Physiological Effects of the High "G" Environment

The Physiological Effects of the High "G" Environment of Aerobatic Pilots-by
TX-Eco's Girl

She is a slacker, she had to interview someone for this, she figured she would choose a topic that she could use a conversation with me as the interview.
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In order to understand the physiological concerns of an aerobatic or combat pilot we must first look at the environment in which he/she is performing. This environment includes elevated altitude, but most aerobatic maneuvers occur in low-level situations, so the effects of very high and low G levels as well as rapidly changing G levels is of greater physiological importance. This combination is quite significant because in a typical airshow performance maneuvers may lead to a range of +12Gz to -8Gz. (Goldsmith, 2004). The high G environment is classified into various types of "G" based on the direction relative to the hydrostatic column between the aortic valve and the eye or brain. (NAMI, 1991) There are three arbitrary types of acceleration, Gz, Gy, and Gx, with only Gz being of physiological significance in aerobatic situations. Of most concern in aerobatics is the cardiovascular system, which secondarily affects both proper neurological and respiratory function, and of less concern are musculoskeletal effects. (NAMI, 2000)

Gz, or "eyeballs up/down" is acceleration in the same line as the heart-brain hydrostatic column. This allows great influence on the blood being transported through this column, and thus leads to the most significant physiological effects. +Gz, otherwise known as "eyeballs down¡" is in the normal G direction. With increasing levels of +Gz acceleration the pooling of blood in the extremities and lower body results in less blood available for the brain and it is increasingly difficult to pump it against the +Gz force and ischemia (lack of blood flow) and/or hypoxia (lack of O2) may occur. The consequences of ischemia and hypoxia and the body's recovery mechanisms are of concern when looking at Gz acceleration.

-Gz, or "eyeballs up" acceleration, is opposite to the normal G direction. This type of acceleration pushes the blood toward the head, thus increasing the flow in and decreasing the flow out of the head, leading to a pooling of blood in the brain. Physiological research into the effects of -Gz has been limited by pilot/volunteer discomfort and researcher fears of untoward side effects. (NAMI, 2000) Reasonable estimates have suggested limits of -4.5 Gz for 15 seconds, and -3Gz for 30 seconds. (Christy, 1971). Aerobatic pilots may attain higher values than these, but typically for shorter durations. These exposures are generally not thought to be of extreme concern to the pilots who fly the maneuvers because the duration and magnitude of the forces are usually kept in check by pilot discomfort and experience working up to the loads, or aircraft design limitations. The -Gz loadings may, however, be accompanied by the bursting of blood vessels and capillaries in the eyes or forehead, though this is not thought to be a significant concern. (Goldsmith, 2004). More significant effects such a cerebral hemorrhage could occur at higher loadings and durations, or in individuals with a pre-existing physiological conditions, however as stated earlier there is no known range of -Gz load and duration specific to this event that I was able to locate.


Gx is also known as "eyeballs in/out" acceleration, but is not of much concern to aviators except upon crash landings or during carrier operations. Gy, acceleration along the third possible axis of direction is called "eyeballs right/left", but is not of concern, and not even discussed in papers thus far examined. (NAMI, 2000) Aerobatic pilots do encounter small lateral loadings in maneuvers such as knife edge flight (approx 1 Gy), or tumbles , but these values are usually less than +/- 1.5 Gy and lead to no discomfort or obvious significant physiological response during the exposures that pilot encounter in certain types of aerobatic maneuvers. Aerobatic aircraft design also limits the amount of Gy acceleration that a pilot can induce (in flight), and these values are in the range as previously stated. (Goldsmith, 2004)

Aerobatic flying demands the best of both aircraft and pilot. The aircraft must be highly maneuverable, yet tolerant of G-loads. The pilot must possess skill and physiological stamina. (Beaudette, 1984) That the aircraft must be of high quality makes sense, but why exactly are these maneuvers so demanding of the human body? At the most basic level it is because the body's circulatory system works based on the ability of the heart to pump blood, which is effected by partial pressure gradients, the aortic valve/eye column height, baroreceptors and the effects of gravity or accelerations. All of these mechanisms normally work in a +1Gz environment and when the G environment is changed so are the foundations of how our blood is pumped, thus altering the nutrient supply to the brain. Without this supply our brain can cease to function properly or at all in a conscious manner. If the initial statement is true, then it is clear why the effects of the high G environment can be so important to study and understand, because without proper functioning of the brain it is quite hard to perform in a highly demanding manner.

