**1.2 Hypoxia:**

Definition of hypoxia

Predisposing and contributing factors

Types of hypoxia (hypoxic, histotoxic, hypemic/anemic,
ischemic/circulatory)

Signs and symptoms of hypoxia

Phases of hypoxia (indifferent, compensatory, impairment, critical)

Time of useful consciousness (TUC) and effective performance time EPT)

Effects of hypoxia on human performance (including night vision, color
vision, reaction time, decision making, etc.)

Prevention and treatment of hypoxia

Oxygen systems (physiological requirements including pressure breathing,
oxygen toxicity, description of the various types of oxygen systems and
their operational limits \[high pressure, low pressure, LOX, chemical,
and OBOGs\], types of oxygen masks and regulators, properties and
manufacturing requirements of aviation oxygen)

Cabin pressurization systems (physiological requirements, description of
isobaric and isobaric-differential pressurization systems, advantages
and disadvantages of cabin pressurization, limits to cabin
pressurization)

Cabin decompression (common causes, types \[slow, rapid, explosive\],
emergency procedures following a cabin decompression, implications of
rapid decompression on TUC, variables that affect the rate and severity
of cabin decompression, etc.)

Difference between aircraft and space vehicle environmental systems

Pressurized suits (partial and full pressure)

Basic physiology and clinical aspects of mountain sickness

Section II, 1.2 HYPOXIA

Hypoxia is a condition in which there is a deficiency of oxygen
delivered at the tissue level.

There are four types: hypoxic, histotoxic, hypemic, and stagnant.

Hypoxic hypoxia results from a deficiency of O~2~ (or reduced partial
pressure) in the air being inspired. This occurs with altitude because
of decreased barometric pressure and is, therefore, most germane to
aerospace medicine. Because hypoxic hypoxia is so closely associated
with aviation, it will be given the most emphasis in this section.

Histotoxic hypoxia is caused by poisons that disrupt the cytochrome
oxidase system

which impedes the utilization of O~2~ at the cellular level. The classic
example is cyanide

poisoning. Exposure to cyanide could occur inflight if there were a
fire; it could also occur in

the aerospace maintenance work place.

Hypemic hypoxia occurs when the capacity of the blood to transport O~2~
is reduced. For example, CO has an affinity for Hb over 200 times that
of O~2~. Consequently, if an individual is exposed to CO, it will
displace O~2~ on the Hb molecule thereby reducing the amount of O~2~
delivered to the tissues even if the Hb and RBC levels are normal. This
could occur due to inflight fire or due to gas leakage into the cockpit.

Stagnant hypoxia occurs if blood circulation is inadequate. This can be
caused by illness such

as congestive heart failure, but it can also be caused by accelerative
forces inflight (+Gz) and by

positive pressure breathing (PPB). In the former, positive G loads will
force blood to the legs and decrease venous return. Likewise, PPB will
inhibit venous return by increasing intrathoracic pressure. In both
cases, cardiac output will be reduced thereby decreasing the
transportation of oxyhemoglobin.

Because the physiological effects of hypoxia were described in a
previous section, we will

now turn to its signs and symptoms. As shown in Table VII, the signs and
symptoms of hypoxic

hypoxia are extremely varied. Furthermore, there is significant
interindividual variability. In any

event, with increasing altitude and an increasingly deficient amount of
ambient O~2~, the more

severe the effects of hypoxia. However, even hypoxia with mild signs and
symptoms is

unwelcome in any cockpit because it can cause performance decrement.

TABLE VII

HYPOXIA

SIGNS AND SYMPTOMS

[Signs]{.underline} [Symptoms]{.underline}

Hyperventilation Headache

Cyanosis Dizziness

Intellectual Impairment Fatigue

Poor Judgment Reduced Visual Field

Behavioral Changes Decreased Night Vision

Delayed Reaction Time Drowsiness

Unconsciousness Paresthesias

Tingling

Interestingly, an individual exposed to an hypoxic environment will very
consistently

develop the same spectrum of symptoms. For example, if a pilot develops
headache and perioral

tingling in an hypoxic environment, the same symptoms will usually occur
on another

day under the same altitude conditions. This phenomenon is very
important to aviators because

there may be a time when a pilot is unknowingly becoming slowly hypoxic.

The danger of hypoxia is its insidiousness, causing increasingly subtle
performance decrement, followed by overt incapacitation. By recognizing
his or her symptoms before becoming significantly impaired or
incapacitated, corrective action can be taken by the pilot. It is for
this reason that altitude chamber training is so important because most
altitude chamber training protocols require experiencing hypoxia. This
is done by taking subjects to altitude (25,000-35,000 ft.) and having
them remove their O~2~ masks. Within minutes, symptoms can be

expected to occur.

