ForHumors

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Jul 8, 2013
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  1. Medical Student
    Hey guys, I have something that I can't seem to find well-explained. Venous return and cardiac output curves use right atrial pressure on the X-axis. Costanzo says that it is used as a proxy for end diastolic volume but does not explain why right atrial pressure can be used instead in this instance. BRS Costanzo, firecracker/FA, internet don't explain it either. I had a TA say that it can be used because the pressure difference between the right atrium and the ventricle is the driving force for blood to fill the ventricle, but I want to get confirmation of this from a more reliable resource. Has anyone heard this explanation/read it before? Thanks!
     

    DrPicard

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    Sep 22, 2015
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    1. Medical Student
      Pressure and volume get reflected backwards. eg if the RV overloads with fluid, this increased volume and pressure get reflected backwards since the RA is connected to the RV, and the RA will also suffer a rise in volume and pressure. Since the entire circulatory system is connected, pressure and volume won't increase in an isolated fashion in a specific area (unless theres some local pathology).

      So for EDV, ideally, we should measure the EDV of the ventricle. But we can used "indices" in place of the EDV, which afford us a good estimate. eg, we could use the EDP of the ventricle. Volume and pressure generally increase or decrease together, so you can see why this idea works. Or, since pressure from the ventricle is going to be reflected back into the atrium, we could use the the EDV or the EDP of the atrium. Going even farther back, a rise in JVP is also indicative of rising pressures/volumes.

      The idea is:
      1. The farther back you go from the ventricle, the less accurate the estimate becomes
      2. Getting a reading at a point before the ventricles is clinically easier, which is why its done

      This is why clinically you can use CVP to estimate right heart pressures and PCWP to estimate left heart pressures. They might not be 100% accurate, but the estimate they afford us is good enough to give us the information we need.
       
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