women might react similarly, However, for the
high energy expenditures seen in many men at
work and sports, this concept is unacceptable.
Since heart rate can only increase to approxi-
mately three times its resting rate under maxi-
mum physical demands, the blood flow under
such conditions would be completely inade-
quate without augmentation of the stroke vol-
ume. In fact, past and most recent experimen-
tations with “normal” men in good physical
condition have left no doubt that the heart does
use both alternatives simultaneously during
strenuous work. Stroke volumes have been
found to increase up to almost twice the resting
value. Thus, maximum cardiac output of 5.to
6 times resting blood flow rates were measured.
The definite limitations observed consisted
either in the attainment of a maximum pulse
rate without any further increase in stroke vol-
ume or a marked diminution of stroke volume
at heart rates in excess of 180-190 beats per
minute,

It. was interesting to observe that the rou-
tinely measured changes in blood pressure cor-
related well with the changes of stroke volume
during physical work. In the light of these
findings, the telemetered, or otherwise recorded
physiological data of pulle rate and blood pres-
sure may allow for a close estimation of the
pilots and astronauts’ functional responses in

light, and how far the reserve capacity of their
circulatory systems might become engaged in
coping with stress situations.:

Functional work capacity is also limited by
the strength and endurance of the respiratory
muscles to force the rate and depths of inspir-
ation and expiration, in pther s, the
maximum breathing capacity under high de-
mands, Usually, the well-conditioned man can
increase his resting ventilation approximately
20 times, the poorly conditioned man only 10;
to 15 times. In addition, the pulmonary vent
lation is more efficient in the former 'thanyin
the latter when compared on the basis of actual
energy exchange. .

Both the circulatory as well as the respiratory
limitations, can be expressed in one single crite-
rion: the maximum gxygen intake. Whenever
at further increasing work intensities oxygen
* consumption reaches a constant level, the func-

tional adaptability is outdone, For the physio-
dogist or physician familiar with the evaluation

of physical fitness, any individual’s maximum
oxygen intake, expressed in ml/min/kg, means
a certain place on the functional] fitness scale.

B. Metabolic Limitations

The metabolic limitations of work capacity
can be assessed during strenuous but submaxi-
mal work of longer duration, The body's gly-
cogen stores are usually running low after close
to two hours of strenuous physical activity. Fat
depots are also ‘utilized for energy supply as
long as carbohydrates are available in sufficient
amounts, .An exhaustion of carbohydrates ter-
minates the capacity for further physical work.
Individuals in poor physical condition may
have carbohydrate reserves of only ‘100-150
grams while exceedingly well-trained men may

ve stored about 400 g of glycogen in the liver
and muscles.” The latter cannot only work at
a much higher rate and for a longer period
than the former because of the higher glycogen
stores but also because of the ability, probably
“learned” in training, to utilize to a greater
extent the body fat as an energy source.’ This
greater fat mobilization was ascertained by
higher blood lipid levels during strenuous exer-
cise of long duration. :

C. Environmental Effects on Work Capacity

Tolerance for physical work is a variable de-
pending upon periods of inactivity or increased
activity, upon fatigue or latent infectious dis-
ease and, certainly not to the least, upon
environmental effects. In the following, three
particular environmental stresses are selected

ecause of their specific demands on one or the
other physiological function,

1. Heat

The assumption is made that a man, engaged
in a regular conditioning program, is accus-
tomed to,run at moderate temperatures of
around 22°C (=72°F) a course of 5 miles in
40 minutes. With a body weight of 75 kg
“(167 Ibs), the intensity of work involved in
running at that speed demands an oxygen in-
take of nearly 3400 m1 /min and, accordingly,
a cardiac output of approximately 30 1/min.
These values are assumed to represent this
individual’s tolerance limits for maximum
steady state work,

-Bw-



B. Learn to tolerate pain, fatigue, hunger

and thirst, sleeplessness, heat and cold,

the discomfort of high altitude —and
‘frustrations and failures.

‘ C, jAnticipate all possible emergency situa-
tions, and arrive, mentally, at the most
A Ptoper response.
Di Learn to relax completely, confiding into
your subconscious the readiness for any
4 demands.

ERANCE FOR GRAVITATIONAL
IRCES

Acceleration

The tolerance for acceleration has been de-
termined in human and animal centrifuge
studies. The positioning of the body relative
to the direction of the increased gravitational
forces is found to be critical. In an upright
position (headward to footward forces) gravi-
tational shifts of blood may leave the brain
cells without adequate blood and oxygen sup-
ply, causing “gray-out” or “black-out” at 4-6 gs.
If, on: the other hand, the accelerating forces
are encountered in a right angle to the longi-
tudinal axis of the body, the general distribu-
tion of blood is less affected and g-loads
up to 10-12 times Earth gravity can be tolerated
for 2-3 minutes.” Moving or lifting any part
of the body against such high centrifugal forces
is restricted because of the disproportion be-
tween the appropriate muscle groups and the
immensely increased weight a the body parts
to lift. Even breathing, which involves an ac-
tive lifting of the chest and/or pbdominal wall,
becomes an enormously laborious task. Con-
sequently, the decigase in, ventilatory activity
(hypoventilation), ‘cuts down adequate a
genation of the blood, first, by not aerating the
lung alveoli sufficiently with fresh inspired air
and, secondly, by clamping down on the re-

turn of venous blood to the heart, thus restrict- -

ing pulmonary blood flow. Understandably,
even in the recumbent or semi-recumbent posi-
tion, the astronau¥’s tolerance for acceleration
“is limited because of the severe oxygen lack de-
veloping in the most vitally important organic

« “systems.

B. Deceleration Tolerance and Effects of
Weightlessness

The tolerance for sudden deceleration may
play an important role in atmospheric higl

speed emergency escape or during a severe
landing shock. Truly heroic experimental
studies on the biological effects of sudden de-
celeration have been made by Dr. Stapp on a
rocket sled. Accelerated to. supersonic speeds,
the sled was abruptly sto ped. The experi-
ments demonstrat. at the human organism
can survive, although with severe injuries, a
sudden landing shock from a speed of about
100 mph “provided the properly restrained
subject is suspended within a capsula in the
aftlacing, semi-supine, transverse, orientation,
and provided the capsula does not fail, crush-
ing in on him.””

