DAvMed 44
Sustained accelerations
KCL 2011
Sustained accelerations
1- Describe how "G Force" is produced in an aircraft.

A "G" is the ratio of the applied acceleration by the gravity
applied acceleration
o G
g
 An aircraft which is in motion (both on ground and in the air) is under the influence of several
forces.
o The force of gravity, which is directed toward the centre of the Earth

o The force of lift (when flying), which is perpendicular to the plane of the wing, directed
"upwards"
o The force of thrust, which is directed forwards
o The force of drag, which direction is opposite to that of the thrust.
 The variations applied to these forces are responsible for the accelerations that are applied to the
aircraft. These accelerations are placed in a 3-axis coordinate system based on the human body:
o Headwards acceleration
⇒ "head to foot" inertial force
⇒+Gz
o Footwards acceleration
⇒ "foot to head" inertial force
⇒-Gz
o Forwards acceleration
⇒ "chest to back" inertial force
⇒+Gx
o Backwards acceleration
⇒ "back to chest" inertial force
⇒-Gx
o Right lateral acceleration
⇒ "right to left" inertial force
⇒+Gy
o Left lateral acceleration
⇒ "left to right" inertial force
⇒-Gy
 a G Force may be produced in an aircraft:
o by the thrust when accelerating on the runway +Gx
o but mostly by the variation of the lift force when the aircraft is turning or looping:
 to turn, the aircraft will bank (or roll) toward one side.
 this bank will change the direction of the resultant force (sum of all forces applied
to the aircraft) creating a centripetal acceleration.
 this centripetal acceleration has a centrifugal reaction, which is seen as "head to
foot" inertial force (+Gz) by the subjects seated in the aircraft.
o by the yaw (±Gy) essentially on the ground and very rarely in flight.
2- What is G Force and how is it measured.


A "G" is the ratio of the applied acceleration by the gravity
applied acceleration
o G
g
o The acceleration due to gravity "g" is a physical constant: g = 9.81 m/s2
o The G Force is measured in how many times the applied acceleration is greater than that
due to gravity.
3- What is G onset rate.



The G onset rate is the rate of change of acceleration.
It is typically expressed in units of G/s
When considering the human response to an impact, the rate of change of acceleration is
commonly termed jolt.
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Sustained accelerations
KCL 2011
4- What are the subjective effects of exposure to +Gz acceleration.



Change in weight:
o pushed down in the seat
o the arms feel heavy
o difficulty in moving the limbs
o the combined mass of the head and the helmet makes head movements difficult
o above +3Gz, unassisted extraction from the aircraft is impossible
o raising the arms is barely possible @ +8Gz
Visual effects:
o first overt manifestations of the cardiovascular effects of acceleration exposure
o loss of peripheral vision (grey-out) after ≈ 5 seconds (⅔ of subjects) usually @ +3 to +4Gz
o or dimming of the vision or other patterns (⅓ of subjects) usually @ +3 to +4Gz
o Black-out @ at mean acceleration of +4.7Gz ± 0.8G (with preserved consciousness)
Unconsciousness (subjective?)
o G-induced Loss of consciousness (G-LOC) with or without prior visual symptoms.
5- What is a hydrostatic pressure gradient.

Considering a column of fluid, the hydrostatic pressure p of the fluid resulting of its exposure to an
acceleration is given by:
o p  hg
(kPa)
 where h is the height of the column
 g is the acceleration to which it is exposed
 ρ is the density of the fluid

 Considering 2 different levels a and b on this column of fluid (where a is placed above b), the
hydrostatic pressure p will vary between a and b in regard to their respective heights.
 The hydrostatic pressure gradient ∆p is:
o ∆ p = p b - pa
6- Describe how +Gz acceleration affects blood pressure in the arterial tree.

