aahy1978
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Diffusion Impairment vs. Pulmonary (right to left) Shunt
both of them seem to have an A-a gradient...
is the only difference b/w these two is that the A-a Gradient of the Pulmonary Shunt will not be fixed by supplemental Oxygen, while the Diffusion Impairment will be fixed.
are there any other clinical differences that i should be aware of b/w the two... ?
thnx D
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| mjl1717
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Ficks law gas diffusing thru a tissue slice)
V.gas=A/TxDx(P1-P2)
A anT are area and thickness
D is diffusion constant
P1 and P2 are partial pressures on both sides of the tissue
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| aahy1978
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hey mjl1717,
first of all, lemme start off by saying that i've seen many of ur posts thru out this forum and ur name DOES bring some credibility to what u have said... so, i am delighted u took interest in this q.
Now, this is what i understand from this so far:
DIFFUSION IMPAIREMENT: this could be caused by fibrosis and that would increase thickness and then therefore reduce the rate of diffusion. Basically, anything that would decrease the rate of diffusion would cause this impairement (i think)... which would cause less oxygen to be diffuse into the pulmonary blood vessel... and this would cause a greater A-a gradient then normal (i think normal is about 5-10)
PULMONARY SHUNT: this is basically bypassing the alveolar exchange totatlly... so there is no oppurtunity for the exchange... and this would also cause an abnormal A-a gradient
So, if u give the Diffusion Impairement person Oxygen, then this would improve the A-a gradient, however, if u give oxygen to the Pulm. Shunt individual, then there would be no significant change.
So, theoretically, will the pateint be presenting any unique symptoms to distinguish them--or is the only way to really distinguish them is by giving oxygen to see if anything is improving or not.
am i totally off... or is there a better way to look at this... maybe it is just theoretical... thnx for reading and would appreciate any comments... thnx 
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| ssrpk
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well i agree with aahy ....so true about mjl
anyway this topic is really confusing and here is how i understand it.!
it's true wht u said abt diffusin impairment.....if we adminitsre supplemental oxygen then it will widen the A-a graqdient further but because of increasing gradient there occurs a point when this gradient will overcome the diffusin impairment and arterial PO2 will get back to normal......
in case of pulmonary shunt....increasing the alveolar PO2 by supplemental oxygen will only increase the end capillary PO2 ....however this increase will occur only in the prtial pressure ,not in the total content as 100mmHg contains only 0.3 vol % which is fairly low....so when this blood in end capillary with high PO2 meets with the large amount of shunted blood with very low total oxygen content & PO2 ......increase in PO2 does'nt equivocally make up for the deficit of oxygn content.....and sys.arterial PO2 does not increse back to normal!
oxygen supplementaion prvides a guud preliminary assessment b/w shunt and diffusion impairment
hope it helps!
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| emhava
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I'm reviving this topic cause I have the same question like aahy1978.
Besides what about PaCO2 in both conditions???
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