In deceleration experiments at the Wright Air
Development Center, several subjects have tol-
erated 3 g for one hour, and 4 ¢ for not quite
that long. It is questionable, however, if the
same individuals would be capable of toleratin;
a similar “re-entry stress” after they had ad-
justed to weightlessness in a space flight of ex-
tended duration. The evidence accumulated in
water immersion experiments Supgests that the
cardiovascular reflexes necessary for maintain-
ing adequate blood circulation during ortho-
static changes, deteriorate considerably when
relieved from functioning for any greater length
of time.” How the complete disorientation of
our circulatory system under zero-gravity con-
ditions might affect theteardiodynamics durin,
and after the return to the normal gravitational
field can only be told after actual space flights.
The more receht events along the space efforts
seen to indicate that a space flight of short
duration results in much less biological dis-
turbances than a flight of longer duration.
There is much speculation about the physio-
logical ‘effects of weightlessness itself. The
space physiologist in the orbiting medical lab-
oratory of the future might consider this the
umber one research problem. ‘

i
IV. THE PSYCHO-PHYSIOLOGIC
PROBLEM OF HYPERVENTILATION

At this point it appears necessary to turn at-
tention to a specific medical problem associated

~3—



comes dizzy and “scared” while using the pas-
sive inflation technique;. two or three breaths
employing ‘the forced inhalation technique let
him regain full consciousness and a feeling of
security.

Although the application of pressure breath-
ing is apparently complicated by the high de-
gree of hypoxia present at extreme altitudes, and
consequently by the excessive hyperventilation
required for maximum altitude tolerance, ex-
perimental evidence has been presented that
pressure breathing can raise the altitude ceiling

considerably without harmful risks to the in-
dividual. “” Table 111 demonstrates to what ex-
tent “normal” and altitude acclimatized men
are able to increase their ceiling when inhaling
oxygen under pressures found to be feasible and
tolerable. Any lowering of the atmospheric
pressure below the “tolerable” or “critical”
levels, shown in Table II, has to be compen-
sated for by an oq ivalent increase of oxygen
pressure in the mask, Knowing a man’s altitude
tolerance and his tolerance for pressure breath-
ing, his “safe” and/or “critical” ceiling can
readily be predicted.

TABLE I
A, B. Px B A,
: Feet mm Hg Feet

Normal man:

“Tolerable” Hypoxia 45,500 108 +20 88 50,000
“Critical” Hypoxia 48,000 96 +25 val 54,000
Acclimatized Man:

“Tolerable” Hypoxia 50,000 87 +27 60 58,000
“Critical”. Hypoxia 52,000 79 +35 44 64,000

Nore: A, and B, are altitudes and pressures traced from Table Il; P,, = oxygen pressure in. the mask; B,
and A, are minimum barometric pressure and maximum altitude attainable.

The “critical” values, as shown, are of the-
oretical interest only. During pressure breathing
full consciousness has to \be preserved in
order to prevent damaging’ lung expansion.
Approaching unconsciousness { would disable
man from adequately controlling the required
coordination of delicate activities.

The figures in Table III, obtained throtgh
actual experimentation with several subjects
indicate what benefits regarding altitudd/toler-
ance are to be expected from altitude acclima-
tization and proper training.

D. Pressure Suit

A sudden. exp&sure to altitudes in excess of
50,000 feet requires the protection afforded
by a pressure suit, The time of useful con-

. stiousness could be counted in seconds when
less than 100 per cent oxygen was inhaled for

a greater length of time before the incident.
Even an ‘immediately initiated nose-down of
an airplane might, under such conditions, not

event several crew members and passengers
From going fhrough a temporary state of
unconsciousness. Breathing of oxygen prior
to an incidental loss of cabin pressure has a
protective effect due to the almost complete
elimination of gaseous nitrogen which other-
wise would compete with oxygen, carbon
dioxide and water vapor in filling the alveolar
spaces, After prior breathing of 100 per cent
joxygen, in a case of sudden decompression, an
automatic delivery of oxygen under pressure,
compensated for by an automatic inflation of
the pressure suit, should permit a continuation
of the mission — if all crew members were
likewise protected and had followed strictly
the oxygen discipline.

—8-



dilatation elsewhere, might be reduced in in-
tensity after hyperventilation training,

Vv. HIGH ALTITUDE TOLERANCE
A. Breathing Normal Air

The decrease of atmospheric pressure, or
more precisely the decrease of the partial pres-
sure of oxygen in the atmospheric air, sets limi-
tations for human operations at high altitude.
At about two-thirds of the sea level pressure,
equivalent to an altitude of about. 12,000 feet,

the ability to work is markedly reduced, and -

a great number of people become temporarily
“sick.” Fortunately, a long stay at such altitude
leads slowly but steadily to full acclimatization,
a complex process involving mainly the blood
and the blood regenerating organs, the circula-
tory and the respiratory system, and the kid-
neys. At the peak of acclimatization, the func-
tioning of the organs has returned to a level
almost identical with that at sea level. Adequate
altitude acclimatization can provide complete
comfort at rest, and satisfactory capacity for
strenuous work, up-to altitudes of about 18,000
feet, at which the atmospheric pressure is one-
half of that at sea level. Beyond this altitude
level, however, the physical discomforts during
rest, but more so, of course, during any work
situation, become increasingly greater until at
about one-third of the normal atmosphere, or at
an altitude of approximately 28,000 feet, even
the optimally acclimatized man has to abstain
from greater physical efforty. The atnazing
fact has to be pointed olut, howeyer, that several
individuals have been/active atfa rate of twice
or three times their testing metabolic require-
ments at an altitudetbf 28,500 feet which woyld
Kill a non-acclimated sea-level man within
minutes.” !

