cerebral perfusion question?? plz help

[bluelamin]

can someone explain why low cerebral blood flow leads to low ICP?

this is from pg 501 in FA where they're explaining the effect of therapeutic hyperventilation.

i get that hyperventilating -> decreases pco2 -> vasoconstriction -> decreased cerebral blood flow

but why does decreased cerebral blood flow -> decreased ICP????

i thought cpp=map-icp so shouldnt cerebral blood flow be indicative of map and not icp? im so confused.

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[treatem-and-streetem]

So to answer "why does decreased cerebral blood flow ---> decreased ICP"...when there is fluid in the system there is less pressure in the system. I think you are trying to combine different concepts. Cerebral blood flow is indicative of MAP but when the brain is vasoconstricting this shunts blood away from the head decreasing pressure. You also have to think of the context in which therapeutic hyperventilation is being used (aka really high ICP like in bleeds). You are trying to keep ICP from becoming close to MAP, so you hyperventilate to reduce blood volume in the brain (bc there is already a TON of blood in the brain).