There is no single definition of “brain death” in the world and even with the help of certain guidelines, there is significant variability in the determination of this condition. A widely accepted mechanism of death is the irreversible cessation of function of the entire brain.
In general, the following criteria have to be met (local guidelines may differ) (only an excerpt, not meant to be complete):
- an acute catastrophe of the central nervous system (the cause should be known)
- exclusion of confounders like severe electrolyte or circulatory or other disturbances
- exclusion of drug intoxication or poisoning
- (near) normal core temperature
- normal blood pressure
A neurologic examination should demonstrate:
- the absence of several brain-originating responses and reflexes
With a valid diagnosis of “brain death”, somatic death will follow usually within a few days even with life support.
Brain death is not equal to a biological death, where whole organ-systems lose their functions permanently and finally, the body disintegrates.
Brain death is a legally accepted definition of a person’s death in many countries and discontinuation of life support or taking organs for transplantation is legally warranted in this situation.
Is this definition of brain death compatible with a Buddhist perspective of death? The definition I found is from MN 9, MN 141, SN 12.2 (all nearly the same):
The passing of beings out of the various orders of beings, their passing away, dissolution, disappearance, dying, completion of time, dissolution of the aggregates, laying down of the body—this is called death. - MN 9, BB
Not all items mentioned above seem to be met with the definition of brain death, but one interesting point might be the dissolution of the aggregates. With the permanent cessation of the (entire) brain function, it seems that feeling, perception, sankhārā, and consciousness are unable to arise again in this remaining rupa. With this I think, we see here the final stage of dying where the “person” is gone and the breaking up of the body is imminent.
In conclusion, I think, with a proper diagnosis of brain death, discontinuation of life support is neither euthanasia common sense nor a breaking of the first precept.