In order to understand the changes associated with abnormal G loads, the physiological responses, and the effects of those responses, we must first examine the methods of regulation in the normal +Gz environment. Of primary concern is the control of blood pressure (BP), which is under the control of the autonomic nervous system. On a short-term basis in normal +1Gz conditions the parasympathetic nervous system (PNS) is dominant, which reduces sympathetic nervous system (SNS) controlled increases in BP that baroreceptors of the thoracic and carotid area might induce. (Banks, 1995).


Baroreceptors are stretch receptors that are able to sense both change and rate of change in the transmural pressure of the upper thoracic and carotid areas. (Banks, 1995) When a signal is sent to the brainstem by a baroreceptor due to a +∆Ptransmural, caused by an increase in BP in the head and thoracic during -Gz, the PNS sends an efferent signal through the Vagus nerve which causes both cardiac deceleration and decreased cardiac contractility. These responses act to reduce BP at the site of the baroreceptors towards a normal level. The SNS is responsible for vasoconstriction, therefore inhibition of the SNS leads to inhibition of vasoconstriction, and vasodilation results from the PNS induced relaxation of the vascular smooth muscle. In connection with these results there is a decrease in heart rate (HR), decreased cardiac filling, and decreased cardiac contractility, and ultimately reduced BP in the upper thoracic and head. (Banks, 1995)


If there is a -∆Ptransmural sensed by the upper thoracic or carotid baroreceptors, this will be interpreted by the PNS center in the medulla as a decrease in BP. This decrease in BP will cause a decrease in SNS inhibition and thus an increase in SNS stimulation on HR, and contractility of both the heart and vascular smooth muscle. SNS stimulation causes an increase in HR via stimulation of the SA node of the heart, and increased contractility of cardiac and vascular smooth muscle which results in increased contractile forces, and vasoconstriction of vessels in both skeletal muscle and splanchnic beds. An increase in HR, heart contractility, and vasoconstriction results in an increase in cardiac output (CO) leading to increased BP. It is also important to note that BP is tightly regulated so that only very small variations occur, meaning that reactions to changes in BP occur very rapidly, which will become important later in a discussion of the push-pull effect. (Banks, 1995)


+Gz acceleration results in reduced blood flow to the head and upper thoracic, and therefore in a -∆Ptransmural, which leads to stimulation of the SNS, leading to increased heart rate via the glossopharyngeal nerve. This response occurs at much slower rate than the decreased heart rate due to the +∆Ptransmural. (Banks, 1995). For the aerobatic pilot this results in a perceptible physiological change in response to the +∆Ptransmural, but little to none to the -∆Ptransmural, this becomes significant when rapid changes are made between -Gz to +Gz loadings. (Goldsmith, 2004)


Without these compensatory mechanisms the tolerance to +G loadings results primarily from the height of the previously mentioned hydrostatic column between the heart and the head. The greater the height of this column the lower the G tolerance because the greater the height of the column blood, the higher the cardiac output must be to maintain sufficient blood flow to the head, especially during elevated +Gz exposure. Thus, reclined seat positions which act to decrease the vertical component of the distance between the heart and head, are one method of increasing Gz tolerances. (NAMI, 2000)


But, of course our bodies do have compensatory mechanisms that increase (and sometimes decrease) our G tolerance, as well as anti-g suits, and anti-g straining maneuvers (AGSM). Stimulating the SNS causes a response that attempts to increase the BP, and the body has three basic mechanisms of doing so. Under +Gz loads, blood tends to be pushed towards the feet, resulting in a general tendency for blood to pool in the extremities, which is why the first mechanism is peripheral vasoconstriction. By constricting the blood vessels in the periphery, that is areas away from the central body core and brain, or the extremities, an adequate blood volume is maintained in the vital organs. There is still a problem with the brain, because even though the blood is within the core that doesn't mean that enough gets up to the brain. Thusly there is an increase in heart rate and increased heart contractility, these mechanisms attempt to increase the flow of blood through the brain.


