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Brain death is the complete and irreversible loss of brain function (including involuntary activity necessary to sustain life). Brain death is one of the two ways of determination of death, according to the Uniform Determination of Death Act of the United States (the other way of determining death being " irreversible cessation of circulatory and respiratory functions"). It is not the same as persistent vegetative state, in which the person is alive.
Brain death is used as an indicator of legal death in many jurisdictions, but it is defined inconsistently. Various parts of the brain may keep living when others die, and the term "brain death" has been used to refer to various combinations. For example, although a major medical dictionary says that "brain death" is synonymous with "cerebral death" (death of the cerebrum), the US National Library of Medicine Medical Subject Headings (MeSH) system defines brain death as including the brainstem. The distinctions can be important because, for example, in someone with a dead cerebrum but a living brainstem, the heartbeat and ventilation can continue unaided, whereas in whole-brain death (which includes brain stem death), only life support equipment would keep those functions going. Patients classified as brain-dead can have their organs surgically removed for organ donation.
Traditionally, both the legal and medical communities determined death through the permanent end of certain bodily functions in clinical death, especially respiration and heartbeat. With the increasing ability of the medical community to resuscitate people with no respiration, heartbeat, or other external signs of life, the need for another definition of death occurred, raising questions of legal death. This gained greater urgency with the widespread use of life support equipment, as well as rising capabilities and demand for organ transplantation.
Since the 1960s, laws on determining death have, therefore, been implemented in all countries with active organ transplantation programs. The first European country to adopt brain death as a legal definition (or indicator) of death was Finland, in 1971. In the United States, Kansas had enacted a similar law earlier. In the 1970s, the Supreme Court of the state of New Jersey ruled that patients and their families have the right to decide when and whether to remove life support.
An ad hoc committee at Harvard Medical School published a pivotal 1968 report to define irreversible coma. The Harvard criteria gradually gained consensus toward what is now known as brain death. In the wake of the 1976 Karen Ann Quinlan controversy, state legislatures in the United States moved to accept brain death as an acceptable indication of death. In 1981 a Presidential commission issued a landmark report – Defining Death: Medical, Legal, and Ethical Issues in the Determination of Death  – that rejected the "higher brain" approach to death in favor of a "whole brain" definition. This report was the basis for the Uniform Determination of Death Act, which is now the law in almost all fifty states of the US. Today, both the legal and medical communities in the US use "brain death" as a legal definition of death, allowing a person to be declared legally dead even if life support equipment keeps the body's metabolic processes working.
In the UK, the Royal College of Physicians reported in 1976 and 1977, rejecting the whole brain death criterion as scientifically worthless, and adopting the notion of irreversible brainstem dysfunction as an indicator of death.
A brain-dead individual has no clinical evidence of brain function upon physical examination. This includes no response to pain and no cranial nerve reflexes. Reflexes include pupillary response (fixed pupils), oculocephalic reflex, corneal reflex, no response to the caloric reflex test, and no spontaneous respirations.
It is important to distinguish between brain death and states that may be difficult to differentiate from brain death (such as barbiturate overdose, alcohol intoxication, sedative overdose, hypothermia, hypoglycemia, coma, and chronic vegetative states). Some comatose patients can recover to pre-coma or near pre-coma level of functioning, and some patients with severe irreversible neurological dysfunction will nonetheless retain some lower brain functions such as spontaneous respiration, despite the losses of both cortex and brain stem functionality; such is the case with anencephaly.
Note that brain electrical activity can stop completely, or drop to such a low level as to be undetectable with most equipment. An EEG will therefore be flat, though this is sometimes also observed during deep anesthesia or cardiac arrest. Although in the United States a flat EEG test is not required to certify death, it is considered to have confirmatory value. In the UK it is not considered to be of value.
The diagnosis of brain death needs to be rigorous, in order to be certain that the condition is irreversible. Legal criteria vary, but in general they require neurological examinations by two independent physicians. The exams must show complete and irreversible absence of brain function (brain stem function in UK), and may include two isoelectric (flat-line) EEGs 24 hours apart (less in other countries where it is accepted that if the cause of the dysfunction is a clear physical trauma there is no need to wait that long to establish irreversibility). The widely-adopted Uniform Determination of Death Act in the United States attempts to standardize criteria. The patient should have a normal temperature and be free of drugs that can suppress brain activity if the diagnosis is to be made on EEG criteria.
Also, a radionuclide cerebral blood flow scan that shows complete absence of intracranial blood flow must be considered with other exams – temporary swelling of the brain, particularly within the first 72 hours, can lead to a false positive test on a patient that may recover with more time. Zack Dunlap in 2008 had a false positive of this type, likely due to temporary cerebral edema.
Brain death may result in legal death, but with vasopressors helping the heart beat, and with mechanical ventilation all other vital organs may be kept completely functional,[unreliable medical source?] providing optimal opportunities for organ transplantation. When mechanical ventilation is used to support the body of a brain dead organ donor pending a transplant into an organ recipient, the donor's date of death is listed as the date that brain death was diagnosed.
In some countries (for instance, Spain, Belgium, Poland, Wales, Portugal, and France), everyone is automatically an organ donor, although some jurisdictions (such as Singapore, Spain, Wales, France, Czech republic and Portugal) allow opting out of the system. Elsewhere, consent from family members or next-of-kin may be required for organ donation. In New Zealand, Australia, United Kingdom (excluding Wales) and most states in the United States, drivers are asked upon application if they wish to be registered as an organ donor. In California, a licensed driver is presumed to be a donor by default, or may state that they do not wish to choose, but they are given no option to refuse to become an organ donor.
In the United States, if the patient is at or near death, the hospital must notify a transplant organization of the person's details and maintain the patient while the patient is being evaluated for suitability as a donor. The patient is kept on ventilator support until the organs have been surgically removed. If the patient has indicated in an advance health care directive that they do not wish to receive mechanical ventilation or has specified a do not resuscitate order and the patient has also indicated that they wish to donate their organs, some vital organs such as the heart and lungs may not be able to be recovered.