Can someone explain Chronic AV fistula affect on cardiac function curve?

[chillaxbro]

Uworld question 1625

Based on the changes on this graph, what happened?

Uworld's explanation of the changes

· there is increase in cardiac output because of the increased height of the cardiac function curve (I can see this)

· there is a decrease in total peripheral resistance because of the increased slope of both the cardiac function curve and venous return curve (why does slope = TPR?)

· there is increase in mean systemic pressure because of the rightward shift of venous return along the x-axis (Google tells me that mean systemic pressure is where VR curve crosses X axis. Why is this so?)

Answer: These 3 changes are seen in chronic AV fistulas. What is AV fistula? Is this like in dialysis? Why do these changes occur?

Thanks!

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[worldbeater]

This is good question, I just memorized the diagram after I missed it and went on with it, bad habit. So let's break it down:

. What is AV fistula? Is this like in dialysis? Why do these changes occur?

You are correct, this is occurs during dialysis, specifically in the elbow. Other arteriovenous malformations you can categorize in this list is a PDA, in the brain (Von-Hippel Lindau) and in the lungs (Osler-Weber-Rendu)

I'll try to simplify the logic of what UWorld is saying from what I think is correct, I don't know the answers for your specific questions. Hopefully this will help. In an AV fistula, you are in a "low volume state", meaning your body thinks your losing blood.

When viewing these type of graphs, we should follow the venous return first, and then view the cardiac output. So we see the venous return has shifted to the right and the cardiac output has increased. Why would the venous return increase, when the body is losing blood?

Your body has entered a sympathetic state to maintain perfusion. 60% of your blood is pooled in your veins and you vasoconstrict (alpha 1 receptors in the veins I think) to bring it up to the heart. So there is your shift along the x-axis to the right for the venous return portion (although they list venous return on the y which makes it confusing).

So we are in a sympathetic state, so your B1 receptors in the heart increase as well. That leads to your cardiac output (stroke volume x heart rate) increasing, increasing your shift of your y-axis.
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For the specifics:

· there is a decrease in total peripheral resistance because of the increased slope of both the cardiac function curve and venous return curve (why does slope = TPR?)

I don't know the answer to this, but they are talking about a decreased TPR in an acute state, this graph is for a chronic state (increased TPR)

· there is increase in mean systemic pressure because of the rightward shift of venous return along the x-axis (Google tells me that mean systemic pressure is where VR curve crosses X axis. Why is this so?)

Alpha 1 constriction of your veins to bring up that 60% of pooled blood to the heart, I don't know the specific answer of venous return crossing the x-axis

 

[chillaxbro]

My current understanding: AV fistula = there is an abnormal connection from arterial side to venous side so blood goes directly from left to right side. As a result your body thinks you have low volume. To fix it, you increase sympathetics -> veno and vaso constriction -> increased CO and VR and TPR

they are talking about a decreased TPR in an acute state, this graph is for a chronic state (increased TPR)

So I agree chronic TPR should be increased because of increased vaso/venoconstriction from sympathetics. The full explanation says decreased TPR for chronic, then later it says decreased TPR also for acute. I'm guessing the first part should say increased and not decreased (they made a typo?)

 

[worldbeater]

The full explanation says decreased TPR. Is that a typo?

Yeah I saw that, they are just showing the progress of what happens acutely in a AV fistula (they have a graph that's different you can click on the hyperlink). That's your decreased TPR or "low volume state" initially.

Then it progress to a chronic state, which is the current question is about. This is where the body compensates and does all the stuff to maximize cardiac output as you outlined above. That's your increased TPR.

They should have made the explanation between acute and chronic into two paragraphs so it's easier for the student to read.

 

[TUVIX]

If cardiac output is zero then then aortic pressure falls to the x intercept. If there is zero CO then there is zero VR. Does that make sense? In other words if mean systemic pressure is the same as RAP then there would be no VR. When RAP is lower than MSP then blood flows into the right side of the heart and you have a corresponding CO. Another way of saying it is the x-intercept is the RAP at which there is no CO.


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