In the human body, pressure is produced @ heart level and is transmitted to the arterial tree.
o Exposure to +Gz acceleration increases the hydrostatic pressure in the column of fluid
determined by the arterial tree (aorta & carotids).
o Increased hydrostatic pressure above heart level leads to a fall in pressure at the top of the
column.
 Assuming that the heart to head distance is 30 cm, that the density of blood is 1.06, the effect of
+Gz acceleration on head level blood pressure can be calculated:
o p  hg
o p  0.3  1.06  10 3  9.81  3.12 kPa (= 23.4 mmHg)
o Therefore, for every +1Gz increase in acceleration, head level blood pressure falls by about
23.4 mmHg

o Considering that heart to the common iliac artery distance is also 30 cm, for every +1Gz

increase of acceleration, common iliac artery level blood pressure increases by about 23.4
mmHg.
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Sustained accelerations
KCL 2011
7- How does +Gz acceleration affect the distribution of blood volume in the body.


The changes in intravascular pressure induced by +Gz acceleration have an effect on the size of
the blood vessels.
o The size of the blood vessels is determined by:
 the vascular transmural pressure (∆ between intravascular and extravascular
pressure)
 the distensibility of the vessel
 the amount of blood available to fill it.
o Exposure to +Gz acceleration increases the hydrostatic pressure of the venous vessels
below heart level.
o The increase in venous pressure in the regions below heart level causes dilatation of the
capacitance vessels.
o Blood volume contained in lower limbs increases "venous pooling" (≈ 60ml @ +5Gz)
o
The rise of pressure within the capillaries of the lower limbs also causes a transudation of fluid
from the blood to the tissues (≈ 200ml/min @ +4Gz), so that there is a progressive loss of fluid
from the circulation.
8- Describe the sequence of events that occurs in the cardiovascular system following exposure to
+Gz acceleration.





Hydrostatic pressure changes with +Gz acceleration (instantaneous)
Maximum venous pooling occurs a little later (6 to 12")
Venous return improves after peak venous distension achieved
Transudation into lower limbs (≈ 200ml/min @ +4Gz) throughout +Gz exposure
all of which reduces the flow of blood to the right heart.
o
o
o
o
o
o
Reduced flow of blood to the right heart
⇒ Central Venous Pressure reduced
⇒ Decreased ventricular filling pressure
⇒ Reduced contractile energy
⇒ Reduced stroke volume
⇒ Arterial hypotension.
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Sustained accelerations
KCL 2011
9- Describe the baroreceptor reflex and its function during +Gz exposure.


The blood pressure changes and blood volume redistribution induced by exposure to +Gz
acceleration produce reflex responses involving the arterial baroreceptors in the carotid sinus
and the aortic arch.
The baroreceptor reflex provides a compensatory mechanism to preserve head-level blood
pressure under increased acceleration.
Exposure to +Gz acceleration
⇓
➘ Arterial pressure
⇓
➘ Arterial baroreceptor activity
⇓
Medulla cardiovascular nuclei
⇓
⇓⇐⇐⇐⇐⇐⇐⇐⇐⇐⇐⇐⇐⇐⇐⇒⇒⇒⇒⇒⇒⇒⇒⇒⇒⇒⇒⇒⇒⇓
⇒ Vagus
⇓
⇒➚Heart rate ⇒
⇓⇐⇐⇐⇐⇐
⇒➚Stroke volume
⇓
⇒➚Sympathetic
nerve activity
⇓
➚Vascular
resistance
⇒➚Cardiac output
10- What is the physiological basis for the changes in vision during +Gz acceleration.



The normal intra-ocular pressure is between 10 & 20 mmHg.
Under +Gz acceleration, the arterial pressure at eye level falls due to the modified hydrostatic
pressure gradient.
o If the +Gz acceleration is such that the arterial pressure at eye-level falls below that of the
intra-ocular environment, blood flow into the eye and hence retinal flow ceases.
o As the overall surface area of the peripheral retinal vessels is greater than that at the
papilla, the pressure in these vessels is lower.
o Therefore the blood supply to the periphery of the retina fails first under +Gz acceleration,
producing the classical "grey-out" tunnelling loss of vision (≈ ⅔ of the subjects).
o However this doesn't fit the true anatomy of the retinal vasculature or the alternative
patterns of visual loss reported by about ⅓ of the subjects.
A higher arterial pressure is required to perfuse the retina than that for the cerebral circulation in
regard to the same +Gz acceleration level.
o The retinal flow ceases therefore before the cerebral flow at same perfusion pressure.
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Sustained accelerations
KCL 2011
11- What factors influence cerebral blood flow during +Gz acceleration.

