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Pressure-volume loops and decreased venous compliance

Discussion in 'Medical Students - MD' started by rsweeney, Mar 27, 2004.

  1. rsweeney

    rsweeney Senior Member
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    When analyzing the pressure-volume loop for decreased venous compliance, would afterload be increased or decreased? Here is where I am at mentally. Decreased venous compliance-->decreased pooling-->greater amount of blood that returns to the heart--> increased filling-->increased preload-->frank-starling mech. activated so contractility increases thus you have an inc. in stroke volume-->increased cardiac output--> higher areterial pressure thus increased afterload.

    BUT: When you analyze the pressure-volume loop for decreased venous compliance the end diastolic volume is increased [as it should be] and end systolic volume is decreased [as it should be due to inc. cardiac output]---but according to my reasoning afterload should be increased due to inc. cardiac output.

    I thought that on a pressure-volume loop diagram afterload-increases are represented by a shift to the RIGHT along the end-systolic volume-pressure (ESVP)curve. But the actual PV-loop diagram for decreased venous compliance has a shift to the LEFT down the ESVP curve. What's going on or what am I missing??

    So an increase to the right or left along the ESVP curve won't necessarily tell you whether you have an increase or decrease in afterload? Is this true?
    -Thank you for any help

    -Richard
     
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  3. CYP2E1

    CYP2E1 An Enzyme
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    Hi Richard

    Background:

    Most of your reasoning is good. However here are a few issues to consider.

    With regard to the arterial system:
    It is important to make sure that the ?venous compliance? you are referring to is the venous capacitance vessels not the ?vascular compliance? (i.e. arterioles) now remember that the balance to the increased arterial blood volume is the elastic nature of the arterial tree. Remember (change in pressure) dPa/dt = Change in flow (read increase in volume) / Ca (arterial compliance). Therefore increased CO is absorbed by the compliant vessel walls. In young people (20 -24) a 200% increase in volume increases the pressure by only 25-40%. As you know the rate of arterial pressure increase (NOT magnitude) is what is changed with varying Ca.

    With regard to the venous system:
    "permanent" relocation of blood from the venous to the arterial side does increase the arterial pressure about 19 times that of the corresponding drop in venous pressure in rigid tubing however there is an important regulatory mechanism, the baroreceptor reflex alters arterial resistance and hence pressure (CO*TPR) is maintained (not increased)

    The basic problem with your reasoning is that you have assumed that increase CO will increase afterload, this is not the case as the body regulates the peripheral resistance to match.

    ?I thought that on a pressure-volume loop diagram afterload-increases are represented by a shift to the RIGHT along the end-systolic volume-pressure (ESVP)curve?

    No ? Increases in afterload manifest as increased ESV and decreased SV (look up ESPVR in Lilly)

    ?So an increase to the right or left along the ESVP curve won't necessarily tell you whether you have an increase or decrease in afterload? Is this true??

    No- movement to the L or R of the ESPVR line correlates well with increases and decreases in afterload.
     
  4. rsweeney

    rsweeney Senior Member
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    CYP2E1,
    Wow, thank you very much for that detailed answer! Your time is very much appreciated, as is the time of others who have taken the time to help me out in other threads!

    -Much obliged CYP2E1!

    -Richard
     

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