The main concern with these mechanisms are that they take about 6-10 seconds to occur after baroreceptor induced SNS stimulation, which is far to slow to be useful to the typical aerobatic pilot. There may be more immediate vestibular responses to changes in G loadings, however they do not appear significant in their ability to increase tolerance for the aerobatic pilot. Physiological changes in response to long term G responses may slightly increase maximal G tolerance in pilots who train daily, however because the elevated G loadings are short in total duration the effects are small. Thus an important issue for an aerobatic pilot is the rate of G onset, and the duration of high G loads. If the rate of onset is greater than the time required for compensatory mechanisms to take effect, then neurological consequences may begin to occur, and failure of the mechanisms can lead to G-LOC, or G-induced Loss Of Consciousness. It is also common to see dysrhythmias in these situations including sinus arrhythmia, premature ventricular contractions, and premature atrial contractions. Many people have worried that such dysrhythmias and extreme loads on the heart can lead to serious damage, and while there has been reports of endocardial hemorrhage ages in pigs exposed to high G loads, damage to human hearts have not been reported due to G exposure within tolerance limits on either a chronic or acute basis. (NAMI, 2000) Given the physiological response time to extremes of +Gz the straining maneuvers become an important factor in +Gz tolerance, for -Gz encounters relaxation before and during -Gz are the methods that aerobatic pilots employ and rely on for the most part. (Goldsmith, 2004)


There are two basic effects on the respiratory system due to experiencing a high G environment. Primarily, G's cause a change in perfusion of the lungs, resulting in higher perfusion of the lower lung and an increased dead space volume. This situation lowers the partial pressure of oxygen, which decreases the oxygen available for the brain, and again lowers performance abilities. The other problem is G-induced oxygen absorption atelectasis, though this problem is primarily associated with 100% O2 breathing systems used by the Navy, and not applicable to aerobatic pilots in particular. Another system that can be effected by G's is the Musculoskeletal system, but it is primarily due to G limiting movement. With increased G loads the body simply weighs more and thus it is harder to move the aircraft controls or pick things up such as your arm, which if it weighs 10 lbs on land, at 5 G's it would effectively weigh 50 lbs. Weight training and maintaining musculoskeletal fitness can help alleviate the severity of this effect, and may augment the straining maneuvers effectiveness. (Goldsmith, 2004)


Neurological effects are only of secondary importance to cardiovascular effects because they are caused by the cardiovascular effects. The two main neurological areas of concern are the eyes and the brain. When undergoing high performance maneuvers in a 3-dimensional environment it is obviously of utmost importance to both see what is going on and to remain conscious. The brain and eyes fail on different time scales, but both recover if blood flow is returned after only a short period of time. The difference in failing time scales can be very useful to aerobatic pilots because the loss of visual acuity will be the first effect, and the pilot may use this effect to limit his/her G loadings. The eyes are more sensitive to blood flow loss than the brain, resulting in vision loss at a rate of about 0.7 G below the G level where the brain fails. (NAMI, 2000) The usefulness decreases with extremely high performance aircraft such as military fighters and unlimited level aerobatic aircraft that operate at such high speeds or that have extremely low stick forces per G, that even small inputs to the control surfaces can lead to high rates of G onset as well and G loads that may be in excess of the pilot¡¦s capacity to tolerate. To combat the risk of airframe damage, or pilot incapacitation when at very high speeds, most modern fighters utilize fly-by-wire computer systems which automatically limit the available Gs that a full stick deflection can induce in a given configuration. In most aerobatic planes the speeds are much less though the maneuverability is much higher and they lack the luxury of G limiters, allowing for extreme G loadings, though most all aerobatic and fighter aircraft have a G meter which will give the pilot at a glance specific G loading information (Goldsmith, 2004). Because a pilot¡¦s G tolerance may vary day to day, between aircraft (seat angle and G onset rates), the G meter itself can not be used to determine if the pilot is within his/her tolerance level, and attention to physiological cues is critical. (Goldsmith, 2004)


While the eyes fail and recover quite smoothly, allowing for the failure to be a G limiting guide, the brain is not as smooth in recovery. Once blood flow to the brain has ceased for a period of about 5 seconds a condition know as G-LOC sets in, that is unconsciousness. It takes about 10-30 seconds to recover from G-LOC, followed by seizure-like activity as well as confusion which usually lasts around 12 seconds. So far that is a maximum of 47 seconds to recover enough to even realize what is going on and try to regain controls, and reports say up to 2 minutes are required to regain normal performance abilities. This can be quite unfortunate because if G-LOC occurs, most likely it was during unusual positioning and combined with high G maneuvering, it is quite likely that when cognitive performance is regained, the pilot will have difficulty recovering from the situation regardless of their mental state.


