Tiff
Forum Guru
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A 24 year old medical student was studying one night for the boards and decided to take a break to eat. While eating quickly, he was reading something that made him laugh. A piece of food got stuck and he started having trouble breathing. At the emergency department, an x-ray film showed a peice of chicken stuck in his trachea. This meant that there was no air getting to the alveoli, but blood was still flowing. Which of the following best describes the relationship between ventilation and perfusion in the lungs?
A. Compared to the apex, the ventilation/perfusion ratio in the base is much greater.
B. During exercise, the ventilation/perfusion ratio in the apex does not change.
C. In a complete airway obstruction (shunt), the ventilation/perfusion ratio approaches infinity.
D. In hypoxic vasoconstricted areas, the ventilation/perfusion ratio approaches zero.
E. Relative to the base, the apex arterial partial oxygen pressure is less and arterial partial carbon dioxide pressure is greater.
F. The ventilation/perfusion ratio in the apex of the lung indicates wasted perfusion.
G. Ventilation is greatest at the base of the lungs.
*Please explain your response
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| Dr_Cosme
Forum Junior
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I would choose G
even though V/Q ratio is greater in the apex than the base, but both ventilation and perfusion are greater in the base than at the apex.
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| grechita12
Forum Senior
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G definitely
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| new_n_lost
Politically InCorrect
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Choice A is false it should be the other way around.
Choice B is False as it does change as the blood flow increases.
Choice C False
Choice D True it will approach zero as blood flow is always shunted in the lungs where there is less PO2
Choice E its the oppostie
Choice F nope its wasted ventilation
Choice G though essentially true but in comparison its lower than that of the apex due to the maximum amount of perfusion in the base.
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| grechita12
Forum Senior
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ventilation and perfusion are greater at the base of the lung that at the apex of the lung. plus all the others are incorrect. (First Aid 2006 pg. 413)
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| Tiff
Forum Guru
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So when there's an airway obstruction it would be the same as it is in normal conditions? In normal conditions isn't ventilation and perfusion both greater at the base than apex? 
By the way, the correct answer is G. Just don't understand why.
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| wyb
Forum Newbie
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My understanding is:
A is false, because V/Q ratio at the base is similar as that at the apex, although blood flow and ventilation are greatest at the base and decrease toward the apex.
B is false, because ventilation increase more than cardiac output during the exercise, so that V/Q ratio is greater than 0.8.
C is false, because the value of ventilation iss almost zero in a complete airway obstruction, so that V/Q approaches zero.
D is false, because the value of perfusion is almost zero in hypoxic vasoconstricted areas, so that V/Q approaches infinity.
E is false, usually PaO2 and PaCO2 of the base vs apex are similar because their V/Q ratios are similar.
F is true or false. I dont know whether the answer should be matched to the situation above or not. Usually, V/Q at the apex approaches normal value 0.8 or sometimes >0.8 if under the situation of hypoxic vsoconstriction, yet wasted ventilation. Based on the situation above, F seems correct.
G is true. I think that it is associated with the change of intrapleural pressure during inspiration at the base, and the alveoli at the base is more compliant than those at the apex.
For your first question, absolutely not the same case .
For your second question, yes, both ventilation and perfusion at the base are greater than at the apex in the normal condition. But V/Q ratio at the base vs at the apex is similar.
Please check out and correct them for me. Thanks!
Edited by wyb on 03/19/08 - 08:51 AM
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| Tiff
Forum Guru
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Thx for the explanation. It seems clear why all the others are incorrect. I'm still though trying to understand the mechanism behind why ventilation is greatest at the base of the lungs during both airway obstruction and during normal conditions.
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| wyb
Forum Newbie
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Tiff, you are welcome.
I think that you are right, it is impossible for the ventilation existing whether at the base or at apex in the condition of airway obstruction. However, the correct answer you post is G. In my opinion, the question is asking about the normal physiological condition, which means that this question is not related to that scenario. Yes, I feel alittle confused too.
Happy studying! 
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| yarab99
Forum Senior
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hi.... wyb
just wanted to correct something u said...and plz correct me if im wrong : A-V/Q ratio in the base is NOT similar to that in the apex ... V/Q in the apex is > base cuz there is wasted ventilation ( not perfusion which makes F wrong)
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| sidiyaa
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| Dr_C
Forum Newbie
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Well Tiff, i would say in an obstruction u will have a low Po2 and a high Pco2 thus a low Ph therofore we will have a low VA/Q ratio, thus Blood flow will be greater than ventilation but at the apex blood flow will be lower by the effect of gravity for the present ventilation even in the obstruction. That's the way I see it. Hope that will help.
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