co2 hco3 graph

[Tummy]

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Could someone pls break down for me how to read such a graph? Something like this was just presented to me on UWorld (question #2072). Where do I find the normal values and how do I get the values of the patient?

I usually don't have problems when I have the values but such graphs just make me pass out. How is it even possible to have two different variables on one axis?

We didn't get graphs like that in our school and neither are they in my (german) physiology book. The Davenport diagram in Kaplan notes also looks somehow different (and quite easily understandable).

I only need to know how to get to the values, don't need acid-base basics. Sorry for the poor drawing, didn't want to do any copyright infringement...

 

[MilkmanAl]

That just looks like Winter's equation to me (i.e., CO2 and bicarb scale linearly). The weird curvature is because pH is logarithmic.

 

[Tummy]

Thx alot for your fast answer.

However, I understand what Winter's formula is about, but still don't get the graph.

More specifically, where do I see the pH, HCO3 and PaCO2 values of the patient (in respect to normal values)? They gotta be somewhere in there... How do I see if a value of them is increased or decreased?

Sorry for being such a *******, but I just don't get it...

 

[MilkmanAl]

Just judging by that graph you posted (my UW subscription lapsed last summer, thankfully), it's purely a conceptual question. As long as you understand that bicarb and CO2 will both change in the same direction with changes in pH, you're good. Are the normal values not in UW?

edit: Oh, maybe you're supposed to Henderson-Hasselbach it or something. That's the only way I can think of getting the pH if you have one of the values but not the others or vice versa. If they don't give you anything, I'm going with the above and saying it's a conceptual problem.

 

[pathtopenn]

Hi!

The graph you posted is actually a davenport diaphragm containing only 1 isobar. It is there to show how the different variation of the relationship of the acid and base during different situations.

We always want the line in the middle the isobar to be at the ideal 7.4. So when we vary one of the three variables we will be trying to come back to the 7.4.

So if any value is given you can use that to trace it on the Davenport to see where it lie. For example, if they give you CO2 and tell you that Bicarb is lost due to diarrhea, then you can trace it on the CO2 given and find match it to the Bicarb value to get the pH in question.

Or for example, if pH is given and they tell u the pt is vomiting, and they give the value of the CO2 you can calculate the bicarb value.

 

[Tummy]

Thanks, but there is no value given in this question (at least I can't see it). They just give this diagram (and new-onset confusion & lethargy), show this diagram and then ask about the acid-base status of the patient.

Ok, I finally got it. In the explanation of a distractor, they come up with the fact that these two lines in the middle are actually forming an arrow. So everything's clear (pH down, hco3 down, co2 down). I think they could have made this a little more obvious, but so be it.

Thanks again and sorry for bothering you 😉