The incidence of G-LOC is much lower than would be expected in regard to the sole changes in the
hydrostatic pressure gradient.
o The hydrostatic pressure gradient is applied on both the arterial and venous vessels in the
neck
 This creates a siphon effect in the jugular vein which maintains the cerebral
circulation as long as the vein doesn't collapse under the surrounding tissue
pressure.
o There is an active vasodilatation of the arterioles of the cerebral circulation:
 ⇒ The resistance to flow through the cerebral circulation is reduced.
o The cerebral vessels and tissue are enclosed in a rigid box and surrounded by
cerebrospinal fluid (CSF).
 The hydrostatic pressure gradient of the column of CSF also changes under the
influence of the +Gz acceleration, in parallel with the reduction of vascular pressure
at head level.
 ⇒ The pressure difference across the walls of the intracranial vessels remains close
to normal and the blood flow remains.
12- Describe the effects of acceleration on heart rhythm.


Benign cardiac dysrhythmias frequently occur during & immediately after exposure to +Gz
acceleration.
o Premature ventricular contractions ++
o Premature atrial contractions ++
o Rarely
 supraventricular tachycardia (SVT)
 atrial fibrillation (AF)
 asystole & heart block
These disturbances are probably related to the rapid of both the sympathetic and
parasympathetic stimulations on the heart rate during and following the acceleraion.
13- What is the physiological basis of the "push-pull" (or negative to positive G) manoeuvre and its
effects on G tolerance.

The "push-pull" or "negative to positive G" is the exposure to a -Gz acceleration immediately
followed by a +Gz acceleration.
o Exposure to a -Gz accelerations has cardiovascular effects:
 Profound rapid onset bradycardia
 Vasodilatation
 Reduced cardiac output
o The immediately following exposure to +Gz happens just as the cardiovascular reflexes and
the distribution of blood volume has been reset disadvantageously:
 ⇒ profound fall in blood pressure
 ⇒ Reduced tolerance to the +Gz acceleration.
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Sustained accelerations
KCL 2011
14- Describe the features of G induced Loss of Consciousness (G-LOC) and the recovery period.





G-LOC is not black-out, as in the latter, the consciousness remains. Black-out occurs at a lower
level of +Gz than G-LOC.
G-LOC occurs without warning symptoms or signs.
G-LOC may be preceded by visual symptoms (grey or black-out).
The first sign is unconsciousness
o First period of total incapacitation
 Up to 15 second period of unconsciousness
o Followed a period of relative incapacitation
 15 to 30 seconds of impaired consciousness
 with "symptoms" during recovery:
 dreamlike condition / euphoria (most common)
 confusion
 Disorientation
 Muscular spasms which are not epileptic in nature
 light headed feeling
 tingling
 fear / panic
 flush
 amnesia
 considered ejection (least common)
The total incapacitation time (where the subject is unable to control the aircraft) can last a minute
or longer and is sufficient for a fast moving aircraft to impact with the ground.
15- What is the incidence of G-LOC.


The different studies assess an incidence of G-LOC of about 20% of pilots.
A study considering only the F-16 aircrew has shown an incidence of 30%.
16- Which aircrew groups are most likely to suffer G-LOC.

G-LOC occurrence is most common in training aircraft (70% of incidents)
o with inexperienced aircrew
o without anti-G system
o pilot unprepared for high +Gz exposure
o "baby pilot" snatching G on instructor without warning
o evasive manoeuvres in air combat / training.
17- List some factors that impair G tolerance.