B. Breathing 100 Per Cent Oxygen

An altitude of 28,000 feet, in space terms, is

only an ant hill. More impressive might appear
man’s adaptabilitTMo high altitude if one con-
‘siders the equivalent altitude he can tolerate
when breathing, instead of normal atmospheric
« air, 100 per cent oxygen through a mask. For

example, the mountaineer who, by adequate
training and altitude acclimatization, became
capable of climbing the highest peaks on
Mother Earth without the aid of oxygen may
enjoy flying in an unpressurized aircraft at an
altitude of 50,000 feet with the aid of breathing
100 per cent oxygen. Hypothesizing on an
eventual space event, the well-acclimatized as-
tronaut, after landing on Mars, could leave the
protection of his space ship equipped with the
same set of oxygen apparatus aviators are using
today, and do some first-hand explorations of
the new Planet, The atmospheric presssure on
Mars approximates, according to estimation, an
equivalent altitude of slightly more than 50,000
feet on Earth. All of this has considerable
bearing also upon survival in the event of super-
sonic transport decompression at the 50,000
foot level. a

A short elaboration on basic facts of respira-
tory physiology might help to fully understand
the biological limitations of man’s conquest of
space. Life is a process of continuous energy
exchange requiring primarily the presence of
oxygen. For adequate transfer of oxygen to the
living cell, the driving force ‘of a substantial
pressure gradient is essential. The normal
pressure of oxygen in the lung alveoli is around
100 mm Hg, while its lowest pressure com-
patible with survival is around 30 min Hg for
“normal” man, and around 22 mm Hg for in-
dividuals optimally acd§matized to high alti-
tude. In the process of inhaling oxygen through
a mask, the gas within the mask will be pre-
sented under pressure conditions close to those
of the surrounding atmosphere, The inhaled
gas will then be humidified on its way down
the respiratory pathway, and the admixture of
water vapor molecules causes a reduction in the
partial pressure of inspired oxygen by the dis-

‘placement of oxygen molecules. At normal

body temperatute the water vapor tension is
47mm Hg, |’ ‘

The following examples, depicted in Table I,
might help to illustrate the interrelationships of
altitudes, gas pressures and biological compli-
cations in the act of breathing air or 100 per
cent oxygen.at very high altitudes.

. -5_



with a variety of stresses, namely to the un-
conscious increase of breathing activity as re-
sponse to psychological or environmental dis-
turbances. It was mentioned before that in
situations of high suspense, that is under ex-
citement, tension, anxiety and fear, an urge
exists to overbreathe, Also, overventilation — or
hyperventilation — may occur as response to
physical stresses such as milder forms of accele-
ration, heat or cold exposure, and high altitude.
Only in the latter case, at high altitude or under
any other condition causing lack of oxygen,
hyperventilation has merits of its own. In
most other cases it is rather detrimental to man’s
well-being and performance capacity.

Hyperventilation is understood as an alveolar
overventilation, at any given metabolic rate,
causing a fall in alveolar carbon dioxide tension,
and as a result an increase of the CO, tension
gradient between tissues, blood and alveoli.
Carbon dioxide is then expired from the lungs
faster than metabolically produced, and the CO,
tension in the blood and tissue is lowered—
bringing to pass the so-called state of “hypo-
capnia”, Such event is usually accompanied by
a series of alarming symtoms such as dizziness,
tingling, impairment of vision, spontaneous
tightening of muscle groups known as tetany,
and even loss of consciousness. Many of these

Symptoms are identical with those observed in

/ hypoxia, thé-state of general lack of oxygen.
That may come as a surprise but can readily be
understood: hypocapnia is known to cause a
constriction of the capillary bed in some parts
of the organism, es; ially in the brain. y This

_ vasoconstriction may cause a marked detrease

\, in blood flow and oxygen supply and, therefore,
the symptoms typical of hypoxig,

A manifestation of more severe hypocapnia
are the muscle spasms known as tetany, The
diagnosis of “hyperventilation” becomes obvigus
to the close observer if he notices carpopédal
spasms, a development of muscle cramps fn the
forearms, fingers, and feet. To the remote ob-

server the same diagnosis is justified when .

ayylio-communication becomes increasingly dif-

ficult: spasms of the facial and neck muscles

interfere with the ability to form sounds and

words to such extent that the severely hypocap-

nig individual cannot transmit any verbal
“" messages.

This tetany, occurring in a progressed state of
hypocapnia may be a self-defense of the body,
to antagonize an excessive loss of carbon diox-
ide by increased CO, production through muscle
spasms.

Hypocapnia can be accompanied by a marked
reduction or, occasionally, complete loss of per-
formance capacity. The first alarming symptoms
might be observed when an individual, by over-
ventilating at better than twice his metabolically
required rate, has lowered his alveolar carbon
dioxide tension to about 25 mm Hg (from a
normal of 40:mm Hg). A three to fourfold
increase of breathing above the normally re-
quired rate usually lowers the alveolar carbon

lioxide tension to about 15 mm Hg and causes,
in most cases, a severe reduction in genera] per-
formance capacity. :
-Eheuindividual susceptibility, to .hypocapnia
varies . wi Some rope ae observed to
faitif after’a Tew fast and deep breaths..- Others,
exposed to a maximum of voluntary overbreath-
ing for relatively long periods of time, never go
beyond the stage of minor giddiness. Volun-
tary repetition of excessive over-ventilation is
experimentally found to increase the tolerance
for hypocapnia, to almost limitless -values, This
is of great importance to astronauts, who might
be more on edge at times than jet pilots. In-
flight studies of the latter have shown that in-
cidents of hyperventilation have accumulated
directly in proportion to the greater speed and
performance of jet airplanes.” Nevertheless,
the accident rate did not go up with the higher
incidence of hypocapnia. e conclusion is
drawn; therefore, that the pilots must have ad-
justed to this biologically unbalanced situation.
Such: adaptation to hyperventilation is con-
firmed in experiments in which several volun-
teer subjects practiced forced breathing from
30 to 60 minutes daily for a period of two weeks.
Although ventilation was finally increased to
5-6 timés above the normal rate, and alveolar
carbon dioxide tensions as low as 10 to 14 mm
Hg were observed, the experimental subjects
eveloped no intapacitation for complex psy-
chomotor tasks. The only explanation for the
phenomenon of such adaptability is the uncon-
firmed hypothesis that the effects of hypo-
capnia upon, local blood circulation, such as
vasoconstriction of brain capillaries and vaso-

—4—

_ tees



TABLE IT

Altitude B Pao, Paco, Pulm. Vent.
Feet mm Hg (normal = 100% )

Normal man:
“Tolerable” Hypoxia 45,500 108 36 25 250%
“Critical” Hypoxia 48,000 96 30 19 300%
Acclimatized man:
“Tolerable” Hypoxia 50,000 87 27 13 400%
“Critical” Hypoxia 52,000 79 24 8 550%
B = barometric pressure = the sum of the alveolar oxygen, carbon dioxide and water vapor tensions.

