Brownout (medical)
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224838993
2008-07-10T17:22:38Z
Valce1
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{{Unreferenced|date=August 2006}}
{{otherusesof|brownout}}
A '''brownout''', or '''grey-out''', is a transient loss of vision characterised by a perceived dimming of light accompanied by a brown hue and a [[tunnel vision|loss of peripheral vision]].{{Fact|date=February 2007}} It is a precursor to [[fainting]] or a blackout and is caused by [[Hypoxia (medical)|hypoxia]], a loss of [[blood pressure]] or restriction of blood flow to the brain. It is commonly experienced when suddenly standing up (see [[orthostatic hypotension]]), especially if sick, [[hangover|hungover]], or suffering from [[hypotension|low blood pressure]] or [[Shock (medical)|shock]]. Usually recovery is rapid and a brownout can be readily reversed by lying down.
A brownout may be brought on intentionally or unintentionally through [[hyperventilation]] (See [[fainting game]] under ''self-induced [[hypocapnia]], and ''[[shallow water blackout]].''
A brownout, or grey-out, may also be experienced by aircraft pilots pulling high positive [[g-force|''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 brownout 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 [[joystick]]. Continued, or heavy G force will rapidly progress to [[g-LOC]] (G force induced Loss of Consciousness).
Another common occurrence of brownouts are in [[roller coaster]] riders. Many roller coasters put riders through high positive G forces, particularly in [[vertical loop]]s and helices.{{Fact|date=February 2007}} Roller coasters rarely have high enough negative G forces to induce redouts, as most [[Zero G roll|low-G elements]] are designed to simulate weightlessness.
Also, grey-out may occur after donating blood, resulting in a loss of blood pressure and insufficient blood reaching the brain. However, the onset of such grey-outs is usually gradual and preventable by lying down so that the cardiovascular system does not need to work against gravity to reach the brain.{{Fact|date=April 2007}}
[[Category:Visual system]]
[[Category:Eye]]
[[de:Redout]]