The effects of hypoxia can be categorized into four stages according to
altitude: indifferent

stage, compensatory stage, disturbance stage, and critical stage. It
should be borne in mind that

there is not exactitude in these stages and there is individual
variability.

Indifferent Stage (sea level to 10,000 ft.)

In this stage, compensatory mechanisms are activated with an increased
heart rate,respiratory rate, and tidal volume. There is a decrease in
night vision (dark

adaptation) and performance decrement in accomplishing novel tasks, the
latter of

consequence in the event of an emergency requiring initiative and quick
action.

Compensatory Stage (10,000-15,000 ft.)

The compensatory mechanisms become more active and some signs and

symptoms may appear according to individual variability. Drowsiness,

increased reaction time, and slowing of mental processes could affect
ability to perform in the cockpit.

Disturbance Stage (15,000-20,000 ft.)

Overt signs and symptoms of hypoxia are manifest with inadequate
response

of physiological compensatory mechanisms to overcome this physiological
stress.

Performance decrement becomes inevitable, endangering flight.

Critical Stage (over 20,000 ft.)

The CNS and CVS are severely compromised with eventual loss of

consciousness and possibly death.

Effective performance time (EPT) is an aviation concept that defines how
long a crew

member can [effectively]{.underline} perform cockpit tasks when in a
state of hypoxia (Table VIII). (The old

term for EPT was time of useful consciousness or TUC.) If the EPT
becomes too short, flying

safety will be adversely affected because there may not be enough time
for the crew to take

remedial action. The EPT will vary depending upon a number of factors
including individual

variability. It will be shortened with increasing altitude, longer time
at altitude, or a higher

rate of cabin decompression.

TABLE VIII

EFFECTIVE PERFORMANCE TIME

(EPT)

[Altitude (ft.)]{.underline} [EPT (min.)\*]{.underline}

18,000 20-30

25,000 3-5

30,000 1-2

40,000 \<.5

\*May be reduced 50% with rapid decompression.

In other words, if an aircraft cabin slowly decompressed at an altitude
of 30,000 ft., the pilot

would have 1-2 minutes of EPT. After that there would be significant
performance decrement

and soon after loss of consciousness due to hypoxia. Matters would be
far worse if there were a

rapid decompression of the cockpit (\<1 sec.) because EPT could be
reduced by 50%, leaving

the pilot as little as 30 seconds to take action.

Although everyone is susceptible to the effects of hypoxia, there are
predisposing factors that

will increase susceptibility including illness and self-imposed stress
such as smoking and

alcohol. Excessive physical exertion at altitude also reduces one's
tolerance to an hypoxic

environment.

There are a number of countermeasures available to prevent or minimize
the effects of

hypoxia. In commercial aviation it is the pressurized cabin which keeps
passengers and crew

below a tolerable 8,000 ft. regardless of aircraft altitude. Only a
decompression would put the

crew and passengers at risk, although emergency O~2~ should provide
protection.

For general aviation aircraft that are un-pressurized, supplemental O~2~
would be necessary particularly when flying over 10,000 ft. Other
countermeasures primarily utilized by the military are pressure
breathing and pressure suits.

Utilizing supplemental 100% O~2~, it is possible to maintain a sea level
PAO~2~ with altitude as

high as 34,000 ft (Table IX). Below 34,000 ft, depending upon what level
of PAO~2~ one wishes to

maintain, a lesser percent of O~2~ would be sufficient. For example, if
one wants to maintain a sea

level PAO~2~ at 10,000 ft., the gas delivered by mask need only be 30%
O~2~. Of course, with

increasing altitude, the % O~2~ delivered must be increased accordingly
as shown in Table IX.

TABLE IX

OXYGEN REQUIREMENTS

TO MAINTAIN A SEA LEVEL PAO~2~

[Altitude (ft.)]{.underline} [Barometric Pressure (mm Hg)]{.underline}
[O~2~ Requirement(%)]{.underline}

S.L. 760 21

5,000 632 25

10,000 522 30

15,000 428 37

20,000 349 45

25,000 281 56

30,000 225 70

34,000 187 100

Consequently, whatever O~2~ delivery system is used, an increasing
percent of O~2~ must be

delivered in order to maintain a sea level PAO~2~. (It must not be
forgotten that even with a sea

level PAO~2~, the PaO~2~ may still be abnormal if the crew member has
pulmonary pathology such

as COPD which prevents normal O~2~ and CO~2~ diffusion between the
alveoli and pulmonary

vasculature.)

