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Brain death is the complete and irreversible loss of brain function. In most cases, it is most commonly associated with physical death but does not have to be. Instead, there are times when a body can be kept alive, usually by artificial or mechanical means, even though there is no brain function. This is when the term brain death, or being brain dead, is most often used.
Through the advancements of modern science, humans have learned how to keep a body alive beyond a point that natural means could achieve by themselves. However, doing so comes at a price. Usually, for the person involved, this is a last-ditch effort to try to save them when medical doctors may think recovery is still possible. Therefore, in most cases, the use of such machines is only temporary until further evaluations can be done and a diagnosis of brain death can be made.
Determining brain death is usually done through the use of an electroencephalogram. This device measures electrical impulses in the brain, which is how brain cells communicate with one another. If there are no electrical impulses detected, at least in certain regions of the brain, there is no communication. If there is no communication, a brain death diagnosis becomes a very likely scenario.
Brain deaths can occur when the person is close enough to the hospital that medical staff are able to keep the heart beating and oxygen and nutrients flowing to the body cells, even though there is no brain activity. As stated before, this is usually done before it can be determined whether brain activity has ceased. Once that takes place, there is very little likelihood of a recovery. However, the decision to take a person of life support machines is usually left up to the family after they consult with the doctor.
A brain death can occur from many different factors, but oxygen is a key to all of it. Though they may not be related at all, they may cause similar things to happen in the brain. For example, different forms of trauma can cause bleeding and swelling in the brain. This could cause the brain to cease functioning. If the brain does not get enough oxygen, a condition known as anoxia, it will begin to die as well. In an indirect way, anoxia is responsible for all brain deaths. For example, in cases where the brain swells, intercranial pressure may cause blood flow to be cut off, resulting in a condition where oxygen can no longer reach the brain.
@JimmyT - Well, in reality someone could be dead for a couple hours and then brought back to life, if they happen to get the heart started again, but they would be brain dead.
As far as how long someone can be dead, from say a heart attack, and be brought back without brain damage is only a matter of a few minutes.
The longest I have ever heard of someone being dead and being brought back without brain damage is fifteen minutes, but damage can occur after only a few minutes.
I have also heard that brain death can be slowed if you lower the persons body temperature, like say if you cover them in ice, assuming that can be done quick enough.
I have heard a lot recently about people having heart attacks and dying, but have to be zapped back to life in a certain amount of time in order to prevent brain death.
I read somewhere that there are guidelines as to how long a person can have their heart stopped before they can have brain death.
It is possible to bring someone back to life, like say after an hour, but they will probably be brain dead at that point or at least very low functioning.
I do not know how long though a person could have their heart stopped, before brain death occurs.