2007-08-31

The Slow Road Down

For many unfortunate people (more all the time), death begins long before the heart stops beating and the lungs stop breathing. The unique personal identity is eroded by loss of memory and disordering of personality from aging, injury, or some other cause or condition.

But for those who manage to make it intact to prolonged cardiac arrest and cessation of breathing, how long does the brain last? How long is there anything left in the brain that future medical technology could use to bring the person back to who they were before?

A person can be brought back even today after 5 minutes, under the best circumstances. Slightly longer times are possible without brain damage if the head or whole body is cooled, by a procedure called clinically induced moderate hypothermia.

If more than 5 minutes have passed and hypothermia is not used, the person is in big trouble. Restarting the heartbeat and breathing can cause a serious brain injury called reperfusion injury. The body may continue heartbeat and breathing, but the brain may never fully recover. The brain may never even receive circulation from the heartbeat. People put on a ventilator may be "brain dead" in a true sense -- the brain is seriously injured and begins the process of death.

If a person is "lucky" enough to be treated as "dead" -- without reperfusion injury being introduced, the process of death proceeds at a pace determined, at least in part, by temperature. Heat provides energy to the various reactions taking place. If there is no circulation, the body's drop to ambient temperature actually helps slow the process. The body being put in a cooler at near freezing temperatures slows it even more.

From there begin a series of changes to the brain. A neat summary (though based on the brains of dogs) is the following:

(Figure from Haines, D. E., & Jenkins, T. W. J. Comp. Neur. 132: 405-418. Studies on the epithalamus: I. Morphology of post-mortem degeneration: The habenular nucleus in dog.)

In the next post I'll talk about the significance of the changes...

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