**2.7 Sustained Acceleration Exposure:**

Definitions of speed, velocity, and acceleration

Types of acceleration (linear, angular, Gx, Gy, Gz)

Variables affecting acceleration

Sources of acceleration forces in aerospace operations

Physiological effects (cardiovascular, pulmonary, CNS, urinary,
musculoskeletal)

Symptoms and signs of acceleration exposure (including definitions of
G-tolerance, G-induced loss of consciousness or G-LOC, red-out,
gray-out, black-out)

The G-LOC Syndrome

Effects of acceleration exposure on human performance

Predisposing and contributing factors to decreased G tolerance

Countermeasures and preventive measures (anti-G suits, anti-G maneuvers,
positive pressure breathing, counter-pressure jerkin, seat tilting)

Emergency ejection issues (type of acceleration exposure, effects,
protective measures)

James E. Whinnery Ph.D., M.D.

Aeromedical Research Division

Civil Aerospace Medical Institute

Sustained Acceleration

Numerous stresses exist for humans here on Earth. By virtue of their
very presence here on Earth humans, and the other currently surviving
organisms, have successfully evolved to cope with the routine stresses
that they encounter. These successful organisms, including humans, have
evolved psychophysiological mechanisms that can adapt to stressful
changes in the environment to maintain short and long term viability.
These mechanisms ensure that the organism has the ability to maintain
homeostasis in the face of terrestrial stresses. Some types of stress
are only occasionally present while others are always present. The
singular stress that has constantly been present for all of us on Earth
is gravity. We have adapted fairly well over evolutionary time to this
ever-present gravitational stress.

Assuming that life on Earth has existed for 2 billion years and land
animals have existed for 275 million years, we humans have had
considerable time to evolve as upright creatures that can tolerate the
Earth's gravitational stress. Based on these estimates of time, we can
define the apparent rate of onset of gravitational stress we humans have
adapted to as we have arisen to an upright posture in the Earth's
gravitational field. The rate of onset of that gravitational stress
would be estimated as approximately 1g of stress developing over 275
million years.

In terms of onset rate of the gravitational stress over time we could
say that this represents 0.00000000000000011 G/s over evolutionary time.
This has evidently been an adequate time for humans to adapt such that
we have few problems with gravitational stress on a daily basis. It is
only rarely that we experience symptoms, such as being lightheaded or
dizzy when we rise too rapidly in the gravitational field. These
represent acute symptoms associated with exceeding the evolutionary
mechanisms existing in the upright human exposed to the Earth's
gravitational stress.

In some individuals, the inability of the normal psychophysiologically
protective mechanisms to respond can be compromised and other acute
symptoms, such as loss of consciousness, may develop. Ultimately, in the
long run of a lifetime, we humans all succumb to this relatively mild
but constant level of gravitational stress. We are all familiar with the
skin, cardiac, vascular, and skeletal system manifestations of the
long-term effects of a lifetime in the constant gravitational stress
field on earth. At least part of the effects of aging on the body is due
to the ever-present gravitational stress.

To enter the aerospace environment, above the Earth, exposes humans to
additional stresses that as terrestrial creatures we have had very
little time for protective mechanisms to evolve. It is therefore not
surprising that we humans are susceptible to more frequent and severe
symptoms and potential problems with maintaining normal homeostasis. To
even enter the aerospace environment is life threatening. We have become
so accustomed to flying in shirtsleeves we sometimes forget this fact.

Immediately, the exposure to altitude requires that we be supplied
oxygen to prevent loss of consciousness or death. Protective equipment
and techniques must be provided when we leave the terrestrial
environment in which we have evolved. Increased gravitational stress,
which we call acceleration stress, is also one of the several stresses
that are experienced when we humans navigate in the aerospace
environment. The most effective approach to understanding tolerance to
acceleration and the problems associated with exceeding tolerance to
acceleration is in terms of the evolutionary aspects of how the body has
adapted to gravitational stress.

Physics and physiology

One of the most interesting and perplexing forces that exists in nature
is gravity. It has occupied the minds of the most talented scientists
for centuries. Aerospace medical specialists should be counted in the
list of those who have at least a basic understanding of gravity and
acceleration.