In these experiments, the ascent to high al-
titude was accomplished gradually over a period
of several minutes, The adapta ility to a very
rapid onset of severe hypoxia would have been
less favorable.

C. Breathing Oxygen Under Pressure

The altitude ceiling can be further raised by
providing oxygen under a pressure exceeding
the surrounding atmospheric pressure. This
technique: has never been applied to greater
extent because of certain drawbacks, The nor-
mal breathing mechanics are geared in such a
way that inspiration is an overwhelming active
muscular process while expiration, usually, is al-
most entirely passive, e negative pressure
occurring during the inspiratory enlargement of
the chest cavity promotes the return of venous
blood into the thorax and, thts, into the heart.
During the expiratory phase th¢ intrapulmonic
pressure does not increase subsfantially enough
to restrict venous return markedly.’ In positive
pressure breathing the inspiration is usu:
passive and the respiratory gas is forced jnto
the lungs. Venous return, therefore, is as-
sisted by the development of a negative in-
trapulmonic pressure during the inspiratory

phase and is furthermore inhibited by the con- «

siderable pressure within the thorax during
expiration. Positive inspiratory pressure can
result in an overst%tching of the diaphragmatic
muscle and of lung tissue—if it is not properly
counterbalanced by a continuous tension of the
« abdominal and the other muscle groups usually

contributing to forced expiration. The con-
tinuous contraction of these muscles is, of
course, progressively fatiguing, especially at
the higher mask. pressures.

For a while the physiological disadvantages
of pressure breathing were overemphasized, In
fact, complete circulatory failure was predicted
at positive mask préssures of about 30 mm
Hg.” Logically, in further work on providing
protection of the aviator in emergency situa~
tions at very high altitude, ways and means
were sought and found to counteract the posi-
tive oxygen pressure in the helmet or mask by
providing a sufficiently high pressure on the
body as a whole.

Certainly, pressure breathing without the as-
sistance and/or protection of a pressurized gar-
ment is limited and means hard physical work
of usually neglected muscle groups. Like any
other strenuous work, it can be ept up fora
limited length of time only, _ However, the
circulatory embarrassment, caused by the men-
tioned restriction of venous blood return, can
sufficiently be taken care of by applying a
breathing’ technique different from the one
described: “ instead of letting the thorax pas-
sively expand under the positive gas pressure —
which gives a brief moment of subjective re-
faxation for the straining expiratory muscles!
~short but active inspirations are made after
forced expirations. The difference between the
two techniques can be felt immediately: at a
more critical positive mask pressure, about
30 mm Hg, an_ experimental subject be-

-7-—



oh . .
,  FEDERAL/AVIATION’ AGENCY. os
+. Civil ical Research Institute ¢ .

. AERONAUTICAL GENTER
|, OKLAHOMA CITY, OKLAHOMA


These experiments demonstrated certain
things. First: subjective symptoms such as diz-
ziness, far-away feeling, headache and dyspnea
are very poor indicators of the true physiological

limitations for hypercapnia, Second: the cardio-
circulatory system, although definitely respond-
ing to CO, accumulation, is not stressed to full
capacity, as Table V indicates.

TABLE V
Ventilation Blood Pressure Pulse Rate
Inspired gas 1/min mm Hg F/min
Air 27 145/80 96
10% CO, in air 124 200/100 130

Third: the best criterion for the tolérance of
carbon dioxide was the attainment of the maxi-
mum breathing capacity, whereby the latter
essentially equalled that observed during maxi-
mum physical efforts. Experiments with the
same individuals before and after altitude
acclimatization demonstrated that the critical
CO, concentration in the inspired air reached
10 per cent in both experimental phases. The
ventilatory response in the control experiments
was 124 1/min (BTPS) while it was 158 1/min

, after acclimatization. The difference between
these two values of maximum breathing ca-
pacity must be explained as an effect of

VIII. TOLERANCE FOR TEMPERATURE

EXTREMES ' i

During many of the imaginable space opera-

tions of the future, the adaptability of man.to -

extreme variations in temperatures might be-
come of considerable importance. The human

tolerance limits for environmental,temperatures ’

depend on the critical body temperatures

which the functioning of the living cell rapidly
deteriorates. At the lower end, cold narcosis
sets in when the body temperature decreases
to 32-30°C; at the higher end, core tempera-
tures of about 44°C agg incompatible with life.
Within the relatively narrow zone of plus or
‘minus 5-7°C the body has to regulate its tem-
perature in response to potential environmental
tdinperature changes from such lows as about

adaptation to high altitude, -There, because
the demands on pulmonary ventilation are con-
siderably | increased, the greater breathing
efforts, especially during physical work, develép
greater strength and endurance of the’ respir-
atory muscles, Hence, a greater maximum
breathing capacity is achieved during altitude
exposure. Fourth: despite the increase in sensi-
tivity of the respiratory center to CO, the
tolerance for the altitude acclimatized. man re-
mained, practically, unaltered because he was
able to respond with increased. pulmonary
ventilation, as discussed previously.

—35°C to such highs as 50°C, that is within a
zone from around —70 to +13°C on both sides
of the body’s 37°C. It follows that man should
be more adaptable to cold than to heat. Never-
theless, more people apparently die from cold
than from heat exposure. That, apparently, is
directly related with the capacity for physical
work because in absence of any efficient exter-
nal protectién the body protects itself against
loss of temperature by heat generation through
muscular activity, Increased energy metabol-

‘ism ‘depends, however, on adequate energy:

sources within the body or on sufficient energy
supply. In the cold, shivering can increase the
metabolic rate up to four times above basal con-
ditions, “” The greater part of our population
can only tolerate a threefold increase for a few

-~10—



There are practically no altitude limitations
for a man fitted with — and properly indoc-

