WBR0580

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::Experimental studies suggest that age-related dementia may be caused by chronic cerebral ischemia resulting from decreased cerebral perfusion. A new investigational drug is being evaluated for its possible role in preventing dementia. Cerebral perfusion studies following the administration of the investigational drug reveal vasodilation of the cerebral arteries. The mechanism of vasodilation of the drug is similar to that of which of the following physiological mechanisms?]]
Answer A	AnswerA::Increase in arterial CO2
Answer A Explanation	AnswerAExp::Increase in arterial CO2 leads to decrease in the cerebral blood pH and subsequent cerebral artery vasodilation.
Answer B	AnswerB::Decrease in arterial CO2
Answer B Explanation	AnswerBExp::Decrease in arterial CO2 leads to increase in the cerebral blood pH and subsequent cerebral artery vasoconstriction.
Answer C	AnswerC::Increase in arterial O2
Answer C Explanation	AnswerCExp::Increase in arterial O2 does not regulate cerebral perfusion.
Answer D	AnswerD::Decrease in arterial O2
Answer D Explanation	AnswerDExp::Decrease in arterial O2 does not regulate cerebral perfusion unless the partial pressure of O2 is less than 50 mmHg.
Answer E	AnswerE::Increase in arterial CO2 and decrease in arterial O2
Answer E Explanation	AnswerEExp::Increase in CO2 causes vasodilation; decrease in O2 does not affect cerebral blood flow unless the partial pressure of O2 is inferior to 50 mmHg.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Cerebral perfusion is highly regulated by the cerebral blood pH that is determined by the CO2 partial pressure in the cerebral arteries. During ischemia, CO2 increases leading to an acidic pH that causes vasodilation and consequencently leads to an increase in the cerebral blood perfusion. CO2 seems to affect cerebral effusion until the partial pressure of CO2 reaches 90 mmHg, beyond which CO2 no longer regulates cerebral perfusion. Educational Objective: Cerebral perfusion is regulated by the pH of the blood, which is determined by the partial pressure of CO2 in the cerebral blood. CO2 seems to affect cerebral perfusion until the partial pressure of CO2 reaches 90 mmHg, beyond which CO2 no longer regulates cerebral perfusion. The partial pressure of O2 does not affect cerebral perfusion unless it is extremely low (<50mmHg). References: First Aid 2014 page 280]]
Approved	Approved::No
Keyword	WBRKeyword::Cerebral flow, WBRKeyword::Chemoreceptor, WBRKeyword::Hypoxia
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::