Brain Death

Brain death has been defined as "irreversible unconsciousness with complete loss of brain function."^[1]It has also been described as "irreversible brain damage as manifested by absolute unresponsiveness to all stimuli, absence of all spontaneous muscle activity, including respiration, shivering, etc., and an isoelectric electroencephalogram for 30 minutes, all in the absence of hypothermia or intoxication by central nervous system depressants." It is also called irreversible coma and cerebral death.^[2]To establish fact and time of death, brain death has to be demonstrated.^[1]

Signs

Signs of brain death include absent pupillary, corneal, gag reflex, oculo-vestibular reflex and oculocephalic reflex. The person shows no reaction to pain and doesn’t breathe when the ventilator is switched off.  Also, there is no brain activity at all on an electroencephalogram test.^[3]

The Whole-Brain Approach

According to the whole-brain criterion, "human death is the irreversible cessation of functioning of the entire brain, including the brainstem." The human brain can be thought of as a structure with two portions: First is the higher brain  which consists of cerebrum, the chief responsible for conscious awareness and the cerebellum which controls the coordination of voluntary movements. Second is the lower brain or the brainstem which includes medulla that controls spontaneous respiration, the reticular activating system which is likened to an on/off switch that enables consciousness without affecting its contents as well as midbrain and pons. Whole-brain death involves the destruction of both the higher brain and the brainstem. ^[4]       

From this approach, it is thus clear that when the entire brain is non-functional but other functions continue such as cardiopulmonary as assisted by respirator or other life-supports, there is false appearance of life. ^[4] 

Brain death is different from persistent vegetative state. In the latter, the person has lost only the higher brain functions but the undamaged brain stem still allows essential functions like heart rate and respiration.^[3]

The Uniform Determination of Death Act (UDDA) adopted by many states in the United States states that “an individual who has sustained either (1) irreversible cessation of circulatory and respiratory functions, or (2) irreversible cessation of all functions of the entire brain, including the brainstem, is dead.” ^[4]    

The acceptance of whole-brain criteria for death enables retrieval of still-viable organs while respiration and circulation continue. ^[4]     

References:

1. http://www.deathreference.com/Bl-Ce/Brain-Death.html
2. http://euthanasia.procon.org/view.answers.php?questionID=000197
3. http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Brain_death?open
4. http://plato.stanford.edu/entries/death-definition/