A serious threat to high altitude flying is the
development of severe symptoms of decom-
pression sickness in the incident of cabin pres-
sure loss, A similar threat would exist for the
astronaut in such a case — if pressure and gas
composition were kept at a normal atmospheric
level in the space capsule. Even the space suit
would not prevent. the occurrence of bends,
chokes or other symptoms, occasionally taking

a’ very serious development. The individual

susceptibility to decompression sickness’ may
depend on age, body fat, general physical
condition, and on neuro-muscular tensions,
Breathing 100 per cent oxygen for’ several
hours, or living for, 1-2 days under atmospheric
conditions equivalent to 14,000. feet, afford
complete protection against bends, etc.,“” at
barometric pressures .as low as 50-60 mm Hg,
and therefore, of course, also against the occur-
rence of decompression sickness when exposed
to space conditions but protected by the space

’
'

me

suit. Those considerations are important for
providing the adequate gas composition and

n trinated in the use of — a pressure or space pressure conditions in the living quarters of

B suit, Under space conditions, an altitude any extraterrestrial vehicle.

i acclimatized man might be sufficiently effective

c in a space suit pressurized to 115 mm Hg or to VH. TOLERANCE FOR HYPERCAPNIA

- sli A idk al wc tan 2p Ss while the “normal In space operations, the loss of oxygen pres-

A of i low Mig jue an oxygen Pressure sure or the accumulation of carbon dioxide in

b oF at least 3 psi. the sealed crew compartments could be one or
| ARIS the other main cause of an environmental

; | VI. arEARISM OR DECOMPRESSION emergency situation. While considerable pro-
4 tection against severe hypoxia can be obtained

by altitude acclimatization, the latter appears,
on first sight, not to be compatible with suf-
ficient tolerance for higher levels of carbon
dioxide in the inspired air. The reason for
that is the increased sensitivity of the respira-
tory center to carbon dioxide, after biochemical
adjustments. to chronic hyperventilation had
been made during altitude exposure. In work
experiments. done at sea level before and
immediately after acclimatization to about
14,000 feet, pulmonary ventilation for the iden-
tical work intensity was 125 and 160 1/min,
respectively."” Ini these experiments metabolic
carbon dioxide production, and therefore the
quantitative CO, stimulus for respiration, were
identical.

In another experiment, carbon. dioxide was
allowed to accumulate, during light physical
exercise, by rebreathing an oxygen enriched
gas mixture. The results are shown in Table IV.

TABLE Iy
} 0 i y Ventilation during work of 400 Kgm/min
% CO, ‘ / liters/min (BTPS)

i in air . Pre-Acclim. 1 Post-Acclim.
05 ' 27 31
5.0 « 56 ca 62

a if 7

? 75 77 \ 95

160 124 158



TABLE I
Altitude Inspired Pulm. Ventilation 3 B-PaH.o PAN; Pao, Paco
Feet Gas % of “normal” mm Hg

500 Air 100 T47 700 555 105 40
34,000 0; 100 192 145 — 105 40
18,000 Air 100 380 333 262 30° 40
44,000 0: 100 117 70 -—- 30° 40
44,000 0, 250 117 70 _ 45 5
25,000 Air 300 281 234 185 30° 19
48,000 0, 300 96 49 —_ 30° 19

Nore: B= Barometric pressure in'mm Hg; Pax,o = alveolar water vapor tension at body temperature
= 47 mm Hg;.PAN,, etc. = alveolar gas tensions of nitrogen, oxygen, and carbon dioxide, respectively.
“Around the “critical” alveolar oxygen tension of 30 mm Hg, in normal man, loss of consciousness is imminent.

At an altitude of 34,000 feet breathing of 100
per cent oxygen insures, physiologically, the
same conditions as breathing normal air at sea
level. For 44,000 feet on oxygen the equivalent
altitude on ambient air i& 18,000 feet. At these
altitudes, without adequate regulation of the
ventilatory activity, the alveolar oxygen tension
attains the “critical” level of 30 mm Hg. At
this point the “normal” man loses consciousness.
Fortunately, a dangerous ‘lowering of the
oxygen tension in arterial blood is recog-
nized by an appropriate sensing device Built
into the system of regulating pulmonary ven-
tilation, and breathing is intensified beyond the
metabolic needs, With a reflexlvely or con-
sciously induced ovéryentilation. of about 2-3
times the normal rate, the alveolar oxygen ten+
sion is raised sufficiently to insure not only sy
vival but even a tolerable amount ,of work
efficiency.

The altitude limits of “normal” individuals
are between 18,000 to 25,000 ft, while breathing
air, or between 44, to 48,000 ft, breathing
oxygen, depending entirely on the individual
respiratory response to oxygen lack and on the
amount of hyperventilation which can be toler-

“ated. In the previous section, dealing with the

problems of hyperventilation, the detrimental
effects of hypocapnia were discussed. The
hypocapic vasoconstriction of brain capillaries
could offset the advantages of increased alveolar
oxygen tension.

The man acclimatized to high altitude has
several factors in his favor for withstanding
greater oxygen lack: First, his critical alveolar
oxygen tension is markedly lower; secondly, the
respiratory center has become more: sensitive
to oxygen lack and regulates ventilation more
adequately according to: needs; thirdly, the
continuous ‘hyperventilation associated with ex-
tended exposure ‘to high ‘altitude has reduced
or abolished the deteriorating effects of severe
hypocapnia.

The interrelationship of the alveolar gas ten-
sions at altitudes where a sufficiently effective
level of mental and' physical working capacity
is maintained, and at other altitudes where the

_ critical blackout point is close at hand, is shown
in/Table II. “Normal” individuals, never hav-
ing been exposed to high altitude for any con-
siderable length of time, are compared with men
optimally acclimatized to altitudes. The figures
presented are based on actual high altitude ex-
perimentation breathing 100 per cent. oxygen:

—6—

—

aS



Earth’s atmosphere may at first be comparable
to the early attempts of crossing oceans in tiny
vulnerable vessels, In these early voyages,
many of the sailors preferred to be non-
swimmers so that they were spared the agony
of fighting certain death in event of a catastro-
phe. Such a fatalistic attitude, similarly, could
be understandable amongst future aeronauts
and astronauts, since minor technical failures
and incidents affecting the supersonic transport
and space craft could lead to inevitable dis-
aster. However, as many strong and powerful
sailors have survived the wreckage of their ‘ship
and the resulting hopeless situation because of
their superior physical condition, so might the
survival of some aeronauts and astronauts de-
pend on a maximum of biological reserve and
‘unctional capacity.