Physics

A free-falling object is an object that is falling under the sole
influence of gravity; such an object has an acceleration of 9.81 m/s/s,
downward (on Earth). This numerical value for the acceleration of a
free-falling object is known as the acceleration of gravity - the
acceleration for any object moving under the sole influence of gravity.
This important quantity has been given a special symbol to denote it -
the symbol "g." According to Newton's 3^rd^ Law of Motion, for every
action (an acceleration) there is an equal and opposite action (an
inertial force).

It was only in the last century that the relationship between gravity
and acceleration was refined by Einstein in the Principle of
Equivalence. Einstein proposed that no experiment can distinguish
between the acceleration due to gravity and the inertial acceleration
due to a change of velocity. The Theory of Relativity considers that the
gravitational force of acceleration is identical to the inertial force
of acceleration.

Acceleration in an aircraft as it maneuvers generates centripetal
acceleration that is opposed by an equal and opposite inertial or
centrifugal force. By constantly changing the direction of the mass of
the aircraft (and the pilot), a *centripetal* force is produced that
results in a *centrifugal* force on the pilot that stresses their normal
homeostasis. We quantitate this inertial force on the restrained pilot
in multiples of the acceleration due to gravity (multiples of 9.81m/s/s)
and describe it in dimensionless units of "G." For the 3 axes of the
body, the nomenclature has evolved to symbolize the physiological
effects that result from the inertial forces. +G~z~ (the inertial force)
is produced from *head-to-foot* as the aircraft maneuvers in a tight
"inside" turn. --G~z~ would result from an "outside" turn
(foot-to-head). ±G~x~ and ±G~y~ refer to the direction of transverse and
lateral G on the body, respectfully as illustrated in Figure 1.

![](media/image1.jpeg){width="1.9756944444444444in" height="2.30625in"}

**Figure 1.** Conventions used to describe the inertial forces on the
body; +G~z~ = Head to foot inertial force; +G~y~ = Left to right
inertial force; +G~x~ = Front to back inertial force.

[]{.mark}

Physiology

In aerospace medicine it is important to understand that it is the air
or spacecraft that generates the acceleration "g." The human within the
craft is indeed accelerated along with the craft; however, it is the
equal and opposite action, the inertial force "G", that alters
physiology and can cause problems that affect aerospace safety.

In this aerospace medical discussion, we are interested in the effects
of +G~z~ because we are focusing on the stress that can more frequently
cause symptoms in pilots, including loss of consciousness in civilian
aerobatics. Neurologic symptoms, including loss of consciousness
resulting from +G~z~ (G-LOC), develops as a result of the differential
location of the central nervous system (CNS) and the heart within the
+G~z~ field. The effects of +G~z~ are such that blood flow to the CNS
locations above the heart can be compromised and thereby produce
symptoms that cause operational problems for the pilot of an aircraft.
The heart must generate higher driving pressure to maintain perfusion in
the head as acceleration stress increases. When inadequate perfusion
pressure occurs the neurological tissues become ischemic and symptoms
result.

It is convenient to recognize that each integral increase in G reduces
eye-level blood pressure by approximately 20 mmHg (actually by 22 mmHg
for each increase in +1G~z~-stress). If the blood pressure at heart
level is 130/80 mmHg (mean arterial pressure being 105 mmHg) at rest in
the Earth's normal gravitational environment of +1G~z~, then mean
eye-level blood pressure would be about 85 mmHg. This eye-level mean
arterial perfusion pressure is decreased by 20 mmHg for each increase of
+1G~z~. If no physiological responses are generated, this would mean
that eye-level mean arterial pressure would be close to 0 mmHg around
+5G~z,~ see Figure 2. We would expect neurological symptoms to result if
inadequate arterial perfusion (ischemia) persists for a sufficient
period of

![](media/image2.jpeg){width="4.170138888888889in" height="2.45625in"}

**[Figure 2.]{.mark}** Heart and eye level blood pressures at resting
+1G~z~ and +6G~z~.

time. Our evolutionary development has thus provided a considerable
cushion against exceeding tolerance to +G~z~-stress, specifically about
4G worth of buffer before neurological symptoms occur when exposed to
+G~z~-stress. This is a generous cushion when we operate within the
terrestrial +1G~z~ environment. It is an inadequate cushion when a high
performance aircraft generates +9G~z~ within one second and sustains it
for a long time.