Heat
Hypoglycemia
alcohol
intercurrent illness
empty stomach
hypoxia & hyperventilation
time off from flying
dehydration
individual variation
(excessive aerobic fitness)
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 incorrect AGSM
Sustained accelerations
KCL 2011
18- What is A-LOC.



A-LOC = Almost Loss Of Consciousness
A-LOC is an altered state of awareness
o sensory abnormalities
o amnesia
o confusion
o euphoria
o difficulty forming words
o Disconnection between cognition and the ability to act on it (++)
Has the potential to cause significant loss of aircrew performance.
19- How does G onset rate affect the risk of G-LOC.



A very slow onset of acceleration allows cardiovascular reflexes to develop, but visual symptoms
or even loss of consciousness will eventually occurs if the acceleration is maintained long enough
(10-15 seconds or more) at +4 to 5Gz or higher.
There is a slightly increased tolerance after 8-10 seconds, due to action of venous returns and that
of the baroreceptors.
A very rapid onset exposure to high levels of acceleration (+10Gz/s) will be tolerated for a short
period (up to 3-4 seconds) without visual symptoms, due to the brain functional reserve in O2. But
if the acceleration is maintained, G-LOC will occur without any premonitory grey-out.
20- How does +Gz acceleration affect the lung volume subdivisions.


Exposure to acceleration up to +5Gz causes little respiratory embarrassment.
o Total lung capacity (TLC) and vital capacity (VC) are unaffected by accelerations up to
+3Gz.
o but TLC and VC are reduced by about 15% by exposure to +5Gz. (increased weight of the
chest wall)
Exposure to +Gz exposure acceleration causes:
o descent of the abdominal content and diaphragm
o increase in the functional residual capacity (FRC) — 500ml @ +3Gz
o increase in the expiratory reserve volume (ERV)
o decrease in the inspiratory capacity (IC)
o little change in the residual volume (RV)
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Sustained accelerations
KCL 2011
21- Describe the distribution of ventilation in the normal lung — how is this changed by +Gz
acceleration.


There exists, due to gravity, a pleural pressure gradient from the apex to the base of the lung
when upright.
o Pleural pressure increases at 2.5 mmH2O/cm/G.
o The pleural pressure gradient leads to a ventilation gradient down the lung, due to the
elastic properties of the lung tissue:
 the alveoli in the apices are distended at all time, and thus have little margin to
distend further during inspiration.
 The alveoli in the bases are less distended, due to the higher pleural pressure, and
thus can fully distend during inspiration.
Under increased +Gz acceleration:
o the pleural pressure gradient becomes steeper:
 induces greater differences in the distension of the alveoli down the lung
 alveoli in the apices are more distended
 alveoli in the bases are closer to their minimum volumes
o the ventilation gradient also becomes steeper.
 Due to +Gz acceleration, the ventilation of the alveoli at the base of the lung ceases:
 they attain their minimal volume and their associated airways close.
22- Why does the distribution of ventilation in the lung change during +Gz acceleration.

Under increased +Gz acceleration:
o the pleural pressure gradient becomes steeper:
 induces greater differences in the distension of the alveoli down the lung
 alveoli in the apices are more distended
 alveoli in the bases are closer to their minimum volumes
o the ventilation gradient also becomes steeper.
 Due to +Gz acceleration, the ventilation of the alveoli at the base of the lung ceases:
 they attain their minimal volume and their associated airways close.
23- What is airway closure.


Under increased +Gz acceleration, pleural pressure at the bases may exceed airway pressure:
o Leads to the collapse of small airways
o Gas is trapped in the alveoli distal to this closure
o These alveoli don't subsequently take part in ventilation.
Airway closure is dependent on:
o location in the lung (apex / base)
o lung volume (airways start to close as they get near to RV)
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Sustained accelerations
KCL 2011
24- How does regional perfusion in the lung change during +Gz acceleration.

