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Physiology question

Discussion in 'Step I' started by rsweeney, Mar 27, 2004.

  1. rsweeney

    rsweeney Senior Member
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    How would decreased venous compliance affect the function of the cardiac cycle in terms of preload, afterload, stroke volume, cardiac output, etc.---especially afterload. What would the PV-loop look like compared to the control loop?
     
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  3. Idiopathic

    Idiopathic Newly Minted
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    Theoretically, decreased venous compliance would initially cause an increase in preload. I would imagine that before you would get a noticeable change in afterload, you would see edema from the increased pressure on the capillaries. Since nitrates dilate veins and decrease venous return, I would imagine that increased venous compliance would have the opposite effect, but I dont really know of a situation where this would happen.
     
  4. Idiopathic

    Idiopathic Newly Minted
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    And I realize now that my post couldnt possibly have been any help.
     
  5. Dr. J?

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    Decreasing venous compliance would increase preload to the heart; which would increase SV d/t the Frank-Starling mechanism; increasing SV would increase CO (b/c CO=SV * HR); increasing CO with no change in TPR (remember arterial side vascular smooth muscle tone is the main determinant of TPR) would eventually increase afterload. Of course, there would be compensation by the baroreceptors which would slow the HR and bring the CO back down to a steady-state which would be slightly above the original steady-state d/t the increased blood return to the heart (b/c of the decrease in venous compliance.

    Hope this helps.
     
  6. doc05

    doc05 2K Member
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    do your own homework.
     
  7. rsweeney

    rsweeney Senior Member
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    Doc05,
    Do not presume that I did not do my "homework" before and just because I posted this question on SDN. I am actually doing voluntary "homework" by breezing through my undergrad physiology notes, going to the library to research my weak points, and then I post on SDN what I absolutely cannot find through other outside sources. Plus, I have not started school yet--I start in August--and I have already graduated from undergrad, so actual homework does not apply yet. I normally don't take things personally, but doing my own "homework" is what I am best at, and I work hard at it.
    :mad:

    Nevertheless, this forum is MEANT for members to help each other with difficult concepts.

    There is no need for you to take offense to this post, as I am just acting on principal.

    -Richard
     
  8. jed2023

    jed2023 Senior Member
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    Decreasing venous compliance shifts blood from the venous compartment to the arterial compartment, raising blood volume in the arterial tree. This directly increases end diastolic volume, increases pre-load, and increases stroke volume (via the Frank-Starling mechanism, as pointed out earlier). You can consult any phys book to see what the curve for increased preload looks like.

    You would expect a transient increase in afterload (reflected in increased blood pressure), but I think it would be corrected too quickly by a decrease in total peripheral resistance to have much of an effect.

    For a test question, I'd probably answer "increase in preload and stroke volume with no change in afterload." I'd only add in "change in afterload" if the question specifically said "immediately afterwards" or if my professor was a hard-baller.
     
  9. rsweeney

    rsweeney Senior Member
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    !I'd only add in "change in afterload" if the question specifically said "immediately afterwards" or if my professor was a hard-baller.

    Those darn baroreceptors! When you say "immediately afterwards" I would imagine you mean on the second heart beat--or the compensatory effect. Here is my take from what you suggested:

    A decrease in venous compliance will shunt all the blood from the veins and into the heart. Thus:

    First heart beat:
    Venous compliance decreases-->More blood in heart --> increase preload-->increase stroke volume for the first beat-->increased cardiac output directly after first beat [yet before the second beat].

    Second heart beat:
    Increased cardiac output after first beat-->increased arterial pressure after first beat-->increased afterload [ASSUMING NO CORRECTION]-->thus on the second beat the stroke volume will decrease [due to the increase in afterload which resulted from the high cardiac output and arterial pressure from the first beat]-->cardiac output will decrease

    Third heart beat:
    Since the stroke volume was decreased on the second beat [due to the increased afterload] then end diastolic volume [preload] will be up again before the third beat. And this cycle will continue over and over again until equilibrium is reached.

    Is this right?

    I think I got it!

    Thank you for your help!!!
     
  10. Dr. J?

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    RSweeney,

    Yup, that's pretty much it.

    Don't sweat the "do your own homework comments". Sometimes it just doesn't click. Grad school is all about efficiency (esp med school). Sometimes it's more efficient to get another perspective when you come to a seemingly intractable prob.

    Work smarter, not harder.
     
  11. rsweeney

    rsweeney Senior Member
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    Thank you for your understanding Dr. J---that's very kind of you! I appreciate your help, as I believe I benefited from it. It's just confusing when you don't know whether the PV-loop represents the primary effect or the steady state effect of the disorder. The devil is in the details for sure. Thank you very much Dr. J!!

    -Richard
     

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