In the following, then, some aspects of
human tolerances: for various stress situations
are considered from the standpoint of dealing
with an “average” individual and/or with an
individual who has utilized all potential train-
ing and conditioning facilities for achieving his
maximum level of adaptive capacity for the
stresses anticipated. To cover all of the com-
plex phystologica! adaptations adequately, a
score of contributions by many authorities rec-
ognized in their special fields of research is re-
quired. In order to keep this report as short as
possible, most of its background is based on
the experimental facts and experiences gained
by the author, both at the U. S. Air Force
School of Aviation Medicine, Randolph Field
and the Civil Aeromedical Research Instifute.
References are kept to a minimum, othetwise
there would be an abundance of them without,
probably, doing justice to the essgntial publica-
tions of many highly, qualified investigators.

v
Il. PSYCHO-PHYSIOLOGICAL STRESS }

Although much of man’s action is governed at
will by the central nervous system, most of the
interplay between the life sustaining organs is
taken out-of voluntary control and regulated by
the*autonomic, or vegetative, nervous system.
The central and augonomic nervous systems are
0 delicately. interconnected that merely a vis-
ual or auditory perception, or a thought, can
spark a vagal or sympathetic response. Such

*yesponses may be accompanied by very un-

comfortable symptoms. Well known is the
pre-show stage fright of experienced actors and
speakers, the uneasiness of athletes before
competition, and’ of soldiers before battle.
Pounding of the heart, the urge to yawn and
overbreathe, unpleasant intestirial upsets, the

eneral feeling of “emptiness” and weakness —
these are some of the symptoms preceding
dangerous or competitive actions or other ac-
tivities with a keynote of high suspense. Al-
though such a state of “stage fright” may be
experienced as a very unpleasant nuisance, it
still is‘an important phase in the individual's
preparation for the imminent event. If geared
properly, the predominant influence of the au-
tonomic nervous system provides an adequate
functional readiness for any responsive action
required. If, on. the other hand, the para-
sympathic and the sympathetic nervous sys-
tems are “getting out.of phase,” then the shifts
of blood flow resulting from vasoconstriction in
some organs and areas, and vasodilatation in
others, are likely to render the individual par-
tially or totally disabled for any quickly re-
quired and well-coordinated action. _

Human tolerances for mental and psycholog-
ical stress cannot yet. be defined in accurate
measures. A given individual might meet sim-
ilar situations occurring at different times, with
different attitudes. Apparently, man’s auto-
nomic nervous system is well adaptable, and
responds well to training. People who volun-
tarily burden themselves with tasks of high risks
and strenuous work — test pilots, mountain
climbers, race drivers, sky and deep sea divers,
explorers of the Arctics or jungles — have
trained themselyes to react, in the. event of
crisis, almost automatically according to pre-
vious ‘experiences or according to reflexes
conditioned while imagining and anticipating
particular crises.

The best psycho-physiological testing might
not enable us to predict precisely an individ-
ual’s reaction to an unforseen emergency situa-

+ tion, A good advice for the potential “explorer

of the unknown” might, therefore, be the
following: ;

(A. Learn to know your limitations for as
many physical and mental stresses as
possible.

-~2-



Supposedly, within a day or two, the outside
temperature climbs to 35°C (=95°F), Our
experimental subject, maintaining his running
routine, feels well on the first mile, starts push-
ing to keep the same speed on the second mile,
reduces the speed considerably on the third
because “he just does not have it today,” be-
comes so “fatigued” on the fourth mile that he
slows down to walking and trotting, and can
make the last mile only by alternatingly paus-
ing and walking.

What had happened? Soon after the first
8-10 minutes of strenuous work the body’s
mechanisms for regulating temperature are
intensified, and extremely so at unusually high
environmental temperatures, As a result, a
much greater part of the total blood flow is
shunted to the body’s surface area and thus
withdrawn from the blood flow necessary to
maintain the aerobic energy exchange in the
musculature, Since cardiac output already was
maximal, no further compensatory adaptations
were possible. Muscular activity had to be
reduced to a level which constituted a compro-
mise between blood flow required for cooling
of the body and that available for continuation
of work. “Becausé of their encroachment on
circulatory limitation’s, the effects of high tem-
peratures have caused reductions of work
capacity by 25 per cent and more.

Of course, there will be immediate criticism
from someone saying that the astronaut will not
be engaged in really strenuous work. In view
of such criticism, it might be worthwhile to
look at the problem from the reversed apgle —
how much circulatory reserye capacityi is left
during a severe heat stress? Heat tolerance
tests are usually terminated when the experi-
mental subject’s pulse rate has risen to a fre-
quency of 160 beats, per minute, almost to three
times the resting value. Bl pressure, at that
point, is usually increased also. Cardiac,/out-
put, therefore, must have been increased/ close
to three times the resting value of about 7 liters
per minute, that is to 21 1/min. Most “normal”

men of “average” physical condition have only ’

a potential capacity for 25 1/min total blood
flow, Therefore, t ere are not many reserves
left at extremely high temperatures and the
slightest additional stress, requiring further cir-
culatory adaptations, might complicate the situ-
«ation beyond possible physiological solutions.

2. ALTITUDE

Work capacity in acute altitude exposure is
reduced because of the higher requirements
imposed upon circulation and pulmonary venti-
lation by the lack of oxygen in the atmosphere.
In men who have become acclimatized to alti-
tude, the circulatory response for any given
work load may have returned to more normal
because of advantageous changes in the blood.
Then only the maximum breathing capacity sets
the limitations for work. Although, as was
discussed in the section on altitude’ stress,
maximum breathing capacity increases during
“active” altitude exposure considerably, from
about 125 1/min to approximately 180 1/min
(BTPS), there it reaches a definite limit. Table
VI may help to explain what reduction in work
capacity must be expected at various altitudes.

Because of an almost 50 per cent rise of
maximum breathing capacity (M.B.C.) after
three weeks of acclimatization to the altitude
of 10,000 feet the work capacity there prac-
tically equalled that previously attained at sea
level. After 6-8 weeks spent at the 14,000
foot level no further increase in M.B.C, was
observed and work capacity was definitely
reduced 13 per cent.

