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Old 06-20-2011
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Lungs Pumonary Vascular Resistance and Vagal stimulation!

Bronchi are richly innervated by autonomic system. Which of the following parameters would increase most as a result of vagal stimulation?

A.) Work of breathing

B.) Pulmonary vascular resistance

The answer is A.)

But I choose B. ) Because my rationale is that if you vasoconstrict the airway, so you decrease the oxygen supply in the lung, thus leads to pulmonary vasoconstriction?? We constirct the airway, and this does not change the length of the airway, so Work should remain the same??
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Old 06-20-2011
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Default I would chose A)

the reason is the hypoxic vasoconstriction is a relative one in the parenchyma to shunt the blood to the relatively well ventilated region, there isn't a logic the whole lungs starts to inadequately perfused in a patient whose ventilation is already not sufficient due to bronchoconstriction, instead a patient with increased resistance to bronchus would have increase work of breathing to meet the ventilation demands..

correct me if i am wrong :- thanks in advance !!
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Old 06-20-2011
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But regardless of inadequately perfused or not, low oxygen really increases pulmonary arterial resistance, isn't it? According to physics, if we want to increase Work, then we must increase the distance, but now we have the same length of airway, so Work should remain the same? Or this question is just a flaw?? Please correct me if I am wrong, thank you : )
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Old 06-20-2011
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Or you can increase the force required. After all, work is the dot product of force and displacement. Bronchoconstriction increases airway resistance dramatically (same logic as blood vessels).
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Old 06-20-2011
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Work is defined as Force x Distance so its possible to increase the work done by changing the force even if the distance has not been changed. However, in this cause, the distance could possibly be changed with vagal stimulation. We know that vagal stimulation would likely increase mucous secretion and bronchoconstriction. Both of which cause some sort of obstruction in breathing. This decreases the diameter of the conducting airways and increases the FORCE needed to pull in the same amount of air. To increase this force, we are likely to use some of our extra-diaghpragmatic respiratory muscles to accomplish this, and likely also expand our chest cavity even more than usual to get the same amount of air into the lungs. So there would most likely be an increase in work done during breathing with vagal stimulation.

However, it COULD be possible that this difficulty breathing COULD cause hypoxia and cause a reflex vasoconstriction of the blood vessels but this would be assuming more information than given in the question and it doesn't say anything other than isolated vagal stimulation.

Although hypoxia COULD occur and there could be an increase in TPR in the vessels of the lungs, it is much more likely that work would increase FIRST and preclude this increase in TPR in the lungs. So A is my choice. Hope that helps! Correct me if i'm wrong
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Old 06-20-2011
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i'd go with A too. Vagus is under parasympathetic influence which releases ach and causes bronchoconstiction which increases the work
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