‐
21
–
©
Copyright
2007
LOBO
May
not
be
copied
without
permission.