In the Himalayan Mountains man_ has
climbed up to altitudes between 28,000 and
29,000 feet without the use of oxygen. In
these events a metabolic rate of 3-4 times basal
metabolic conditions must have been main-
tained for a few hours, requiring, theoretically,
a pulmonary ventilation of approximately 90
1/mm. Actually, the breathing efforts must
have been much greater. Since the acclimatiza-
tion processes reach a plateau approximately
at 23,000 feet, any ascent beyond that altitude
level requires for a maximally acclimatized
individual similar adaptive mechanisms as an
acute ascent of ‘anormal man from sea level to
‘medium altitudes of 10,000 to 14,000, or more
feet. One should expect, therefore, that these
mountaineers, climbing upward on Mt. Everest
at an altitude of 28,600 feet, must have been
working closely at their circulatory and respir-
atory limitations.

* ~B-



(carbohydrates) are exhausted, In considering
many of

ing activities outside his sheltering “home” —
whatever it will be — the limited work capacity
of man under extreme cold situations should be
kept in mind!

The best chances for survival under critical
stress, whatever stress complication might be
encountered, are assured by superb physical
conditions, Exactly at the time this was writ-
ten, the first American astronaut completed
successfully an orbital flight. This was just
the beginning of an endless series of further
probing into deeper and deeper space — with
more and more psycho-physiological demands.
The future will tell us to what extent human
tolerance limits for complex stresses can be 10,
stretched by the very specialized trainin
needed for mastering the extraterrestria
universe,

the spaceman’s future missions requir- 1, 2 3.

iL.
12,
13,

4,

a relatively short time until his energy stores REFERENCES

Stapp, J. P. (1959). In “Man in Space” (K. F.
Gantz, ‘ed.), p. 63 Duell, Sloan and Pearce,
New York.

Graveline, D. E., Balke, B., McKenzie, R. E., and
Hartman, B. (1961) Aerospace Med., 32, 387.

Balke, B., Wells, J. G., and Clark, R. T. (1957),
J. Aviat. Med, 28, 241,

Balke, B., Ellis, J. P,, and Wells, J. G. (1958)
J. Applied Physiol. 12, 269,

Norton, E. F, (1925), The Fight for Everest,
1924, Longmans, Green, New York,

Homberger, W. (1950). In: “German Aviation
Medicine, W. W. II.” Vol. I, p. 484, Dept. of
the Air Force, Washington, D. C.

‘Wilks, S. S. and Balke, B. (1958). J. Aviat. Med.
29, 301,

Balke, B. In: “Bioastronautics, Advances in Re-
search,” p. 122. USAF School of Aviation Med.,
Brooks AFB, Texas.

Balke, B. (1960). In: “Science and Medicine of
Exercise and Sports” (W. R. Johnson, ed.) p. 339,
Harper Brothers Publishers, New York.

Iampietro, P. F., Vaughan, J. A. Goldman, R. F.,
Kreider, M. B., Masucci, F., and Bass, D. E.
(1960). J. Applied Physiol. 15, 632.

Hedman, R. (1957), Acta Physiol. Scand. 40. 305.



- a

.-

HUMAN TOLERANCES

Bruno Balke, M. D.

ABSTRACT

The ultimate limitations in flight performance and in future civil air carrier equipment

are the limitations im

by what may be termed “human

tolerances.” ‘This is particularly
discussion

ay ible to the matter of the .' The of man’s maximum
itptve apes er the maf eas eal eee kas na
transatmospheric flights points to the weakest links in the mari-machine complex of air and

‘space transportation. An attempt is made to -point out the means which the human
tolerances can be maximally adapted to the advanced technology. by

I. INTRODUCTION

In the ‘perpetual stru for survival on this
planet Earth, mdn a aly a remarkable
ability to adapt té a variety of rather adverse
environmental conditions. Although increasing
technology has succeeded in making life more
comfortable and less exposed to nature's perils
and, thus, has reduced man’s resistance to stress
and’ strain ‘in ‘general, the potential Capacity
for the full development of adaptive mech-
anisms fortunately appears preserved in ‘the
greater number of individuals. Through ‘train-
ing and conditioning many normal” people can
achieve considerable reserves for all sorts of
functional demands, and. become: enabled to
ex¢el in conquering new frontiers, to reach the
highest mountain summits, to venture {nto. the
deep sea, to resist extremely hot or¥cold en-
vironments and to perform amazing physical
work feats.

‘

Tn this new era of technology, in which the
human adaptive capacity is eclipsed by the
enormous potéatial of machinery, the full utili-
zation of such superior machinery requires ad-
ditional devices and techniques to compensate
*for the evolutionary shortcomings of man. A

well known example, in this regard, is the de-
velopment of modern aviation: from flying
“by the seat of his pants,” amongst or above
some lovely clouds, to the era of flying around
the ‘clock under all climatic and weather con-
ditions, the aviator has to be surrounded by
more and more equipment. This makes neces-
sary the additional perfection of instrumenta-
tion, particularly navigation, communication,
and..oxygen equipment, and. such things as
cabin pressurization, __ :

Despite all of the above measures, the tech-
nical capabilities of the newest aircraft cannot
always be utilized, because the operational de-
mands have to be kept at such a level that the
crew and passengers have a sufficient chance of
survival in emergericy situations. Almost no
efforts are made to raise , man’s psycho-
physiological Jimitations to a level where they
could match: more readily the limitations. im-
posed by their vehicles, This could have been
possible where flights did not exceed altitudes
of 55 to 60ithousand feet. Now the time has
arrived. when man begins probing near and
outer space.

The supersonic transport, for example, will be
a near-space vehicle. Flights beyond the

. -1-



|
HUMAN TOLERANCES

Bruno Balke, M. D.

Chief, Biodynamics Branch

62-6

FEDERAL AVIATION AGENCY
ye AERONAUTICAL CENTER
- CIVIL, AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA
APRIL 1962



.-

hours before becoming extremely fatigued.
Thus, without adequate shelter, the oorly con-
ditioned individual might not be able to sur-
vive a long period of exposure to temperatures
around the freezing point. The physicall
well-conditioned individual, however, can tol-
erate extremely cold situations for days and
weeks by maintaining just the Proper amount
of activity, About four-fifths of the energy
expenditure in rest and in physical work are
“wasted” on elevating or preserving body
temperature.