G-tolerance in normal humans

As previously mentioned, we have successfully evolved to tolerate the
constant gravitational stress of Earth (+1G~z~ stress on the body). In
just the past 100 years we have not only learned to fly, but we have
developed aircraft that have remarkable maneuverability that can exceed
the tolerance of normal humans. Human tolerance to acceleration is more
complicated than it might appear. It depends not only on the direction
of the stress relative to the body, as mentioned above (+G~z~), it also
depends on the level of the stress, the rate at which the stress is
applied, and the duration of the stress. Finally, tolerance also depends
on the anatomy and physiology of the individual at the time of exposure.

An extremely useful concept for understanding G-tolerance in normal
individuals is the G-time tolerance curve as shown in Figure 3. This
curve is actually a combination of two curves defining neurological
symptoms resulting from various G-onset rates that rapid and exceed
cardiovascular reflex responses and that G-onset rates that are gradual
enough to allow cardiovascular reflex response.

![](media/image3.jpeg){width="3.1819444444444445in"
height="2.3993055555555554in"}

**[Figure 3.]{.mark}** The G-time tolerance curve; +G~z~-level in G
units vs. Time in seconds.

*Level or magnitude*

We shall now consider the average acceleration (+G~z~) tolerance of
"normal" individuals. It is important to note that individuals with
medical irregularities can have normal tolerance significantly altered.
In civilian aviation, as compared to military fighter aviation, the use
of protective equipment (G-suits, for example) is frequently not
present. Assuming that no protective equipment is utilized, the average
human would be expected to tolerate about +4 to +5G~z~. That's not all
that must be considered, however.

*Onset rate*

This average level of tolerance depends on several other things beside
just how high a level one can tolerate, it also includes how rapid the
onset of the acceleration stress is. The average tolerance of a normal
human mentioned above, being about +4 to +5G~z~, assumes that the onset
is gradual enough for the cardiovascular system to respond and augment
the perfusion pressure to the cephalic nervous system. If the onset is
rapid, such that the stress is applied faster than the cardiovascular
system can respond, the average tolerance is decreased to about +3 to
+4G~z~. The cardiovascular system can respond fully if the onset rate is
on the order of 0.1G/s or less, not very rapid even for civilian
aircraft. Rapid onset stress can be as high as 10 G/s or more in
civilian aerobatic aircraft and military fighters. Compared to our
evolutionarily developed tolerance, it is no wonder that 10G/s grossly
exceed our current human design characteristics as previously described.
It is evident that even 0.5G/ is greater than what our human
cardiovascular system can fully respond to.

*Duration*

Normal human tolerance also depends on the duration that the +G~z~
stress is applied to the individual. If the stress is of very short
duration, that is only 2 to 3 seconds of exposure to levels above the
ground level of +1G~z~, then very high levels can be tolerated. If the
duration exceeds 5 seconds then the tolerance would be expected to be
within the levels described above. Sustained acceleration is considered
to be present if the acceleration lasts for greater that 5 seconds.
Fatigue of the musculoskeletal system can result with sustained
acceleration. Fatigue results in the inability of a human to tense their
muscles and allows the adverse effects of acceleration to reduce or
overcome human tolerance. If the leg and abdominal muscles are not
contracted blood will pool in the capacitance vessels (veins) in those
areas. Such pooling of blood reduces the volume of blood that is
available to the central circulation for ultimately supplying the brain
with oxygenated blood. Pooling of the blood in the extremities and
abdomen can reduce +G~z~ tolerance.

*Other tolerance considerations*

We have mentioned only the main aspects of +G~z~ tolerance. Other
considerations include the ability to perform an effective protective
anti-G straining maneuver (training), how long one is able to
effectively perform and maintain the straining maneuver (physical
conditioning/rest), recent/frequent exposure to the high-G environment
(acclimation to the environment), individual characteristics (for
example: anatomy, short may be more tolerant than tall; physiology, very
low blood pressure can predispose to lower tolerance), and a host of
medical conditions that can adversely alter tolerance. On any given day,
a unique combination of events that can reduce tolerance may combine to
reduce an individual's normal tolerance by just enough to cause a G-LOC
episode in-flight.

*Structural Tolerance*

Musculoskeletal and other pathological injury to the body can result
from exposure to acceleration. Neck injuries are frequent in military
fighter pilots exposed to acceleration stress. Movement of the head and
neck while wearing helmets and oxygen masks is a frequent contributor to
neck and back injuries. Neck and back muscular strengthening exercises
are recommended for anyone anticipating high G exposure. Muscular
warm-up and stretching prior to acute exposure is also recommended to
prevent musculoskeletal injury. Exposure to -G~z~ acceleration can
result in hemorrhages above the heart (such as conjunctival hemorrhages)
resulting from breach of integrity of the vascular system. Any existing
structural abnormality in the skeletal or other system could produce a
susceptibility to reduced structural tolerance.