However, beyond 34,000 ft, even 100% O~2~ will be inadequate.
Consequently,

100% O~2~ must be delivered under pressure (see below).

Supplemental O~2~ is usually delivered in a gaseous form, as liquid
oxygen (LOX), or by

chlorate candle as is the case for commercial airlines emergency O~2~
drop- down masks.

Gaseous O~2~ is 99.5% pure, i.e., free of contaminants and is stored in
high pressure cylinders

color coded yellow. It can be delivered by aviator mask or oronasal
cannula. One hundred

percent O~2~ is delivered on commercial aircraft but for military
aircraft the percent of O~2~ is

determined by altitude ( Table IX) in order to conserve the supply. LOX
comes in high pressure

cylinders color coded green and provides the gaseous form of O~2~ to the
user by evaporation of

the liquid.

The LOX system is used mainly in high performance military aircraft
because it is relatively light weight and occupies much less space than
gaseous O~2~ cylinders. Chlorate candles

release O~2~ by a chemical reaction (NaClO~3~ + Fe=FeO +NaCl+O~2~)
initiated by heat once the

system is activated by pulling down on the drop- down mask. Another O~2~
delivery system

adopted by the military services is the onboard oxygen generating system
(OBOGS).

With this system, ambient air is passed through a bed that extracts O~2~
and delivers it to the aircraft system, eliminating the need for a
stored O~2~ supply. Although this is a major advantage of OBOGS, one
disadvantage is that it can produce only 95% O~2~ (not 100% O~2~).

Whatever system is used, the percent and delivery pressure of O~2~ is
controlled by a regulator

of which there are several types: continuous flow, diluter demand, and
pressure demand.

In a continuous flow system such as used in commercial aircraft
supplemental oxygen systems, O~2~ simply flows continuously into the
mask regardless of phase of respiration.

With a properly fitting aviator's mask, this should provide, at least
theoretically, a sea level

PAO~2~ as high as 34,000 ft. (In reality its ceiling is 25,000 ft.
because the 100% O~2~ is diluted in

the lungs by CO~2;~ also, the masks might leak if not well sealed on the
face.) Unfortunately, this

system, while useful for passengers until descent to a safe altitude, is
also wasteful because it

delivers O~2~ even during expiration.

The diluter-demand O~2~ system is superior to the continuous flow system
because it is less

wasteful by allowing O~2~ flow only during inspiration, thus conserving
the supply. Also it does

not deliver only 100% O~2~---rather it provides an increasing percentage
with increasing altitude

with a schedule somewhat similar to the O~2~ requirements outlined in
Table IX. The diluter-

demand system gives physiological protection, i.e., a sea level PAO~2~
to 34,000 ft. if a well

sealed aviators mask is used.

As emphasized above, 100% O~2~ beyond 34,000 ft. will not suffice to
maintain a sea level

PAO~2~ because of the lowered barometric pressure. Consequently, the
100% O~2~ must be delivered

under increasing pressure---positive pressure breathing (PPB)---with
increasing altitude above

34,000 ft. (Table X). For example, if a pressurized aircraft at 40,000
ft. decompresses, the PAO~2~

would fall to 54 mm Hg. In order to increase this to a sea level 102 mm
Hg, 100% O~2~ under a

pressure of 48 mm Hg must be delivered. This added pressure is provided
by the 3^rd^ type of

regulator, the pressure-demand O~2~ system. This type of regulator is
most commonly used in

military high- performance aircraft.

Unfortunately, there are limitations to how much positive pressure
breathing (PPB) can be

tolerated by an airman. Most can tolerate 15-30 mm Hg added pressure but
beyond that, the

system itself causes such physiological stress that other means of
protection become necessary

(see below). With PPB, the airman could become fatigued because of the
necessity to forcefully

exhale in order to overcome the pressure in the mask---this results in
an increased work of

respiration. There is also the tendency to hyperventilate because of
this resistance, which, in

itself, can cause unwanted symptoms. For most airmen, these stresses are
tolerable at 15-30 mm

Hg for ½ hour.

As PPB approaches 30 mm Hg, the increased intrathoracic pressure will
cause venous pooling with a decreased venous return, decreased cardiac
output, and possible syncope. As the PPB approaches 60 mm Hg, there can
be damage to the lungs because of tearing. Although there is individual
variability in an airman's ability to tolerate PPB, the system should
offer at least short time protection to altitudes as high as
43,000-50,000 ft.

