sibility to foster and promote, through education,
an effective and safe mounting of these higher
altitudes,

At CARI, through our Physiological Training
Support Service and utilizing a modern. altitude
chamber complex, private and business pilots are
being given oxygen equipment training and alti-
tude effect demonstrations. Each individual can
determine his own tolerance to hypoxia (low ox-
ygen) in this one day course, Proper breathing
is taught, especially the means of avoiding over-
breathing (hyperventilation) which can result in
uncontrollable muscle spasms (due to too much
loss from the body of carbon dioxide with the
blood becoming too alkaline in character).

Already, CARI has provided indoctrination as
part of the FAA’s public safety education pro-
gram to many private and business pilots, avia-
tion medical examiners, and commercial pilots of
such companies as Mooney, Cessna, Aero Com-
mander, Piper and Beech. The result will be
a safer tomorrow in the upper atmosphere. We
also provide for civilian physiological training at
various Air Force installations around the
country.

We would be remiss if we did not mention
certain factors which aggravate the effects of
thinner air on the individual.

Such conditions as anemias, emphysema (weak-
ened lungs due to premature aging, asthma, etc.),
and illnesses accompanied by fever, can lower the
individual pilot’s tolerance to altitude.

In addition, fatigue, alcohol, antihistamines,
and heavy smoking (with inhalation), also lower
altitude tolerance. Carbon monoxide is one of
the most subtle and vicious factors in lowering
altitude tolerance, since it binds with the blood
far more tightly than oxygen, and, in effect, di-
minishes the total quantity of circulating blood
capable of carrying oxygen. Carbon monoxide
poisoning mimics anemia, therefore.

All persons deteriorate as they ascend in alti-
tude. Some, especially those in poor physical

condition (as not infrequently found in the busi-
ness-man pilot population), deteriorate in per-
formance far more rapidly than others. As a
general statement, night vision deteriorates in all
persons beginning at about 5,000 feet altitude in
the absence of supplemental oxygen. Symptoms
of oxygen deficiency begin in a subtle fashion, and
are frequently unrecognized by the individual
with no direct experience in the matter.

The non-physically conditioned person deteri-
orates faster at high altitudes, because his body
demands more blood flow per unit of metabolic
increase resulting from the work stress of alti-
tude, and, being in poor physical condition, he
cannot deliver as efficiently the required blood
flow.

The frequency of mistakes begins to increase
above 12,000 feet, and mental clarity definitely be-
comes clouded. Since the subjective evaluation
of one’s own performance is compromised under
these circumstances, there is a tendency to believe
one is performing better than is really the case.
Euphoria develops at 16,000 to 20,000 feet, and
all seems wonderful, in spite of the fact that
the speech may be slurred, the motions sluggish
and uncoordinated, and the computations of time
and distance inaccurate. An objective sign of
hypoxia at these upper altitudes is the change in
color of the finger nails from pink to gray.

We are tempted to relate here the half dozen or
so recent tragic experiences of those who came to
grief in light aircraft through lack of knowledge
about and respect for the brain’s oxygen require-
ment.

We shall not do so because our table entitled
“Utilizable Atmospheric Oxygen” speaks for
itself.

Let us then be certain that we ourselves, as
well as our flight colleagues, acquire and bring
to bear the requisite knowledge in our next great
adventure in light aircraft: flight into the higher
altitudes.

UTILIZABLE ATMOSPHERIC OXYGEN

Sea Level 12,000 feet

Available Atmospheric Air Pressure 760 mm Mercury 483 mm Mercury
Lung Air Sac Water Vapor —4T —47

Pressure of Remaining Lung Gases 713 436

Oxygen Portion (20%) x0.20 0.20

Oxygen Pressure in Air Sac 143 87

Minus Air Sac Carbon Dioxide 40 40

Oxygen Pressure in Lung Air Sacs 103 mm Mercury 47 mm Mercury
Provides Arterial Blood Oxygen % of 96% 87%

67-3293
3



normal individual can still be accomplished at
87 percent blood oxygen saturation (assuming
that a blood hemoglobin quantity of normal or
higher value exists—the red-colored chemical,
hemoglobin, contained in the red blood cells, is
the transporter of oxygen), but this figure ap-
proaches the marginal level. At higher altitudes,
as one ascends through 14,000 feet and up, dis-
tinct impairment of mental functions occurs.
This is especially true of mathematical and rea-
soning capabilities.