[Exceeding G-tolerance]{.underline}

The usual criteria utilized for defining G-tolerance are associated with
+G~z~-stress and the ischemic/hypoxic effects on the neurological
system. Although the main effect is the +G~z~--induced ischemia on the
cephalic portion of the central nervous system, a hypoxic aspect results
from the ventilation-perfusion mismatch that occurs in the lungs. The
magnitude of the hypoxic component increases as the duration of the
+G~z~--stress increases. Acceleration stress along the other axes is not
generally signaled by abrupt incapacitation. In the prone position
(-G~x~) the head requires support and in both the prone and supine
(+G~x~) positions chest compression becomes a problem. Lateral
acceleration (±G~y~) stress usually has more of an impact on performance
such as controlling the aircraft. The symptoms associated with exceeding
+G~z~ --stress that are of the most concern are associated with sudden
incapacitation, specifically +G~z~ --induced loss of consciousness
(G-LOC).

The G-LOC Syndrome

The entire symptom complex associated with the loss and recovery of
consciousness has been defined as the G-LOC syndrome. The G-LOC syndrome
complex is described in Table I.

Table 1. The G-LOC Syndrome

Loss of peripheral vision

Tunnel vision

Blackout (complete loss of vision)

Loss of consciousness

Loss of motor control (purposeful movement)/output from the brain

Loss of sensory input to the brain

Lack of memory formation

Electroencephalographic synchronization (slow - delta waves)

Myoclonic convulsions

Vocalizations (occasional moaning or groaning)

Dreamlets

Recovery of consciousness

Neurological reintegration

Self-touching reflex (Sensory-motor integration)

Neurological external environment reorientation

Return of purposeful movement

Transient tingling or slight numbness of the extremities and/or
periorally

Alteration of psychological state (anxiety, confusion, giddiness,

embarrassment)

General preservation of cardiorespiratory function

*[Not part of the G-LOC syndrome:]{.underline}* Loss of bowel or bladder
control; only rarely does tongue biting occur and it is usual in
association with the myoclonic convulsions; respiration is preserved; no
relationship between cardiac dysrhythmias and G-LOC has been observed.

The most common symptom associated with exceeding G-tolerance is a
progressive compromise of vision. The progressive, regional lack of
blood flow to the retina begins with loss of peripheral vision and
increases to tunnel vision and then blackout. Blackout is complete loss
of vision with preservation of consciousness. Vision is vitally
important to aerospace safety, representing 80% of the input necessary
for optimally piloting an aircraft. The visual symptoms, with
preservation of consciousness, result from regional ischemic (hypoxic)
differences within the cephalic nervous system. The eye has an increased
pressure compared to the remainder of the cephalic nervous system,
intraocular pressure that may be around 20 mmHg, which is equivalent to
+1G~z~ reduced tolerance to ischemia/hypoxia. This is important to
recognize because the other G-LOC syndrome symptoms can also be linked
to regional ischemic differences within the cephalic nervous system. The
key system that is immediately necessary for piloting an aircraft is the
neurological system. Vision and consciousness degradation immediately
compromise the safety of flight. Any abnormality or disease, as
discussed below that ultimately affects the neurological system, is a
concern in aerospace medicine because of the potential for compromise of
normal neurological processes.

G-LOC Syndrome Kinetics

It is important to have a detailed understanding of the kinetics of the
G-LOC syndrome. The time relationship of the symptoms induced by
+G~z~-stress provides insight to the anatomic basis of ischemic
compromise of the structures within the cephalic central nervous system.
Accident and incident investigation is also facilitated by the kinetics
of G-LOC. As shown in Figure 4, the key features of the G-LOC syndrome
are shown.

For rapid onset +Gz exposures, which can produce G-LOC without warning,
the loss of consciousness induction time (LOCINDTI) is about 5 -- 7
seconds. This is the time from the onset of the +G~z~-stress to the
onset of loss of consciousness. The period of unconsciousness is called
the absolute incapacitation period (ABSINCAP) and lasts on the average
12 seconds. This is followed by a period of relative incapacitation
(RELINCAP) that also lasts on the average 12 seconds. Together these two
periods make up the total incapacitation period (TOTINCAP) that lasts on
the average 24 seconds.

