Brain death applies to the situation when the heart continues to beat with the breathing maintained mechanically after the brain has permanently ceased to function — and it is an unnatural artefact of medical technology.
Under natural conditions when the brain ceases to function breathing immediately stops, and soon after that the heart stops from lack of oxygen. If breathing is taken over by a mechanical ventilator, oxygenation is maintained and the heart can continue to beat, for days at least, because the heart muscle acts on its own independently of the brain. That death is a process rather than a sudden event is now well recognized. Most often the process is initiated by the heart stopping, and this is then followed by brain failure due to lack of oxygen. Sometimes it is the breathing that stops first, the lack of oxygen leads to brain failure and later the heart also stops.
The relevant part of the brain for the maintenance of breathing, as well as for the activation of higher cerebral function, is the brain stem — the lowest part of the brain above its junction with the spinal cord. Because function in this region is crucial for the whole of the brain, cessation of that function, strictly ‘brain stem death’, is commonly referred to as brain death.
Brain death is the result of unsuccessful resuscitation — the price paid for the many patients whose lives are saved, and who make a good recovery, when a ventilator is used during brain failure which proves to be temporary. When mechanical ventilation was begun it was not known whether or not the brain could recover — only a trial period of ventilation could settle that question.
The problem with waiting for the heart to stop following brain death is that it can go on beating for several days, occasionally for weeks, during which time other organs fail and the extremities may begin to decompose. To continue artificial ventilation is therefore regarded as both futile and undignified. In many countries it has been accepted that when the brain is dead the person is dead; in some jurisdictions laws have been enacted to acknowledge this, but in others it has been considered unnecessary.
This matter has, however, been complicated by the development of organ transplantation. Kidney transplantation was well established before the concept of brain death was widely accepted, because it was possible to use organs from donors whose hearts had stopped beating, but the transplantation of hearts, lungs, and livers is possible only from donors whose hearts are still beating, and therefore only from those who are brain dead.
There is clearly need for strict criteria for the diagnosis of brain death, whether or not there is any question of organ donation, because the consequence of this diagnosis will be the withdrawal of artificial ventilation. This is regarded as discontinuing an inappropriate intervention for a person who is already dead, rather than letting that person die. In the UK the medical Royal Colleges agreed criteria for the diagnosis of brain death in 1976. These require satisfying certain pre-conditions and then undertaking tests to confirm that there is no function in the brain stem. The pre-conditions must establish that the patient is in coma and on a ventilator because breathing has ceased due to irreversible structural brain damage — usually due to severe head injury, brain haemorrhage, or an episode of oxygen starvation of the brain. It is also necessary to exclude reversible causes of failure of brain function, including depressant drugs and hypothermia. The tests for absence of brain stem function require there to be no reflex responses in the pupils or the muscles of the face, throat, or eyes. The final test is to confirm that there has been no recovery of spontaneous breathing by disconnecting the ventilator temporarily, whilst maintaining a passive flow of oxygen to the lungs. These tests are carried out twice and by two experienced doctors. The time of death, for legal purposes, is when the first set of tests were completed, although death is not declared until after the second test. Additional tests are not required in the UK but are used in some countries. These include demonstrating lack of electrical activity in the brain by electro-encephalography (EEG), or lack of blood circulation in the brain using either radioactive isotopes or radio-opaque dyes injected into the bloodstream.
— Bryan Jennett
See also coma; life support; organ donation; vegetative state.
