Hypoxia may be due to:
- Disturbances in the autoregulation of blood supply to the brain.
- Conditions affecting cerebral blood vessels.
When the main blood pressure falls below about 60 mmHg,the autoregulating mechanisms that control the blood flow to the brain by adjusting the diameter of the arterioles fail. The consequent rapid decrease in the cerebral blood supply leads to hypoxia and lack of glucose. If severe hypoxia is sustained for more than a few minutes there is irreversible brain damage. The neurones are affected first,then the neuroglial cells and late the meninges and blood vessels. Conditions in which autoregulation breaks down include:
- Cardiorespiratory arrest.
- Sudden severe hypotension.
- Carbon monoxide poisoning.
- Hypercapnia(excess blood carbon dioxide).
- Drug overdose with,eg,opioid analgesics,hypnotics.
Conditions affecting cerebral blood vessels that may lead to hypoxia include:
- Occlusion of a cerebral artery by,eg,a rapidly expanding intracranial lesion,atheroma,thrombosis or embolism.
- Arterial stenosis that occur in arteritis,eg,polyarteritis nodosa,syphilis,diabetes mellitus,degenerative changes on older people.
If the individual survives the initial episode if ischaemia,then infarction,necrosis and loss of function of the affected area of brain may occur.
