Brain death is defined as the irreversible loss of all functions of the brain. It can be determined in several ways. First - no electrical activity in the brain; this is determined by an EEG. Second - no blood flow to the brain; this is determined by blood flow studies. Third - absence of function of all parts of the brain - as determined by clinical assessment (no movement, no response to stimulation, no breathing, no brain reflexes.)
There are several ways in which a person can become brain dead, these include:
1. Anoxia caused by drowning, respiratory diseases, or drug overdose.
2. Ischemia - Blockage of an artery leading to the brain or in the brain, heart attack (stoppage of the heart for a period of time), bleeding in the brain.
3. Intracranial hematoma - caused by a head injury (a blow to the head) or a ruptured aneurism.
4. A gunshot wound to the head - causes destruction of brain tissue and swelling of the brain.
5. Intracranial Aneurysm - the ballooning of a blood vessel supplying the
brain - can cut off blood supply or rupture.
6. Brain tumors - can destroy brain tissue and increase pressure within the brain.
When any of the above occur, they cause swelling of the brain. Because the brain is enclosed in the skull, it does not have room to swell, thus pressure within the skull increases (this is "intracranial pressure"). This can stop blood flow to the brain, killing brain cells and causes herniation of the brain (pushing the brain outside of its normal space). When brain cells die, they do not grow back, thus any damage caused is permanent and irreversable.
Some points to note:
A persons' heart can still be beating because of the ventilator and medications helping to keep the blood pressure normal.
A person who is declared brain dead is legally dead.
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