M2 Brain death (clinical criteria)

Topic

In brain death, the brain—including the brainstem—ceases to function completely and irreversibly; it is this irreversible and complete cessation of brain function that defines brain death. Brain death is formally distinguished from deep coma by the total and permanent absence of brain activity, rather than merely a reduction in such activity, as occurs in deep coma.

The model formalizes the standard clinical criteria that collectively define brain death. The first of these criteria is the sustained absence of brain activation—specifically, the sustained absence of the same thalamocortical activation previously described in another model regarding the level of consciousness.

The second criterion is the cessation of cerebral blood flow to the brain—specifically, the cessation of the incoming blood flow defined in the neurovascular coupling model. This cessation of cerebral blood flow is associated with cerebral perfusion pressure collapsing below a critical value; it is precisely when cerebral perfusion pressure falls below this critical threshold that blood flow to the brain can no longer be sustained.

The third criterion is the absence of brainstem reflexes in response to maximal stimulation; that is, even the most intense stimulus fails to elicit a reflex response from the brainstem. The fourth criterion is a positive result on the apnea test, which involves verifying the absence of a respiratory response despite progressive hypercapnia—meaning that, even as blood carbon dioxide levels rise progressively (a condition that would normally stimulate respiration with increasing intensity), no respiratory response occurs. The fifth and final criterion is a recording of the brain's electroencephalogram signal that is isoelectric—that is, showing no detectable electrical activity, thereby constituting complete cerebral electrical silence—anticipating the same type of electroencephalogram signal described in greater detail in another diagnostic model. The simultaneous and sustained combination of these five criteria, all pertaining to the same brain, constitutes the operational definition of brain death.

ID:4274