Death in the UK is defined in the 2008 Academy of Medical Royal Colleges (AoMRC) Code of Practice as the irreversible loss of the capacity for consciousness, combined with the irreversible loss of the capacity to breathe. This definition is not contingent on cardiac function — a patient whose heart continues to beat because of mechanical ventilation and vasopressor support is dead if they have permanently and irreversibly lost the capacity for consciousness and for independent breathing.

The basis for this definition lies in the anatomy and physiology of the brain stem. The ascending reticular activating system — the neural network responsible for generating and maintaining the conscious state — runs through the brain stem. The primary respiratory centre, controlling the automatic generation of the drive to breathe, is also located in the brain stem. Irreversible destruction of the brain stem therefore causes both permanent unconsciousness and permanent apnoea. This is the physiological basis for equating brain stem death with death of the organism.

It is important to understand that brain stem death is a clinical diagnosis — it does not require EEG, cerebral angiography, or any other ancillary investigation in the UK. This distinguishes UK practice from some other countries, where additional tests are required. The clinical examination alone, performed by appropriately qualified doctors who have satisfied all preconditions, is sufficient to make the diagnosis.