The TOTINCAP represents the time period from the loss of aircraft
control (loss of consciousness) to the return of purposeful movement at
the end of the relative incapacitation. Myoclonic convulsions are a part
of the G-LOC syndrome about 70% of the time. They occur at during the
last 4 seconds of the absolute incapacitation period and end coincident
with the return of consciousness. The relative incapacitation has an
initial period of 5 seconds where neurologic system reintegration occurs
followed by a period of 7 seconds where reorientation to the external
environment occurs. Once the reintegration and reorientation occurs the
pilot is able to make purposeful movements to control the aircraft.
Short dreams (dreamlets) are often experienced during the recovery
process.

![](media/image4.jpeg){width="4.407638888888889in"
height="2.8118055555555554in"}

**[Figure 4.]{.mark}** G-LOC kinetics, the time relationship of G-LOC
syndrome events. LOCINDTI = loss of consciousness induction time (for
rapid onset profile); G-LOC = onset +G~z~-induced loss of consciousness;
ROC = Return of consciousness; RPM = Return of purposeful movement;
ABSINCAP = absolute incapacitation period; RELINCAP = relative
incapacitation period; TOTINCAP = total incapacitation; Convulsion free
period = Period during unconsciousness where myoclonic convulsions do
not occur; Convulsion period = Myoclonic convulsion period;
Reintegration period = Period where neurological reintegration occurs
following loss of consciousness; Reorientation period = Period where
reorientation to the environment occurs ending in the return of
purposeful movement; Dream period = period where dreamlets often occur.

*G-LOC is a protective mechanism*

The symptoms of the G-LOC syndrome are all part of a protective
mechanism that has evolved to protect the human in a gravitational field
and to ensure the optimum protection of the organ system that is the key
to its evolutionary success on Earth, the brain. First of all, the
neurological system is placed in the safest anatomical compartment we
have, the skull and the spinal column skeletal system.

From the initiation of +G~z~-stress the cardiovascular and neurological
systems have built in cushions of blood pressure and flow such that
functional compromise does not occur easily. A significant increase in
+G~z~-stress above +1G~z~ must be applied before symptoms occur. The
cardiovascular and neurovascular systems have compensatory responses
that can increase tolerance to +G~z~-stress when the threat of exceeding
these cushions occurs.

The visual symptoms of greyout, tunnel vision and blackout warn that the
cardiovascular system cushion and reflex response are inadequate for the
magnitude of the stress and that evasive action is required immediately.
If evasive action is not taken then loss of consciousness occurs. This
occurs only when brain becomes threatened by ischemia/hypoxia and cannot
function reliably. The response is to place the heart and brain at the
same level in the +G~z~-field (horizontal). This action facilitates the
cardiac ability to get the needed blood flow to the brain. The loss of
motor function results in the body falling to the horizontal position.

The brain is placed in a minimal energy expenditure condition with the
loss of sensory, motor and consciousness function. This is just the
optimum condition for the neurons when there is inadequate blood flow.
The electroencephalogram shows a synchronized slow wave pattern. When
blood flow begins to return, myoclonic convulsions occur. This serves to
contract the muscles in the extremities and abdomen thereby enhancing
return of blood to the central circulation and ultimately the brain.

We consider the dreamlets to serve as a mechanism to alert the
individual that the loss of consciousness has occurred. Without the
dreamlet, G-LOC episodes can frequently go unnoticed and unreported by
an individual. If unnoticed, the individual may not recognize the
importance of future threat avoidance with subsequent G-LOC episodes
occurring. The relative incapacitation period serves to ensure that the
sensory, motor and consciousness functions are all thoroughly
reintegrated. The self-touch mechanism serves as a built-it test to
ensure that neurologic reintegration is complete.

Once the reintegration is complete and tested the nervous system
requires a short time to reorient to the external environment to ensure
that no movement is made prior to being capable of making a safe
purposeful move. Most of the +G~z~-protective methodology we have
developed to protect ourselves have their basis in what nature had
already accomplished. G-LOC is a protective response with the components
of the G-LOC Syndrome being normal responses that can be produced in
everyone given a high enough magnitude of +G~z~ exposure.