Actually, under real flight conditions, the pilot
is accomplishing certain physical work above his
basal rest level requirements. Hence, the arterial
oxygen saturation can be even lower than that
conservatively stated above. Additionally, with
work above the basal level, carbon dioxide is ele-
vated in the venous blood, and, consequently, also
in the air sacs. This means that the 40 mm car-
bon dioxide factor can become increased, taking
a further bite into the available oxygen.

The physical laws requiring us gradually to
“firewall” the throttle as we gain altitude in our
nonsupercharged aircraft, apply equally to our
“nonsupercharged” bodies.

The solution to the engine-power problem at
higher altitudes in piston light aircraft is the
employment of a supercharger. The solution
to the oxygen problem for occupants in these
aircraft at altitudes exceeding 12,000 feet, is the
use of oxygen equipment in accordance with well-
established procedures, as available through the
various recognized manufacturers of the aviation
industry. Good portable equipment is on the
market.

It is possible that in the near-future, pressur-
ized light piston engine aircraft (with cabin pres-
sures equivalent to 12,000 feet or less) will become
available (of course, in this case, due to the possi-
bility of rapid decompressions, emergency oxygen
equipment should be available). As the cabin
altitude increases, one can pass through a pres-
sure level beyond which the venous oxygen pres-
sure exceeds that in the air sacs, and oxygen,
in effect, leaves rather than enters the body—this
can occur at the 30,000 foot range of altitude.).

How much oxygen flow should a pilot use at a
given altitude for a given period of time? The
recommended flow rates provided by the manufac-
turers of oxygen equipment serve as the base line,
in a fashion similar to the recommended manifold
pressure and RPM settings for given altitudes to

determine a certain percent horsepower. In the
case of oxygen, we recommend flow rates with
the specific equipment being used which insure an
arterial blood oxygen saturation of 87 to 96 per-
cent.

Let us be cognizant, then, of one salient fact:
the brain is our governing body during flight,
our master organ. It consists of nerve cells
which, unlike other types of cells (for example:
muscle cells, liver cells or bone cells), cannot
function more than about fifteen seconds without
oxygen. Asa matter of fact, as the arterial blood
oxygen saturation level falls from the usual 96
percent to 90 percent and then to 87 percent, the
efficiency of brain function begins to fall as meas-
ured by objective psychophysiologic tests.

There is a wide individual variation in suscep-
tibility to hypoxia, since individuals vary in their
blood vascular efficiency and other biologic char-
acteristics. However, all persons begin to deteri-
orate in alertness and mental efficiency to some
degree above 12,000 feet in the absence of supple-
mental oxygen, and here lies, therefore, the crux
of the matter. Incidentally, air traffic controllers
have witnessed pilots suffering from hypoxia (ex-
ample: slurred speech) between 12,000 and 14,000
feet, who were unaware of their hypoxic state.
Daily exposures to these altitudes during flights
does not produce adaption to high altitude.

As a general rule, it would be wise to use sup-
plemental oxygen above 12,000 feet, and it should
be an invariable companion above 14,000 feet in
unpressurized aircraft. It is well to remember
the advisability of providing passengers with
poor circulatory conditions (arteriosclerosis,
heart disease, etc.), or with poor lung function
(asthma, emphysema, chronic bronchitis, etc.),
with supplemental oxygen beginning at about
8,000 feet, depending upon the severity of the
condition and duration of the flight. A physician
should be consulted on specific cases.

Of course, from the operational standpoint,
there is little percentage in cruising a nonsuper-
charged aircraft at altitudes above 14,000 feet.
The lower engine efficiency, coupled with the high
angle of attack and the slow cruising speed, plus
the time and fuel consumed in climbing to this
altitude, conspire to keep most of the light pres-
ent-day aircraft in the lower altitudes.

