The death of a person does not occur all at once. Different tissues, organs, and people die at different rates. Tissues and organs may be preserved and transplanted into living persons long after the donor has been declared medically dead. Furthermore, some people have a much slower dying trajectory than others, and linger on for years after others who became terminally ill at the same time have long since been dead.
The traditional signs of death-cessation of respiration, heart rate, and sensory and motor reflexes—are still important in establishing time of death, but the major medical criterion now applied in most states is the cessation of all electrical activity of the brain for a certain period of time. This diagnosis of brain death is usually made when the patient has a flat electroencephalogram (EEG) for at least 10 minutes. Determination of brain death and other designations of death listed under the Harvard criteria (Ad Hoc Committee of the Harvard Medical School, 1968) have become especially important in these times of frequent organ transplants. However, the heart and lungs of a person in a coma may continue to function even though neural activity in the higher brain centers has ceased.
Loss of functioning of the heart, lungs, and brain are signs of death but not its actual cause. Rather, death is due basically to a breakdown in the oxygen cycle due to failures of the circulatory and respiratory organs and the respiratory centers in the brain stem. When the oxygen source to the brain is cut off, the cells of the higher brain die first, within 5 to 10 minutes; then, the cells in the medulla of the lower brain stem, which regulate heartbeat, respiration, and other vital reflexes, die.
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