Impaired brain oxygenation results from:
- Reduced arterial oxygen pressure - lung disease.
- Reduced haemoglobin to carry oxygen - anaemia or blood loss.
- Reduced flow of blood containing oxygen (ischaemic hypoxia) - due to reduced cardiac output (with reduced cerebral blood flow).
- Biochemical block of cerebral utilisation of oxygen - rare (e.g. cyanide poisoning). When cerebral arterial Po2 falls below 35 mmHg (4.5 kPa), anaerobic metabolism takes over; this is not efficient and a further drop in Po2 will result in neurological dysfunction. The extent of hypoxic damage depends not only upon the duration of hypoxia but also on other factors, e.g. body temperature - hypothermia protects against damage. The irreversibility of hypoxic damage is explained by the 'no flow phenomenon' - after 3-5 minutes the endothelial lining of small vessels swells - even with reversal of hypoxia, flow through these vessels is no longer possible.
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