UWorld conflicting answers: Cerebral perfusion

[OrthoRehab33]

Would really love it if someone could help me make sense of this confliction within UWorld!
UW ID: 1494
UW ID: 1493
Overall: How does hyperventilation affect cerebral perfusion?
In 1494- it states in second paragraph- "(Because hypocapnia can decrease cerebral blood flow, patients with cerebral edema are often hyperventilated in order to decrease intracranial pressure.)"
Then in 1493 you have a pt with cerebral edema and they tx by increasing his ventilator respiratory rate to lower his pCO2 and they ask what is the most likely effect of this intervention?
Obviously I chose: decreased cerebral perfusion pressure-but that was wrong it says hyperventilation doesnt affect systemic blood pressure..........uhhhh what??

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[OrthoRehab33]

Nevermind- I get it: tricky though

 

[worldbeater]

I didn't look up the ID numbers, but just remember that CO2 is a vasodilator (in addition to being an acid). The reason you breathe through a paper bag during hyperventilation is that you have lost CO2; therefore, you have vasoconstriction and a problem with flow. By breathing in a bag, you are retaining CO2 and vasodilating, increasing the flow or amount of blood reaching the brain.

In the patient with cerebral edema, the vessels are bulging out and hitting structures it should not be. You have too much CO2 and have a problem with vasodilation. To vasoconstrict, you cut down the CO2 below the normal value of 33-45. Now the vessels have vasoconstricted and the edema will be reduced.