TABLE X

PRESSURE BREATHING REQUIREMENTS

Altitude(ft.) B.P.(mm Hg)\* PAO~2~ with 100% O~2~ Pressure to

Maintain S.L.

PAO~2~(mmHg)

34,000 187 100 none

40,000 141 54 48

45,000 111 24 78

50,000 87 0 102

\*B.P. is barometric pressure.

To protect airmen who are exposed to very high altitude, where 100% O~2~
delivered

under pressure becomes intolerable as described above, a pressure suit
becomes necessary.

Although it protects the aviator from decompression illness (DCI) and
low temperatures, its

primary purpose is to prevent hypoxia.

There are 2 types: a full pressure suit and a partial pressure suit.
These suits are necessary for military aviators flying at high altitude
(\>50,000ft.) reconnaissance aircraft and astronauts during
extravehicular activity (EVA).

The full pressure suit can be thought of as a cockpit within a cockpit
because it houses the airman in a controlled environment with
pressurization, an O~2~ supply, ventilation, and cooling (by circulating
air or water).

Athough the physiological support of such a suit may vary with the
manufacturer, most

provide full body pressure of 25,000 ft. (281 mm Hg) with 100% O~2,~
which allows for a sea

level PAO~2~. In other words, the airman is literally housed in a
garment-helmet ensemble that

provides a pressure of 281 mm Hg and 100 % O~2~---this provides a
comfortable physiological

environment and, most importantly, protects against hypoxia even in the
vacuum of space.

To illustrate, if a pilot is at 70,000 ft. in a cockpit pressurized to
22,000 ft. and there is a

rapid decompression, the cockpit will soon be at 70,000 ft (or the
altitude of the aircraft)---this

would be physiologically intolerable with significant hypoxic symptoms
occurring in seconds.

However, with full pressure suit automatic activation, i.e., the suit
would rapidly inflate to

281mm Hg and 100% O~2~ would be provided to the pilot, thereby,
preventing hypoxia. This is a

tolerable situation, permitting the pilot to safely take corrective
action.

Partial pressure suits are a modification of the full pressure suit and
are rarely used in

today's operations. (One exception is the launch- entry suit worn by
astronauts during launch and

reentry.) It is a form fitting suit with air filled capstans that when
inflated will tighten the suit material causing pressure to be applied
to most of the body. There is also a helmet that provides 100% O~2~
under pressure. This suit is not as efficient as the full pressure suit
because it does not cover the entire body, although it will provide
about 30 minutes of "get-me-down" protection at altitudes as high as
100,000 ft.

Perhaps the greatest discovery in air travel is the pressurized cabin.
With a pressurized cabin,

the barometric pressure can be maintained at any cabin pressure
regardless of aircraft altitude.

For example, if an aircraft were flying at 30-35,000 ft., the cabin
pressure might be maintained at

much lower altitudes, such as 5-6,000 ft., thereby protecting all
cabin/cockpit occupants. This

obviates, therefore, the need for special equipment such as supplemental
oxygen, oxygen masks,

PPB, and pressure suits. The risk of DCI is also reduced. .

The FAA and most international regulatory agencies allow for a maximum
cabin altitude of 8,000 ft. which is well tolerated by reasonably
healthy individuals. Today's commercial aircraft flying at

altitudes as high as 43,000 ft. (the Concorde routinely flies at 60,000
ft.) offer crew and

passengers safety and comfort in a shirt sleeve environment.

The only potential danger is decompression of the aircraft due to a
mechanical malfunction, the effects of which will be discussed below.

Different aircraft have varying pressurization schedules, although all
of them can be classified

as isobaric or isobaric-differential, each having its advantages and
disadvantages. With an

isobaric schedule, the aircraft altitude is the same as the aircraft
altitude up to 8,000 ft. In other

words, as the aircraft ascends from the ground up to 8,000 ft., the
inside barometric pressure (and

ambient pO~2~) remain the same as the outside barometric pressure.

However, once the aircraft ascends beyond 8,000 ft., the inside
barometric pressure remains fixed at that altitude. Therefore, the
aircraft could be at any altitude as high as 40,000 ft. but the inside
would be steady at 8,000 ft.

On the other hand, the isobaric-differential system maintains a sea
level barometric pressure

until the aircraft reaches 21,000 ft. Once the aircraft is beyond 21,000
ft., the cabin altitude

slowly ascends while maintaining a constant pressure differential
between the inside and

outside of the aircraft of 8.6 psi (446 mm Hg). Consequently if the
aircraft were flying at 40,000

ft. where the barometric pressure is 2.7 psi (140 mm Hg), the inside
cabin pressure would be

11.3 psi (8.6 psi+2.7 psi) or the equivalent of 7,000 ft (586 mm Hg).

