Obstructive Lung Disease Hypoxemia and Dyspnea sequence - USMLE Forums
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Old 04-08-2011
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Lungs Obstructive Lung Disease Hypoxemia and Dyspnea sequence

Can someone please explain why does Chronic Bronchitis have early-onset hypoxemia and late-onset dyspnea and vice versa with Emphysema?
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Old 04-08-2011
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emphysema is destruction of alvoelar wall septa...two or more alveoli may coalese to form one...now when the air enters during inspiration, it fills up this enlarged alveoli. due to pressure situations within the alveoli, the terminal bronchiole gets plugged and this air cannot escape. now when the patient tries to inspire, the plugged in bronchiole is opened due to the pressure but this new air cannot get into the alveoli due to the air from thr previous breath.thus with every breath patient gets more breathless.
Try this...inspire maximally and dont exhale. now try to inspire again!! can't do it?? this is exactly the case with emphysematous patients leading to dysnea..
chronic bronchitis is inflammation of airways. in the early stages there is not enough hypoxemia to cause dypnea....
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Old 05-02-2012
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To understand this you need to get some concepts straight:

Hypoxemia (dec in Po2 , and ass with cyanosi), is not the same as Hypoxia (dec O2 delivery to the tissues, including chemoreceptors and hence dyspnea)

now V/Q MISMATCH will cause hypoxemia, but if the Q is still adequate there will be no tissue hypoxia and henca dyspnea.

in emphysema, there is parallel destruction of alveolar wall and capillaries so there is tissue hypoxia ( hence early dyspnea) but no V/Q mismatch and hence no early cyanosis. The opposite is true with Chronic bronchitis : there is a VQ mismatch bec excess mucus causes plugging of bronchioles and dead space, so ....cyanosis, however, there is late onset dyspnea because hypoxia does not occur except when hypoxemia is too low.

This is from FA 2011 p 508, with some Goljian explanations from audio. Please correct me if i correlated something wrong
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Old 05-02-2012
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Originally Posted by donofitaly View Post
Can someone please explain why does Chronic Bronchitis have early-onset hypoxemia and late-onset dyspnea and vice versa with Emphysema?
In emphysema hyperinflated lungs limit the air entry as previously mentioned (dyspnea) but theres no CO2 retention that is why hypoxemia develops late in the disease

In chronic bronchitis hyperinflation is not as severe as in emphysema and does not compromise air entry but the CO2 retention causes alveolar PAO2 to fall causing hypoxemia (pao2=pio2-paco2/R)
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Pathology-, Respiratory-

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