Brain death

Brain diagram showing brain stem, upper and lower brain. Courtesy of the Australian Museum Research Library

Brain death (irreversible cessation of all function of the brain) normally occurs after a stroke, or an impact that causes the brain to swell and push against the skull, preventing blood from flowing to the brain. In the absence of oxygenated blood, brain cells quickly die. The dead cells break down and liquefy. Brain death is quite different from reversible coma (unconsciousness) in which living brain cells remain.

A person can remain permanently unconscious with total or partial brain death. A person with death of only the upper brain (cerebral hemispheres) will not have consciousness, memory, knowledge or thought, but the living lower brain (brain stem) allows the heart to pump, the lungs to breathe and the body to function.

To be legally brain dead, all function of both the upper and lower brain must cease. Because the heart will fail on a brain-dead person, certification of death by brain-death criteria (instead of circulatory criteria) will only be needed when the dead person's body functions are being maintained by an artificial ventilator.

To establish that the brain is dead, certifying doctors must ascertain that:

there is no evidence of brain function over a period of time
the loss of function is not a result of drugs, hypothermia (low temperature), hypoglycaemia (low blood sugar) or hyponatraemia (low blood sodium)
the person has sustained a brain injury sufficient to account for the irreversible loss of brain function. Often this is done by CT scan
there are no reflex functions associated with coughing, gagging, eye movement, blinking, or dilation of the pupils
the person makes no attempt to breathe when disconnected from the respirator for several minutes
during the previous test, the carbon dioxide level of the blood has risen above the point at which breathing is normally stimulated

These tests are frequently repeated after a further 24 hours as an assurance of irreversibility. A flat electroencephalogram, indicating an absence of brain activity is often used for verification.

Where the "dead" person's organs are available for donation, two doctors, neither of whom is caring for a potential organ recipient, must undertake the testing and certification.

If the person has received drugs that might invalidate the testing procedure, or if the head is so badly damaged that the tests cannot be performed, death can be determined by total lack of blood flow to the brain. This is done by inserting dyes (angiogram), or radioisotopes into the blood vessels supplying the brain blood vessels, to ascertain that they do not travel to the brain.