Cerebral Blood Flow: How low can it go?
The brain receives blood from the carotid and vertebral arteries at a rate of about 750 ml/min.2 This continuous supply of blood normally provides the brain with enough oxygen and nutrients. The average rate of cerebral blood flow is 50-55ml per 100g of brain tissue per minute. In actuality, though, the gray matter-or the outer surface of the brain-requires up to four times the blood supply of the white matter, or the inner portion. When cerebral circulation is impaired, the supply of oxygen and nutrients to brain tissue can diminish, compromising neurological function. Quick action is needed to restore blood flow before permanent damage occurs. Ischemia with loss of brain function tends to occur when cerebral blood flow drops below 20ml/100g/min. Whether or not permanent deficit will result depends primarily on two things: how low blood flow to a particular area drops, and for how long. The window of opportunity for reversing mild ischemia is generally four to six hours.3 As blood flo
