hyperventilating a pt. with ICP

[jessica84]

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I know that hyperventilating a pt with ICP promotes cerebral vasoconstriction (by decreasing PaCO2) and decreasing ICP.

BUT... does it also prevent tissue hypoxia to the brain?

I am a nursing student, and was asked which of the following does hyperventilating a pt with ICP do:

1. promote cerebral vasoconstriction
2. prevent cerebral hypoxia to the brain

(choices 3 and 4 were wrong) My guess is that I would say #1, but I also have to explain why #2 is wrong.

Any help? I have been reading all my books and I can't find anywhere to explain why 2 would be wrong.

 

[jessica84]

never mind---I got the answer. I guess hyperventilation can actually induce ischemia to the brain (so it wouldn't prevent hypoxia to the brain as the one choice said)

 

[kapMD/PhD]

you are correct that hyperventilating the patient will reduce the PCO2 (the pt is breathing off CO2 more quickly and therefore it is decreased). Blood vessels of the brain are extremely responseive to the level of CO2, and when levels are low, they will constrict as a metabolic response to the decreased levels of CO2 needed to be eliminated (b/c you're already getting rid of it by hyperventilation). When vasoconstriction occurs, this in itself could possibly lead to cerebral ischemia/hypoxia because of decreased blood flow because of the vasoconstriction.
On a side note- it is possible that there can be reduced O2 delivery to the brain during exercise for the similar reasons. When one begins exercise, the stimultion of the sympathetics cause an increase in respiration - which will lead to quasi-hyperventilation/aka breathing off more CO2 than is necessary, and b/c of this the cerebral vessels may vasoconstrict.
Hope this helps!

 

[jessica84]

Thanks for the info. Yes, that helps.