Another situation that can cause G-LOC is due to experiencing the "push-pull" effect. From the pilot¡'s perspective this is an effective reduction to +Gz tolerance following an exposure to -Gz, and may lead to GLOC or visual effects of +Gz at a loading that the pilot usually may not have any difficulty with, as low as +4G even for only short exposures. (Goldsmith, 2004) This push-pull effect is due to conflicting time constants in compensatory mechanisms when experiencing relatively negative G's and then experiencing positive G's in a short period of time. Because these two situations have physiologically opposite responses, when you start the +G period, the starting position is much farther from what is needed to compensate for the +G experience and so it takes much longer for the body to recover. There are four general areas that change due to +/-G experiences that could cause more problems due to conflicts in time constants. First there is the change from bradycardia to tachycardia, second low cardiac contractility to high cardiac contractility, third excess atrial/ventricular preload to normal or reduced atrial/ventricular preload, and forth peripheral vascular vasodilation to peripheral vascular vasoconstriction. While all four of these mechanisms may and probably do so some extent play a part in the push-pull effect, the time constants and thus contribution to the push-pull effect has not yet been worked out for cardiac contractility or filling mechanisms. But, we do know a lot about the mechanisms of heart rate and vascular contractility and their contribution to the push-pull effect.


The first mechanism to consider is that of changing heart rate in response to the two opposite G induced situations. When one experiences -Gs, there is a decrease in heart rate, or bradycardia, and then with +Gs there is an increase in heart rate, or tachycardia. The bradycardia takes about 2-4 seconds of -Gz exposure to develop and with increased -Gz exposure there can be some recovery, but as mentioned earlier long exposure to -Gz is unpleasant and most pilots tend to avoid it, though aerobatic pilots can be an exception. Next it is important to note that tachycardia takes about 6-8 seconds of +Gz exposure to develop and that the rate of bradycardia development is faster than the rate of tachycardia development. Finally there is the 5-7 second substrate latency period that the brain can last without blood flow. The recovery time of 6-8 seconds, the time for tachycardia to compensate for the +Gz load, is greater than the substrate latency period, which means that if full -Gz reaction occurs and then +Gz is experienced the brain does not have enough nutrients to last through the recovery time, and thus G-LOC can develop, which as mentioned earlier can be very dangerous. (NUMI, 2000)


The other mechanism that is known to contribute to the push-pull effect is that of peripheral vascular contractility changes. Recent findings show that the rate of change in vasodilation is faster than the rate of change in vasoconstriction. This is of concern because -Gz causes vasodilation and +Gz causes vasoconstriction, so again the recovery is slower than the initial reaction. Another interesting correlation is that in older pilots whose blood vessels have become a bit more rigid, there is a reduction or even lack of the push-pull effect because the initial compensatory responses to -Gz do not occur on the same time scale and thus the body can react to changes more easily.


Overall, aerobatic physiology is very interesting with more research to be done, such as into other causes of the push-pull effect. But, from what we know the main issue is of awareness. That is, awareness of both a pilot's abilities, and their physiological reactions to varying G loads and rates of G onset. While this sport relies heavily on skill, the skill can not be used if the brain and other systems are not working properly and by simply doing aerobatics these systems can be diminished in their capabilities. This makes it hard to treat any of these issues, but simply understanding them allows a pilot to lessen the risks and consequences.

Bibliography

1. *********, Christina. Interview with Brett Goldsmith, a competition aerobatic pilot. 2004

2. Naval Aerospace Medical Institute (NAMI) under auspices of The bureau of Medicine and Surgery, Department of the Navy. Sustained Acceleration. United States Naval Flight Surgeon¡¦s Manual: Chapter 2: Acceleration and Vibration. 3rd Edition. 1991. Last revised March 2000.

3. D.C. Beaudette, Acting Director of Flight Operations. A Hazard in Aerobatics: Effects of G-Forces on Pilots. Advisory Circular No: 91-61. 2/28/84.

4. R.D. Banks and L.S. Goodman. Neurological Influence in Push-Pull Effect. Defense and Civil Institute of Environmental Medicine, 1995. AGARD Conference Proceedings 579. AGARD, Advisory Group For Aerospace Research and Development. France. 1996.

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