*The Human Centrifuge*

Aeromedical scientists have had the need to have a convenient method to
investigate the response to +G~z~-stress in a safe, controlled
laboratory environment. Although research in aircraft is very important
and safety in the operational environment is the ultimate goal of
aerospace medicine, the cost of flying high performance aircraft is very
high. In addition, the risk of high +G~z~-stress inducing performance
degrading neurological symptoms that could compromise flight safety is
unacceptably high. For these reasons, human centrifuges were developed
to simulate the high-G environment. Scientific investigation of the
human response to +G~z~-stress, in a safe environment has been a
critical discipline in aerospace medicine. Figure 5 illustrates the
simulation of +G~z~-stress on a centrifuge compared to the stress in a
maneuvering aircraft. The majority of our aeromedical knowledge of the
human response to acceleration stress has been generated from centrifuge
research.

![](media/image5.jpeg){width="3.795138888888889in"
height="2.5902777777777777in"}

**[Figure 5.]{.mark}** The simulation of +G~z~-stress on a laboratory
centrifuge compared to the +G~z~-stress in a maneuvering aircraft.

Protection

Military aircraft have rapid-onset (9G/s), high (+9G~z~), sustained
(\>5s) +G~z~ capabilities that can exceed unprotected human G-tolerance.
Individuals in the civilian community now fly many of these high
performance aircraft. In addition, civilian aerobatics can expose pilots
to significant ±G~z~-stress. Protection in the G-stress environment can
be approached by consideration of aircraft design, applying equipment to
be worn by the pilot and by changes to pilot anatomy and physiology.

Aircraft Design

The orientation of the pilot, or other passengers, within the aircraft
is important. Protection against +G~z~-stress can be approached by
orienting the pilot such that the maneuvers of the aircraft reduce the
magnitude of +G~z~-stress and transfer it to +G~x~, ~y~-stress.
Reclining the aircraft seat and lifting the heel line have accomplished
this, as has been done in the F-16. The recumbent (+G~y~-stress) or
prone (-G~y~-stress) positions serve to reduce the +G~z~-stress but they
have not been considered operationally optimal positions to accomplish
the overall mission. Even the F-16 seat configuration (30^o^ reclined
from horizontal) does not significantly enhance +G~z~-level tolerance,
although it does have G-enhancing characteristics.

Protective Equipment

The standard anti-G protection for military fighter pilots flying high
performance fighter aircraft is the G-suit. By covering the lower
extremities and abdomen with inflatable bladders +G~z~ tolerance is
enhanced by preventing pooling of blood below the heart. The most modern
anti-G suit ensembles have assisted positive pressure breathing and
chest counter pressure garments to provide enhanced protection.

Pilot Anatomy and Physiology

There are a lot of factors that combine to determine overall G-tolerance
on a given day. Short pilots have a higher tolerance than tall pilots
based on their respective differences in heart to eye-brain distances.
Individuals with higher blood pressure have a higher tolerance than
those with lower blood pressure. Being sick with dehydration and being
bed-ridden (0G~z~-stress) can acutely reduce G-tolerance.

Military pilots are required to have centrifuge high-G training to
ensure they are proficient in performing protective anti-G straining
maneuvers (AGSM). The AGSM is a combination of tensing the muscles in
the abdomen and extremities while performing a repetitive, respiratory
Valsalva-like maneuver to increase the driving pressure to get blood to
the brain.

Military pilots are required to maintain currency in their aircraft.
This serves to maintain physiological acclimation to G-stress. Physical
conditioning and strength training are important adjuncts to ensure
safety in the acceleration environment.

Finally, a solid base of information concerning G-tolerance, especially
the G-LOC syndrome, is key for all who enter the high-G environment.
G-LOC can be difficult to recognize by a pilot who experiences it. This
is important for all pilots to understand and be alert for such
symptoms.

Medical problems that may decrease G-tolerance

Neurological Problems

Any abnormality of the neurological or neurovascular system should be
reason for concern because of the potential for sudden incapacitation
during exposure to G-stress. Any abnormality that would contribute to
compromising blood supply to the cephalic nervous system is of extreme
concern. In addition, it should be remembered that exposure to high,
sustained +G~z~-stress while performing a vigorous anti-G straining
maneuver can put the blood vessels under increased pressure.

Cardiovascular Problems and Associated Medications

The cardiovascular system is the system that is primarily affected by
+G~z~-stress. Compromise of the cardiovascular system leads to
compromise of the neurological system. Since it can be compromised in
its support of the cephalic nervous system even in normal humans, any
abnormality in cardiovascular anatomy or physiology is reason for
concern in aerospace safety.