As previously mentioned, this picture will
change with the more general availability of
supercharged light aircraft, and it is our respon-




AM 66-28

OXYGEN IN GENERAL AVIATION

Stanley R. Mohler, M.D.

Approved by Released by
James L. Harris P. V. Sreczr,
Curer, ArromepicaL Epucation Feperat AIR ne
Division

September 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151


OXYGEN IN GENERAL AVIATION

Stanley R. Mohler, M.D.*

Our metabolic engine and our aircraft engine
have in common the requirement for oxygen.
They also have in common the feature that
neither can store oxygen.

We shall direct our attention at one table. This
table, entitled “Utilizable Atmospheric Oxygen”,
provides us as surely with the facts of oxygen
requirements in flight as do the power setting
tables in specific aircraft flight manuals.

Note that at sea level, the total air pressure ap-
proximates 760 millimeters of mercury. This
same total gas pressure exists within the tiny air
sacs of the lungs since all of the air tubes leading
from them are open and connect directly with the
atmosphere.

Since the gases within these air sacs are 100
percent saturated with water vapor at a body
temperature of 98.6 degrees F, a water vapor
pressure of 47 mm of mercury results.

Note also that at 12,000 feet, where the total
air pressure approximates 483 mm, the lung air
sacs continue to contain water vapor at a pres-
sure of 47 mm, since the water vapor saturation
and body temperature remain the same.

This water vapor content of the lungs takes
a proportionately larger “bite”, therefore, out
of the available atmospheric air at 12,000 feet,
as compared to sea level. This fact, together
with certain points mentioned later in this paper,
explains why we begin to suffer progressively as
we climb into thinner air (item: a similar situa-
tion concerning water vapor prevails with re-
spect to aircraft engine efficiency and humidity
levels. As the amount of humidity in the air
increases, all other atmospheric factors remain-
ing constant, the power deliverable by the engine
decreases. People and. aircraft engines have,
therefore, certain features in common, in that
the water vapor pressure can be critical to the
function of both).

We now see that subtracting the lung air sac
water vapor, we have at sea level 718 mm gas

pressure remaining (consisting mostly of nitro-
gen and oxygen). Flying at 12,000 feet in an
unpressurized aircraft, our lungs contain (minus
the water vapor) a gas pressure of 436 mm,

Of these two pressures, oxygen represents
roughly 20 percent, which gives us a sea level
lung oxygen pressure of 143 mm and a 12,000
foot oxygen pressure of 87 mm.

Since one of the functions of the lungs is to
clear the blood of carbon dioxide (the gaseous
by-product of metabolism), we find that under
resting conditions the lung air sacs contain 40
mm carbon dioxide pressure (the carbon dioxide
is physically expelled from the lungs with each
exhalation).

Let us now correct the sea level lung oxygen
for the ever present carbon dioxide factor (143
minus 40) and we have an effective oxygen pres-
sure in the lung air sacs of 103 mm.

Correcting the 12,000 foot level lung air sac
oxygen pressure for carbon dioxide pressure (87
minus 40) we have remaining in the lung air
sacs an oxygen pressure of 47 mm.

Here we are, then, piloting our unpressurized
craft along at 12,000 feet, with a lung air sag.
oxygen pressure only 45 percent of that in the
same air sacs at sea level (47 divided by 103 times
100).

This lowered lung air sac oxygen pressure
found at 12,000 feet, provides an arterial blood
oxygen saturation of 87 percent, compared to the
96 percent saturation at sea level.

It is clear from these figures, that an altitude
of 12,000 feet begins to compromise the efficiency
of our metabolism, and especially that of the
brain, which requires a continuous irrigation of
96 percent oxygen saturated blood for peak men-
tal function. Acceptable mental function in the

*Dr. Mohler is Chief, Aeromedical Applications Division,
Office of Aviation Medicine, Federal Aviation Agency, Wash-
ington, D.C.