Acceleration affects the pulmonary circulation in a similar manner to the systemic circulation:
o there is in the lung a hydrostatic pressure gradient
o the pressures are lower in the pulmonary circulation than that in the systemic circulation
 @ heart level, the mean pulmonary arterial pressure (MPAP) is ≈ 15 mmHg
 MPAP @ heart level is unchanged by acceleration
 at the apex, MPAP @ +1Gz (gravity) is ≈ 0 mmHg (≈20 cm above heart level)
 @ +4Gz, MPAP = 0 mmHg only 5 cm above heart level (& throughout the upper half
of the lung.
o ⇒the proportion of the lung that is not perfused increases with increasing acceleration
from the apex @ +1Gz to the whole of the upper half of the lung @ +5Gz
25- What change in the ventilation-perfusion ratio occurs under +Gz acceleration, and what is the
significance of these changes.




A ventilation-perfusion ratio V/Q = 1 reflects the perfect match between ventilation and
perfusion, in which each unit of perfusion is ventilated and each unit of ventilation is perfused.
In the normal upright lung @ +1Gz (gravity), there is a normal V/Q mismatch due to both the
ventilation and MPAP gradients:
o at the apex, V/Q tends to the infinite as MPAP tends to zero
o in the base, V/Q is slightly below 1
Under increased +Gz acceleration,
o there is a part at the extreme base that is perfused but non-ventilated (V/Q = 0).
o there is a larger part of the upper lung that is ventilated but non-perfused (V/Q = ∞)
The O2 tension in the closed alveoli falls rapidly (within 1-2")
o blood flowing through these alveoli no longer takes part in gas exchange
o ⇒forms a right to left shunt
o @ +5Gz, up to 50% of the cardiac output is shunted in this way
o ⇒SaO2 falls to ≈ 85% after 1 minute (desaturation)
26- How does acceleration atelectasis occur, what are the symptoms, and how is it prevented.



Under +Gz acceleration (> +3Gz), air closure occurs in the dependent part of the lung.
o The closed alveoli contain some trapped gas.
o Perfusion through the closed alveoli remains and gas exchange continues between the
trapped gas and the mixed venous blood
o This blood absorbs the trapped gas at a rate limited by that of the least soluble gas (usually
N2.
o If little N2 is present (when breathing 100% O2), all the alveolar gas will be rapidly
absorbed
 ⇒these alveoli will be rendered gas free
 the alveolar walls will be held together par surface tension forces and will remain
collapsed after acceleration has ended.
The symptoms of acceleration atelectasis are:
o dry cough
o substernal discomfort (or pain), exacerbated by inspiration
Acceleration atelectasis is prevented by:
o (+Gz acceleration lower than +3Gz)
o maintain a minimum [N2] of 40% in breathing gas
o Anti-G straining manoeuvre (AGSM)
o (pressure breathing for G protection —PBG)
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Sustained accelerations
KCL 2011
27- Describe how to perform the anti-G straining manoeuvre.

AGSM is a combination of muscle tensing and repeated Valsalva manoeuvres:
o start with muscle tensing especially in the lower limbs sustained throughout the
acceleration exposure, without relaxation during breathing
o followed by a Valsalva manoeuvre rhythmically repeated every 3-4 seconds as long as the
acceleration is applied
o breathing between each Valsalva manoeuvre must be performed as rapidly as possible
28- What is the physiological basis of this manoeuvre, and how do the various components reduce
the risk of G-LOC.



Muscle tensing increases tissue pressure
o the increased tissue pressure is applied to the arteries and arterioles,
 reducing their diameter
 rising the peripheral resistance
 thus increasing arterial blood pressure
o the increased tissue pressure applies mechanical pressure to veins
 limiting venous pooling
o the increased intra-abdominal pressure helps to prevent the descent of the diaphragm
The Valsalva manoeuvre
o increases both intra-thoracic and intra-abdominal pressures
o the pressure increase is transmitted directly to the heart and great vessels
o raising the systemic arterial pressure
o helping to maintain cerebral perfusion
o But short protection (4-5 cardiac cycles)
o thus is repeated every 3-4 seconds.
The brisk respiration cycle between each Valsalva:
o causes an inspiratory negative intra-thoracic pressure
o helps increasing venous return
o and so improve cardiac output.
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KCL 2011
29- Describe how an anti-G suit works. What different types of anti-G suit are there.