Medications that alter cardiovascular physiology should also be viewed
with caution, specifically pharmacological agents that alter blood
pressure and/or cardiac dynamics.

Acceleration is known to be a dysrhythmogenic stress. Propensities for
cardiac rate, rhythm or conduction disturbances that adversely affect
cardiac output are a threat to safety. Tachydysrhythmias (ventricular
tachycardia, supraventricular and frequent premature atrial and
ventricular premature beats) are most common during +G~z~-stress and
although they can also occur following the +G~z~-stress the
bradydysrhythmias (marked sinus arrhythmia, bradycardia, ectopic atrial
rhythm, prolonged periods of asystole) predominate the post-+G~z~-stress
period. The main concern with all the dysrhythmias is the potential for
their compromising cardiac output and the subsequent neurologic symptoms
that result.

Musculoskeletal Problems

The neck and back are of particular concern during +G~z~-stress. Any
anatomical abnormality that decreases neck or spinal strength or
stability should be carefully considered before exposure to
+G~z~-stress. Neck and back muscle strengthening should be considered
for anyone anticipating exposure to high, sustained +G~z~-stress. Any
muscular injury that could compromise the ability to perform a
proficient anti-G straining maneuver should be allowed to resolve prior
to high, sustained +G~z~-stress.

Pulmonary Problems

+G~z~-stress results in ventilation -- perfusion changes (mismatch) in
the lungs that alter optimum oxygenation of the blood. Increasing
degrees of hypoxia do result from sustained +G~z~-stress. Compromise of
neurological function therefore results from a combination of ischemia
and a varying degree of hypoxia. Any abnormality of the pulmonary system
that would contribute to hypoxia should be of concern for reducing
tolerance to +G~z~-stress. Breathing increased concentrations (100%)
oxygen can also cause problems, when wearing an anti-G suit, by virtue
of causing the distal alveoli to collapse. This is a self-limiting
problem known as aeroatelectasis, with symptoms of retrosternal chest
pain and coughing. Coughing itself usually reverses the problem by
re-expanding the collapsed alveoli.

Tolerance to --G~z~-stress

Negative G~z~-stress is typically encountered when a military aircraft
pushes its nose over or in specific civilian aerobatics maneuvers
involving an outside loop. The physiological effects from --G~z~-stress
result from blood being displaced *toward* the head. The body has not
evolved in an environment where --G~z~-stress is common and it therefore
tolerates this type of stress poorly. The blood vessels in the head are
much more fragile than those in the lower extremities where
gravitational stress (+1G~z~) has been ever present. Petechial
hemorrhages produced in the conjunctiva are not uncommon even at
moderate levels of --G~z~-stress (-2 to -3G~z~).

The physiological response to a sensed over-pressurization of the
cephalic nervous system is to rapidly reduce the threat of the
over-pressurization. The carotid baroreceptors inhibit the cardiac drive
causing the over-pressurization, resulting in dramatic slowing of the
heart rate.

Because of the potential for pathologic insult to the cephalic nervous
system and structures in the head, experimental investigation of
--G~z~-stress has been avoided in healthy humans. The specific tolerance
to --G~z~-stress has therefore not been thoroughly investigated. The
symptoms that result from --G~z~-stress have therefore not been
thoroughly investigated either.

A frequently reported symptom such as "red-out" does not have a
well-documented basis. Red-out has been attributed to the lower eyelid
being deviated upward with red vision resulting from light passing
through the lower lid. It has also been attributed from engorged retinal
blood vessels producing red vision.

Exposure to --G~z~-stress is therefore risky especially in the
unacclimated individual. There is evidence that individuals who do
participate in civilian aerobatics do build up a tolerance for short
exposures to --G~z~-stress. Civilian aerobatics does have exposure to
short duration moderate levels of --G~z~-stress.

There is a specific sequence of exposure to  ±G~z~-stress that deserves
special consideration. It has become known as a "push-pull maneuver"
that involves exposure to --G~z~-stress rapidly followed by
+G~z~-stress. The problem that is posed by this sequence is that
--G~z~-stress with its reflex slowing of cardiac response followed by
+G~z~-stress that requires rapid response of the cardiac system could
result in a lower tolerance to +G~z~-stress with G-LOC.