WBR0580

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
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 that results from decreased cerebral perfusion. A new investigational drug is being evaluated for its possible role in the prevention of 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 only
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 only
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 only
Answer C Explanation	AnswerCExp::Increase in arterial O2 does not regulate cerebral perfusion.
Answer D	AnswerD::Decrease in arterial O2 only
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::The brain consumes approximately 20% of the available arterial oxygen. Delivery of oxygen is thus strictly regulated because it is critical to maintain physiologic brain function. Cerebral perfusion is highly regulated by the cerebral blood pH that is determined by the CO2 partial pressure in the cerebral arteries. CO2 is a potent vasodilator that is thought to act by directly affecting extracellular hydrogen ions on vascular smooth muscles along with induction of local prostanoids and nitric oxide secretion in the cerebral circulation. Hypercapnia causes vasodilation of cerebral arteries and leads to increased cerebral blood flow. In contrast, hypocapnia results in vasoconstriction and decreased cerebral blood flow. During ischemia, an increase in CO2 results in an acidic pH that causes vasodilation and consequently leads to an increase in the cerebral blood perfusion. CO2 seems to affect cerebral perfusion until the partial pressure of CO2 reaches 90 mmHg, beyond which CO2 no longer regulates cerebral perfusion. In contrast, cerebral blood flow remains constant until severe hypoxemia is present, where partial pressure of oxygen needs to fall below 50 mmHg for changes in cerebral perfusion to be present. It is thought that acute hypoxia results in a drop of ATP that leads to opening of the K-ATP channels on smooth muscles. As channels open, hyperpolarization and vasodilation ensue. 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::