In contrast, heat exposure demands a reduc-
tion or cessation of physical efforts. The only
physiological defense against overheating is the
evaporative cooling of the body surface, com-
bined. with a highly increased blood flow
through this area, Adequate evaporative cool-
ing. is dependent, of course, on the humidity
of the surrounding air, and on sufficient body
fluids, Functionally, the main load is carried
by the circulatory system and the functional
limitations of the latter, more or less, determine
heat tolerance,

In extraterrestrial operations excessive tem-
perature stresses will invariably be combined
with other stresses such as hypoxia, hypo- or
hypercapnia, high humidity, or high gtorces.
These and other variables—clothing, radiation,
air movement, and possibly a change of phy-
sical condition during long space operations—
makes it impossible to predict accurately, in
terms of centigrades, the tolerable temperature
extremes, The biological space laboratories of
the future will certainly help in adding more
data to the already extensive literatire on the
effects of environmental temperatures under

complex stress situations, 4

Xx. TOLERANCE FOR ee

Why this last section on’ physical worl capa-
city? “Is not the future astronaut apd space
explorer bound to be very inactive because of
narrow living quarters and long periods of
weightlessness? Why should one be interested

how many foot pounds of work a candidate for '

outer-space could do in a given time?

The answer & simple: The capacity for phy-
sical work is the best criterion of physical fit-
ness.. Everyone appears to agree that excellent
‘physical fitness is one of the main prerequisites

for the man who is going to face all sorts of
unprecedented mental, psychological and phy-
sical stress situations and ardships, However,
the agreement stops right there since everyone
has his own ideas about “fitness.” What are the
differences between poor, good, excellent or
superior physical fitness, or physical condition?
Does a clean bill of health, given to a number
of individuals, mean that all these individuals
will have the same Potential capacity to with-
stand successfully the same. life endangering
emergency situations? The question here is not
about differences in skill or training or mental
attitudes or experiences! The only concern here
is: What, in healthy individuals, does con-
stitute differences in their physical condition,
enabling some to survive more or less.unharmed
while others are Perishing, some sooner, some
later? Looking at physical fitness from such a
point of view might help to reach a better agree-
ment on its basic principles.

A. Functional Limitations

The functional limitations of work capacity
can be assessed adequately in relatively complex
but short tests, involving gradually increasing
work intensities which will evoke, finally, maxi-
mum cardiovascular and res; iratory responses.
The best criterion for the adaptive capacity of
the circulatory system should be the maximum
cardiac output. “Unfortunately, direct measure-
ment of blood flow cannot be made in human
experimentation, and even the indirect measure-
ments require elaborate techniques. As an a]
parently poor substitute, pulse rate and blood
pressure can be measured routinely Most re-
cent investigations have indicated that this sub-
stitution does not offer so poor results after all.
In fact, it allowed for a very close estimation of
the relative changes in cardiac output.

Like any other pump, the heart can change
the flow of blood by changes of the stroke,
of the frequency of strokes, and by a combina.
tion of both,’ In recent years much emphasis
was turned toward the concept that the heart
responds to jincreased demands almost entirely
with a speed-up of the rate of contractions,
keeping the stroke volume nearly unchanged.
The basis for this concept was mainly gained
from experiments with animals and/or with
hospitalized male patients. Children and many

* -ll-



TABLE VI

1500 2000

2500 «3000 §=63500 ©4000
30 4 52 68 83 100

vo, ml/min 250 1000
Vent. (STPD) l/min 5 20
Alt, in ft. Ventilation
500 6.5 25
10,000 95 36
14,000 ll 43
18,000 13 52
24,000 18 70
28,500 22 89.

(BTPS)

liters/Min.
37 51 64 84 103 123
54 74 94 123 149181
65 89 112 47 V8
78 107 «135 ITT

105 143182

134 183

Vo, = oxygen intake, under basal metabolic conditions to maximum aerobic work capacity
Vent. (STPD) — pulmonary ventilation at 0°C, 760 mm Hg, dry
Vent. (BTPS) — “true” pulm. ventilation, saturated with H,O at body temp. at the environmental barometric

pressure.

An astronaut, having an identical level of
altitude acclimatization, should be able to per-
form work of 2-3 times his basal metabolic rate
under environmental conditions equivalent to
50,000 feet — when bréathing oxygen.

,

Cc. Corp

The functional capacity for work — as actual
experimentation has shown — is not directly
affected by severe cold. Metabolic work ca-
pacity, however — that is thé maximum energy
expenditure accumulated during steady worl
— is considerably reduced when severe epld
shivering competes — either preceding of in
rest pauses — with the work tasky In a wet-
cold environment the’ exhaustion pf metabolic
reserves can be fatal. The following example
may illustrate the clase relationship between
general physical condition and: tolerance for
combined work-cold stress. | __ /

Three experienced mountaineers’ — one! of
them a mountain guide in top physical con-
dition, the other two, after a long “inactive”
winter “out of shape” — climbed a not difficult
moufitain in the early pre-summer season.
Temperatures were tlose to the freezing point,
the ground toward the peak was covered with
ice and snow, and frequent rain or sleet showers
soaked the clothing. After a two-hour ascent,
‘n rest period was taken by the two weaker

members of the party, while the third climbed
to the peak and returned within one hour. Dur-
ing the rest period the former two ate a little
but were severely harassed by icy winds and
snow flurries. Immediately after the party had
rejoined, descent was begun but it became very
slow because of vehemently progressing
“fatigue” of the two less trained climbers who
shivered violently and became weaker and
weaker. Helping one of them down a more
difficult spot, the leader, suddenly, held a dead
body in his arms.

Concentrating on getting the third man safely
down, the guide used all the tricks of the trade
to force this man’s actions. At the beginning
of an easy trail, leading downhill within a one-
half hour, at a fast pace, to an inhabited shelter
cabin, the man gave up — utterly exhausted.
By now unable to carry his companion to safety,
the guide bedded him down in a place well-
protected .against the weather, covered him
with all the extra clothes available and with a
survival nylon sack — and hurried down to

‘alet a rescue teqny. Two hours later such a

team returned — but found the man lifeless.*
A great many of such incidents happen
annually also in the plains where severe bliz-
zards might trap scores of people. The knowl-
edge we gain; from such tragedies is: The
poorly conditioned man can fight cold for only

-u4—