Any event that stops the oxygen supply can cause hypoxia, including drowning, suffocation, choking, asphyxiation and heart attack.
The brain is particularly sensitive to a reduction in a good supply of oxygenated blood.
Even after prolonged heart surgery when blood circulation has been maintained artificially, sensitive tests have detected minor brain impairment, including memory loss.
Hypoxia of the brain initially causes confusion and dizziness, then unconsciousness and death. If suffocation is the cause of the hypoxia, being allowed to breathe freely again by removal of the obstruction will quickly restore the situation without any damage. But as brain cells can survive for only a few minutes without oxygen, damage to the brain can be rapid. When there is damage to the oldest part in evolutionary terms - the brain stem and the cerebellum, which are located at the base of the skull - a patient will need life support.
These parts of the brain control some of the body's automatic functions, including blood pressure and breathing.
Some reports have suggested that Joy Gardner died of kidney failure. Although it is possible, doctors say hypoxia is unlikely to cause sudden kidney failure in a patient unless there is an existing kidney condition that is being treated. Kidney failure tends to develop slowly.