KEY POINTS
- The first description of cessation of brain functions using a concept similar
to the modern definition of brain death appeared in 1959, although the subject became
more controversial after the development of organ transplantation. Criteria for
brain death were published in 1968, a year after the first heart transplantation.
- The traditional concept of death has used the cessation of cardiac and
respiratory functions as its basis because of the acceptance of simple and nonmedical
concepts: that life begins with the first inspiration after birth, that death comes
with the last expiration, and that cardiac activity ceases within a few minutes of
the last expiration. In contrast, the modern concept of brain death adopts the conclusions
of modern biologic science: that the CNS, including the brainstem, is the control
center for the living organism; that cessation of CNS functions represents cessation
of the harmony of life; and that without CNS control, the living organism is nothing
more than an aggregation of living cells.
- Trauma to the brain or cerebrovascular injury produces brain edema. Because
the brain is covered by a rigid bony skull, edema is accompanied by an increase in
intracranial pressure, which, if sufficiently high, exceeds arterial blood pressure.
When cerebral circulation ceases, aseptic necrosis of the brain ensues. Within
3 to 5 days, the brain becomes a liquefied mass. Such increased intracranial pressure
compresses the entire brain, including the brainstem, and total brain infarction
follows.
- Clinical studies indicate that hypothalamic and anterior pituitary functions
are preserved to some degree for a certain period after the onset of brain death.
The response of the immune system to stimulation is modified considerably after
total and irreversible loss of CNS functions.
- During the process of brain death after head injury or intracranial bleeding,
intracranial pressure increases, and compression of the brainstem leads to marked
hypertension and bradycardia (i.e., Cushing phenomenon). After the establishment
of brain death, different types of autonomic spinal cord reflexes develop, such as
elevation of arterial blood pressure because of bladder distention.
- Determination of brain death confirms the irreversible cessation of all
functions of the entire brain, including the brainstem. Irreversibility means that
no treatment may be reasonably expected to change the condition. The passage of
time is also an essential component in determining that a lesion is irremediable.
Although testing all functions of the brain is conceptually impossible, the cessation
of all functions of the brain is practically determined by loss of consciousness,
loss of brainstem responses, apnea, and confirmatory tests including lack of electroencephalographic
activity.
- Cerebral death, the so-called persistent vegetative state, refers to cessation
of the functions of the cerebral cortices. It is not the equivalent of death.
- Tests to confirm brain death include an EEG, evoked responses, measurement
of blood flow, angiography, CT, MRI, TCD, and PET.
- Because of their intact spinal cord and the presence of somatic and visceral
reflexes, brain-dead patients
require special anesthetic management, including use of muscle relaxants, vasodilators,
and perhaps sedation and analgesia. Anesthesiologists should understand the medical
and legal definitions of death, as well as the ethical concepts behind them.
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