Brownout (medical)
Encyclopedia
A greyout is a transient loss of vision characterized by a perceived dimming of light and color, sometimes accompanied a loss of peripheral vision
. It is a precursor to fainting or a blackout and is caused by hypoxia
(low brain oxygen level), often due to a loss of blood pressure
.
Greyouts have a variety of possible causes:
Usually recovery is rapid and a greyout can be readily reversed by lying down. This way, the cardiovascular system does not need to work against gravity to reach the brain.
A grey-out may be experienced by aircraft pilots pulling high positive g-force
s as when pulling up into a loop or a tight turn forcing blood to the lower extremities of the body and lowering blood pressure in the brain. This is the reverse of a redout
, or a reddening of the vision, which is the result of negative G forces caused by performing an outside loop, that is by pushing the nose of the aircraft down. Redouts are potentially dangerous and can cause retina
l damage and hemorrhagic stroke
. Pilots of high performance aircraft can increase their resistance to greyout by using a G-suit
, which controls the pooling of blood in the lower limbs but there is no suit yet capable of controlling a redout. In both cases symptoms may be remedied immediately by easing pressure on the flight controls. Continued, or heavy G force will rapidly progress to g-LOC
(G force induced Loss of Consciousness).
Surprisingly, even during a heavy grey-out, where the visual system is severely impaired, pilots can still hear, feel, and speak. In other words, complete grey-out and loss of consciousness
are separate events.
Another common occurrence of greyouts are in roller coaster
riders. Many roller coasters put riders through positive G forces, particularly in vertical loops and helices. Roller coasters rarely have high enough negative G forces to induce redouts, as most low-G elements are designed to simulate weightlessness.
Tunnel vision
Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision.- Medical / biological causes :Tunnel vision can be caused by:...
. It is a precursor to fainting or a blackout and is caused by hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
(low brain oxygen level), often due to a loss of blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
.
Greyouts have a variety of possible causes:
- Shock, such as HypovolemiaHypovolemiaIn physiology and medicine, hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma...
, even in mild form such as when drawing blood. - Suddenly standing up (see orthostatic hypotensionOrthostatic hypotensionOrthostatic hypotension, also known as postural hypotension, orthostasis, and colloquially as head rush or dizzy spell, is a form of hypotension in which a person's blood pressure suddenly falls when the person stands up or stretches. The decrease is typically greater than 20/10 mm Hg, and may be...
), especially if sick, hungoverHangoverA hangover describes the sum of unpleasant physiological effects following heavy consumption of alcoholic beverages. The most commonly reported characteristics of a hangover include headache, nausea, sensitivity to light and noise, lethargy, dysphoria, diarrhea and thirst, typically after the...
, or suffering from low blood pressureHypotensionIn physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
. - By pilots or roller coaster riders experiencing positive g-forceG-forceThe g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
. - Paradoxically, hyperventilationHyperventilationHyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
, such as the fainting game, self-induced hypocapniaHypocapniaHypocapnia or hypocapnea also known as hypocarbia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation....
, or shallow water blackoutShallow water blackoutA shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres , when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have...
.
Usually recovery is rapid and a greyout can be readily reversed by lying down. This way, the cardiovascular system does not need to work against gravity to reach the brain.
A grey-out may be experienced by aircraft pilots pulling high positive g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
s as when pulling up into a loop or a tight turn forcing blood to the lower extremities of the body and lowering blood pressure in the brain. This is the reverse of a redout
Redout
A redout occurs when the body experiences a negative g-force sufficient to cause a blood flow from the lower parts of the body to the head. It is the inverse effect of a greyout or brownout, where blood flows away from the head to the lower parts of the body. Redouts are potentially dangerous and...
, or a reddening of the vision, which is the result of negative G forces caused by performing an outside loop, that is by pushing the nose of the aircraft down. Redouts are potentially dangerous and can cause retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
l damage and hemorrhagic stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
. Pilots of high performance aircraft can increase their resistance to greyout by using a G-suit
G-suit
A G-suit, or the more accurately named anti-G suit, is worn by aviators and astronauts who are subject to high levels of acceleration force . It is designed to prevent a black-out and G-LOC caused by the blood pooling in the lower part of the body when under acceleration, thus depriving the...
, which controls the pooling of blood in the lower limbs but there is no suit yet capable of controlling a redout. In both cases symptoms may be remedied immediately by easing pressure on the flight controls. Continued, or heavy G force will rapidly progress to g-LOC
G-LOC
G-LOC, pronounced 'GEE-lock', is the abbreviation of G-force induced Loss Of Consciousness, a term generally used in aerospace physiology to describe a loss of consciousness occurring from excessive and sustained g-forces draining blood away from the brain causing cerebral hypoxia...
(G force induced Loss of Consciousness).
Surprisingly, even during a heavy grey-out, where the visual system is severely impaired, pilots can still hear, feel, and speak. In other words, complete grey-out and loss of consciousness
G-LOC
G-LOC, pronounced 'GEE-lock', is the abbreviation of G-force induced Loss Of Consciousness, a term generally used in aerospace physiology to describe a loss of consciousness occurring from excessive and sustained g-forces draining blood away from the brain causing cerebral hypoxia...
are separate events.
Another common occurrence of greyouts are in roller coaster
Roller coaster
The roller coaster is a popular amusement ride developed for amusement parks and modern theme parks. LaMarcus Adna Thompson patented the first coasters on January 20, 1885...
riders. Many roller coasters put riders through positive G forces, particularly in vertical loops and helices. Roller coasters rarely have high enough negative G forces to induce redouts, as most low-G elements are designed to simulate weightlessness.