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  #1  
Old 08-19-2014
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Default pulmo physio question

A normal volunteer during an experiment takes a 500cc tidal volume breath starting at FRC, then after rest he takes a 500 cc tidal volume breath starting two liters above FRC.
Compared to breathing at FRC, breathing above FRC will be associated with an increase in all of the following except?

a)airway diameter
b)lung compliance
c)work of breathing
d)pulm vascular resistance


please explain
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  #2  
Old 08-30-2014
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Quote:
Originally Posted by jeniaz View Post
A normal volunteer during an experiment takes a 500cc tidal volume breath starting at FRC, then after rest he takes a 500 cc tidal volume breath starting two liters above FRC.
Compared to breathing at FRC, breathing above FRC will be associated with an increase in all of the following except?

a)airway diameter
b)lung compliance
c)work of breathing
d)pulm vascular resistance


please explain
A?
well pulm vasc resis should increase with increased air in alveoli exerting more pressure on the vessels.
work of breathing should also increase i guess,
Lung compliance will also increase in the sense that there would be more air in the lungs compared to at FRC
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Old 08-31-2014
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I think D is correct. Pulmonary vascular resistance will be decreased as intrapleural pressure become more negative. Compliance us defined as deltaV\deltaP, so in case where volume is increased compliance and airway diameter would also be increased. Work of breath should be increased to forse more air into chest.
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Old 08-31-2014
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Quote:
Originally Posted by jeniaz View Post
A normal volunteer during an experiment takes a 500cc tidal volume breath starting at FRC, then after rest he takes a 500 cc tidal volume breath starting two liters above FRC.
Compared to breathing at FRC, breathing above FRC will be associated with an increase in all of the following except?

a)airway diameter
b)lung compliance
c)work of breathing
d)pulm vascular resistance


please explain
I think the answer is: B (lung compliance) which will actually decrease at high lung volumes
I'm very bad at explaining a concept to a colleague, but I'll try it.
If you look closely to the Lung compliance curve, you would notice that that curve will be more flat (more horizontal) at high lung volume. Since compliance is the slope of the curve and in this case the slop is decreasing, so the compliance will decrease as well. If you don't understand my explanation (and of course it's not your problem), please let me know so I could explain it in another way. That of course if my answer is correct after all.
Have a nice day
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Old 08-31-2014
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Quote:
Originally Posted by jeniaz View Post
A normal volunteer during an experiment takes a 500cc tidal volume breath starting at FRC, then after rest he takes a 500 cc tidal volume breath starting two liters above FRC.
Compared to breathing at FRC, breathing above FRC will be associated with an increase in all of the following except?

a)airway diameter
b)lung compliance
c)work of breathing
d)pulm vascular resistance


please explain
It's also worth mentioning why other answers (A, C, D) are incorrect:
A) Airway diameter increases at high lung volumes, which is called "Radial traction". That's actually why patients with high airway resistance (e.g., asthma) learn to breathe at higher lung volumes to decrease their airway resistance (remember that airway resistance is inversely proportional to airway diameter).

C) The work of breathing increases at high lung volumes. Although airway resistance is low at high lung volumes, the lung compliance is at its lowest . That's because the high lung volume would increases the lung elastic recoil. So, the lung tries to collapse (remember that the natural tendency of the lung is to collapse) and the patient would have to exert more work to get more air inside his lung.

D) Pulmonary vascular resistance increases at high lung volumes. That's because inflated alveoli will stretch pulmonary capillaries longitudinally which decreases their diameter and thus increases pulmonary vascular resistance (also remember that vascular resistance is inversely proportional to vessel diameter)
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Old 08-31-2014
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dr awaheed
Your explanation makes lot of sense...
B seems to be the right answer then.....
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Old 09-01-2014
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the correct answer is lung compliance (b)

and i can easily get why other answers are not right, but i`m still missing the concept about why lung compliance is correct

dr_awaheed thanks for trying to explain, but i didn`t quite get it, so if you have another explanation it would be cool to see it

i thought that the more volume you get -> the more you stretch the lung -> the more compliant it gets (there`s a formula for that: compliance=ΔV/ΔP)
But i guess ΔV doesn`t really change in the question, `cause in both cases we changing volume for 500cc

Anyways i`m still messing up with compliance
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Old 09-01-2014
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as i reread your explanation dr_awaheed , i probably started to understand what you meant...
you saying that compliant curve becoming more flat at high lung volume, so this means that when we go to high lung volumes it`ll take a greater pressure to increase volume=you need a bigger pressure to get the same change in volume
(so in this question change in volume was given=500cc, but as we breathing above FRC we need greater pressure to make the same change in volume...now we can use the formula compliance= ΔV/ΔP, ΔV-same ΔP-increased => compliance decreased)

and it`s the same as we trying to blow up a balloon it`s easy to get the air inside in the begging(not taking the starting point), but the more air it`s already inside the ballon, the harder we need to blow to get some more air in

am i right?
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