V/Q mismatch and Hypoxemia in PE

Aclamity

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    maybe this is a n00b question, but I can't seem to understand why a perfusion defect (like a small PE) would lead to hypoxemia. If blood can't get into some ventilated alveoli, wouldn't it just reroute to other alveoli and get oxygenated there?

    Yup, told you it was a n00b question :D
     

    ijn

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      The fact the blood does reroute is why you can correct hypoxemia with pulmonary embolism. If it were a diffusion defect, like say in ARDS, then you couldn't correct the hypoxemia with O2 adminstration.

      I doubt you'd get much hypoxemia with a small PE. With a large PE you get hypoxemia because you can't reroute all of the blood so you get shunting. The low PaO2 causes you to hyperventilate, thus lowering the PaCO2 and causing respiratory alkalosis. You can't completely compensate for a low PaO2 in a large PE without administering high FiO2.
       

      Aclamity

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        The fact the blood does reroute is why you can correct hypoxemia with pulmonary embolism. If it were a diffusion defect, like say in ARDS, then you couldn't correct the hypoxemia with O2 adminstration.

        I doubt you'd get much hypoxemia with a small PE. With a large PE you get hypoxemia because you can't reroute all of the blood so you get shunting. The low PaO2 causes you to hyperventilate, thus lowering the PaCO2 and causing respiratory alkalosis. You can't completely compensate for a low PaO2 in a large PE without administering high FiO2.

        also PE is painful that's why patient begins to hyperventilate taking shallow and frequent breaths

        Great, thanks that's what I thought. So basically large PE -> V/Q mismatch -> hypoxemia + pain -> hyperventilation -> resp alkalosis

        And with a small PE you are able to effectively reroute the blood and avoid hypoxemia

        Thank you!
         
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        johndoe3344

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          Great, thanks that's what I thought. So basically large PE -> V/Q mismatch -> hypoxemia + pain -> hyperventilation -> resp alkalosis

          And with a small PE you are able to effectively reroute the blood and avoid hypoxemia

          Thank you!

          Yup. That's why small PE's are often asymptomatic.
           

          Morsetlis

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            maybe this is a n00b question, but I can't seem to understand why a perfusion defect (like a small PE) would lead to hypoxemia. If blood can't get into some ventilated alveoli, wouldn't it just reroute to other alveoli and get oxygenated there?

            Yup, told you it was a n00b question :D

            Another answer is that areas of high V/Q can't sufficiently oversaturate Hb to compensate for the drop in O2 from areas of low V/Q ratios...

            But yes the first response is hypoxic vascular contraction.
             
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