Other notes

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The term coma describes a severe decrease in mental function due to structural, physiological, or metabolic impairment of the brain. A person in a coma is characterized by a sustained loss of the capacity for arousal even in response to vigorous stimulation. There is no outward behavioral expression of any mental function, the eyes are closed, and sleep-wake cycles disappear.
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Coma can result from extensive damage to the cerebral cortex; damage to the brainstem arousal mechanisms; interruptions of the connections between the brainstem and cortical areas; metabolic dysfunctions; brain infections; or an overdose of certain drugs, such as sedatives, sleeping pills, and, in some cases, narcotics.
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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.
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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.
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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 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.
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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.