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How to understand this concept

Discussion in 'Dental' started by rsweeney, Apr 4, 2004.

  1. rsweeney

    rsweeney Senior Member
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    In aortic stenosis, you get a decrease in stroke volume and cardiac output, thus you eject less blood and thus have a larger end systolic volume [increased residual volume]. But WHY does end diastolic volume and pressure increase? According to texts, the NORMAL venous return adds to the increased residual volume thus increasing preload or end diastolic volume and pressure. But why is this? If you eject less then shouldn't that smaller stroke volume return to the heart thus maintaining end diastolic volume? Since the circulatory system is a closed system, then cardiac output has to equal venous return. Thus, if you have a decreased cardiac output, which you do with aortic stenoisis, then you should have a reduce venous return, thus maintaining your end diastlic volume and pressure [preload].

    So, when texts say that the normal venous return adds to the increased residual volume causing an increase in end diastolic pressure and volume [preload] isn't that wrong? Shouldn't the end diastolic pressure and volume [preload] stay the same since the previous stroke volume was less?

    Following is the link which shows the PV-loop for aortic senosis for your reference:
    http://www.oucom.ohiou.edu/cvphysiology/HD009.htm
     
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  3. ItsGavinC

    Dentist Moderator Emeritus 15+ Year Member

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    Man, you're really enjoying your physio aren't you?

    I have an answer for this but can't post it tonight. Maybe tomorrow.
     
  4. Midoc

    Midoc Senior Member
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    The difficulty here lies in that you are treating a chronic disease as if it were an off/on condition. Aortic stenosis would build up slowly so you could possibly say that the secondary effects (increased preload) happen at the same time as the primary effect (increased afterload). You would have to ask your professor on this one but I'm inclined to think that he will have you assume that both are happening at the same time.
     
  5. rsweeney

    rsweeney Senior Member
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    I feel like a dope--end diastolic volume and pressure do increase, even in the absence of compensatory mechanisms.

    Since the text in the link says,

    "These changes just described do not include cardiac and systemic compensatory mechanisms that attempt to maintain cardiac output and arterial pressure."

    there has to be another reason end diastolic volume and pressure are up, and here it is. It really helped once I drew a picture of the heart as a closed pump.

    Imagine the pump is turned off until all blood inside is at a stand-still. Now, at stand-still, let's say hypothetically you have 5L in the right ventricle, 5L in the left ventricle, and nothing in the veins or arteries. Now you decide to turn the pump on. Since there is nothing prohibiting ejection from the right heart, a normal 3L will be pumped out and 2L will stay in [residual volume]. But, the aortic stenois in the left heart will only permit 2L to be ejected and 3L remains [residual volume]. Therefore, at the end of the first right- left heart cycle after the pump was turned on, the normal 3L that was ejected from the right will build up on the abnormal 3L that remained in the left heart, thus you will have 6L as your end diasotlic volume. Also, the 2L that was ejected from the left heart will add to the 2L that remained in the right heart leaving the right heart with an end diastolic volume of 4L.

    Then, since the left heart now has more blood in it than before [6L], it will eject say 3L instead of 2L thus bringing the end diastolic volume in the right heart back to normal--to 5L. Also, since the right heart had only 4L in it, it only ejected 2L [b/c end systolic volume won't change due to the Frank-Starling relationship and because nothing is wrong with the right heart], thus that 2L added to the 3L that remained in the left heart bringing its end diastolic volume back to 5L.

    Then, I believe, that whole cycle will start all over again. I would imagine that the end diastolic volume does not increase indefinatlely because of my hypothetical situation [right heat ejects more and left heart ejects less then right heart ejects less and left hear ejects more--Since the CVS is a closed system that is. Even with aortic stenosis, a higher end diastolic volume will still result in a greater stroke volume --right?

    -Thanks everyone!

    -Richard
     
  6. ItsGavinC

    Dentist Moderator Emeritus 15+ Year Member

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    Hey Dan, one of your professors from UDM is teaching our Oral Soft Tissues course here at ASDOH.
     
  7. Midoc

    Midoc Senior Member
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    That's cool, who is it? Do the guest lecturers give their own exams? If so I can probably get you some good examples.
     

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