<P> Objective measurements of the severity of cerebral hypoxia depend on the cause . Blood oxygen saturation may be used for hypoxic hypoxia, but is generally meaningless in other forms of hypoxia . In hypoxic hypoxia 95--100% saturation is considered normal; 91--94% is considered mild and 86--90% moderate . Anything below 86% is considered severe . </P> <P> It should be noted that cerebral hypoxia refers to oxygen levels in brain tissue, not blood . Blood oxygenation will usually appear normal in cases of hypemic, ischemic, and hystoxic cerebral hypoxia . Even in hypoxic hypoxia blood measures are only an approximate guide; the oxygen level in the brain tissue will depend on how the body deals with the reduced oxygen content of the blood . </P> <P> Cerebral hypoxia can be caused by any event that severely interferes with the brain's ability to receive or process oxygen . This event may be internal or external to the body . Mild and moderate forms of cerebral hypoxia may be caused by various diseases that interfere with breathing and blood oxygenation . Severe asthma and various sorts of anemia can cause some degree of diffuse cerebral hypoxia . Other causes include status epilepticus, work in nitrogen - rich environments, ascent from a deep - water dive, flying at high altitudes in an unpressurized cabin without supplemental oxygen, and intense exercise at high altitudes prior to acclimatization . </P> <P> Severe cerebral hypoxia and anoxia is usually caused by traumatic events such as choking, drowning, strangulation, smoke inhalation, drug overdoses, crushing of the trachea, status asthmaticus, and shock . It is also recreationally self - induced in the fainting game and in erotic asphyxiation . </P>

Effects of lack of oxygen in the brain