The isobaric-differential system can be set at any differential as long
as the cabin altitude does not exceed 8,000 ft. In military fighters it
is usually much lower than 8.6 psi, e.g., 5 psi in order to minimize the
effects of a rapid decompression due to battle damage. With a lower
pressure diffetrential, penetration of the aircraft by battle damage
would be much less severe.

There are many advantages to aircraft pressurization including the
comfort/safety of a shirt sleeve environment for crew and passengers and
the reduced risk of DCI ( gas expansion and N~2~ evolution). Also there
is no need for supplemental oxygen (except in emergency situations),

aviator masks, and pressure suits.

The main disadvantage is the potential for loss of cabin pressure . If
this occurs, there are significant risks due not only to aircraft
damage, but due to potential performance degradation and partial or
complete incapacitation from hypoxia or DCI.

A pressurized cabin is practical up to 85,000 ft. Above 85,000ft. the
air is so rarified that aircraft compressors can no longer pressurize
the cabin. Hence, a sealed cabin, similar to that employed in space
vehicles becomes necessary. A sealed cabin is entirely self contained
carrying its own O~2~ supply, obviating the need for outside air.

The need for sealed cabins is not a requirement in today's civil
aviation operations, but could be in the future if commercial aircraft
fly suborbital or orbital routes.

As discussed above, the major risk of a pressurized cabin is the
potential for cabin depressurization. This can occur due to a system
malfunction or damage to the aircraft causing a

hole in the aircraft structure that allows cabin air to escape to the
outside, e.g., loss or breakage

of a window, opening of the cabin door, damage from explosives, etc.

The loss of pressurization can be slow possibly due to a small leak
while a rapid or explosive decompression occurs suddenly, usually within
a few seconds. In any event, the severity upon the cockpit/cabin
occupants depends upon a number of factors including the rate of
decompression and the difference in pressure between the inside and
outside of the cabin (pressure differential).

Other factors include the volume of the cabin/cockpit and the size of
the opening. Consequently, the decompression would be most severe if the
rate of decompression is rapid, the pressure

differential is high, the volume of the cockpit/cabin is small, and the
size of the opening large.

When a rapid decompression (R.D.) occurs, there is not only confusion
caused by noise and

fogging, but also the danger of hypoxia, gas expansion, DCI, and
hypothermia. Hypoxia is

particularly compelling at high altitude , because the EPT can be
measured in seconds (Table

VIII).

Should an R.D. occur, corrective action must be taken immediately. Of
utmost urgency is the

donning of oxygen masks. Quick-don masks are available to the crew and
drop-down masks for

the passengers. This will at least give the crew ample time to clearly
think out emergency

procedures which, in most cases, would include rapid descent to lower
altitude and landing as

soon as possible.

Acute mountain sickness(AMS) or high altitude sickness is a syndrome of
ill-defined etiology that can cause a broad spectrum of symptoms ranging
from mild or annoying to life threatening. It most commonly occurs in
mountain climbers or skiers who are un-acclimatized to altitude. The
higher the altitude and faster the rate of ascent, the greater the risk.
Cardinal features include nausea, vomiting, anorexia, headache,
dizziness and fatigue or lassitude. AMS occurs most commonly over 6,000
ft. and usually develops 6-10 hours after ascent.

Because of its features, it is highly doubtful that AMS is a significant
risk to airmen in-flight, although the possibility is there.

The more severe forms of AMS include high altitude cerebral edema (HACE)
and high

altitude pulmonary edema (HAPE). HACE results from cerebral vasodilation
causing

encephalopathy and, frequently, death. HAPE is a form of noncardiogenic
pulmonary edema with

cough, shortness of breath, and hemoptysis. Like HACE, it is a very
serious manifestation of

AMS frequently causing death.

Fortunately, HAPE and HACE are rare.

Although the basic treatment of AMS in its mild forms is symptomatic,
HACE and HAPE

demand more aggressive therapy including cessation of activity at
altitude, 100 % O~2~, and

descent to a lower altitude. Descent can also be simulated in a portable
hyperbaric chamber

(which can be pressurized) if available. Some have advocated
acetazolamide and dexamethasone

for the treatment of HAPE and nifedipine for HACE.

There are preventive measures recommended for individuals who are
particularly sensitive to

AMS. This includes a slow ascent to altitude and a gradually increasing
work (exercise) load to

allow for acclimatization. Acetazolamide given prophylactically prior to
ascent has been found

useful in some cases.