Cerebral blood flow oxygen extraction fraction

Metabolic data from fetal brain studies indicate that moderate hypoxia or asphyxia can be adapted to by increases in cerebral blood flow and/or fractional oxygen extraction (15,111,135-138). Acutely, reductions in cerebral oxygen consumption are small eompared to the reduction in arterial oxygen content (34) however, during severe or prolonged asphyxia, reductions in cerebral oxygen consumption become apparent. 

It follows from the above that the ratio of cerebral blood flow to cerebral blood volume is a measure of cerebral perfusion reserve (Schumann et al. 1998). Below a ratio of approximately 6.0, even if cerebral blood flow is still normal, vasodilatation and cerebral blood volume are maximal and the reserve is exhausted, as shown by a rising oxygen extraction fraction on PET. 

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