An anti-G suit consists in
o a trouser-like garment
o made of non-stretch material
o containing 5 interconnecting bladders
 1 abdominal bladder
 and on each leg, 1 thigh and 1 calf bladder
The five bladders are air-compressed automatically by the anti-G valve when +Gz acceleration is
pulled.
the anti-G suit
o works in a similar manner to muscle tensing
 increases tissue pressure
 increases peripheral resistance
 decreases venous pooling in the lower limbs
o the abdominal bladder
 prevents the descent of the heart
 has an important influence on venous return from the abdominal cavity to the
thorax
Different types of anti-G suit:
o Standard five-bladder anti-G suit, with non-circumferential interconnecting bladders
o Extended coverage anti-G suit, where the limb bladder cover the whole circumference of
the thighs and calves
o Liquid-filled (Sibelle) suit
o Graded pressure suits
o Capstan suit
30- What is the purpose of high G training and how is it conducted.



The requirement for centrifuge-based high G training was identified after the G-LOC surveys in
the 1980s.
High G training objectives are stated by NATO:
o aircrew awareness of potential for G-LOC
o anticipation and recognition of symptoms
o develop efficient and effective AGSM
o develop confidence in ability to sustain high +Gz accelerations
High G training is conducted as following:
o detailed briefings on the physiological basis for acceleration-induced visual disturbances
and G-LOC
o demonstration of good AGSM
o individual centrifuge experience with different profiles:
 simple profile
 simulated air combat manoeuvre (SACM)
o use of occasional G-LOC during centrifuge training to draw attention to
 the risks
 the slow recovery and the confused condition that follow G-LOC
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31- How does physical fitness (aerobic and anaerobic) influence G tolerance.




General health benefits of physical training should not be neglected
AGSM is a fatiguing manoeuvre
There is no evidence that aerobic fitness has any great effect on G- tolerance
o Excessive aerobic activity should be avoided since it induces an imbalance between
sympathetic and parasympathetic activity.
There is no evidence that anaerobic fitness increases absolute +Gz tolerance.
32- Why do fast jet pilots get neck pain, and what can be done to reduce the prevalence;



Fast jet pilots are exposed to
o High +Gz acceleration
o High +Gz sustained for long periods
o High G onset rate
o helmet and its gear (NGV, HUD…)
o which all are predisposing factors for neck pain.
Furthermore, fast jet aircrew is often turning the head in all directions, during all kinds of
manoeuvres and thus often under exposure of high +Gz acceleration.
To reduce the prevalence, one can/could:
o design lighter gear and helmets
o provide support to the head and neck under +Gz (but fails to allow the essential
satisfactory head mobility in air combat)
o neck muscle training (but no evidence that training provides the required protection)
33- Why is pressure breathing for G protection (PBG) used in high performance aircraft, and what is
it physiological mode of action.






PBG, by raising intra-thoracic pressure, allows an effective automation of the Valsalva manoeuvre
and thus reduces fatigue.
The elevation of intra-thoracic pressure acts directly on the heart and great vessels to increase
blood pressure on a virtual one-to-one basis.
The head-level blood pressure is therefore increased, also on a virtual one-to-one basis.
Requires an adequate support of venous return, which is provided by the use of extendedcoverage anti-G trousers.
Over-distension of the lungs is prevented by the use of a chest counter-pressure garment.
The combination of PBG and extended-coverage garments enables most individuals to maintain
clear vision @ +9Gz with little or no straining
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34- What are the advantages and disadvantages of using PBG.


Advantages:
o Higher G-tolerance (> +9Gz)
o Reduced fatigue
o Aircrew can better focus on the control of the aircraft
Disadvantages:
o Cost (extra man-mounted equipment and clothing)
o Heat
o Mobility
o Comfort
o Integration with other equipment
o Mask leakage
o Helmet mounted display stability
o Forearm pain
o Requires specific aircraft systems
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