Figure 137

Brain structures involved in the sleep-wakefulness cycles. The suprachiasmatic nucleus is the site of the major "biological clock."

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Consciousness and Behavior CHAPTER THIRTEEN

Consciousness and Behavior CHAPTER THIRTEEN

TABLE 13-2 Criteria for Brain Death

The nature and duration of the coma must be known.

a. Known structural damage to brain or irreversible systemic metabolic disease.

b. No chance of drug intoxication, especially from paralyzing or sedative drugs.

c. No sign of brain function for 6 h in cases of known structural cause and when no drug or alcohol is involved; otherwise, 12-24 h without signs of brain function plus a negative drug screen.

Cerebral and brainstem function are absent.

a. No response to painful stimuli administered above the spinal cord.

Pupils unresponsive to light.

c. No eye movement in response to ice-water stimulation of the vestibular reflex.

d. Apnea (no spontaneous breathing) for 10 min.

e. Systemic circulation may be intact.

f. Purely spinal reflexes may be retained.

Supplementary (optional) criteria.

a. Flat EEG (wave amplitudes less than 2 ^V).

b. Responses absent in vital brainstem structures.

c. No cerebral circulation.

But a coma—even an irreversible coma—is not equivalent to death. We are left, then, with the question: When is a person actually dead? This question often has urgent medical, legal, and social consequences. For example, with the need for viable tissues for organ transplantation it becomes imperative to know just when a person is "dead" so that the organs can be removed as soon after death as possible.

Brain death is widely accepted by doctors and lawyers as the criterion for death, despite the viability of other organs. Brain death occurs when the brain no longer functions and has no possibility of functioning again.

The problem now becomes practical: How does one know when a person (for example, someone in a coma) is considered brain dead? There is general agreement that the criteria listed in Table 13-2, if met, denote brain death. Notice that the cause of a coma must be known because comas due to drug poisoning are usually reversible. Also, the criteria specify that there be no evidence of functioning neural tissues above the spinal cord because fragments of spinal reflexes may remain for several hours or longer after the brain is dead. The criterion for lack of spontaneous respiration (apnea) can be difficult to check because if the patient is on a respirator, it is of course inadvisable to remove him or her for the 10-min test because of the danger of further brain damage due to lack of oxygen. Therefore, apnea is diagnosed if there is no spontaneous attempt to "fight" the respirator; that is, the patient's reflexes do not drive respiration at a rate or depth different from those of the respirator.

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