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Measuring size of a body cavity

  1. Nov 27, 2013 #1
    I have to measure the size of a cavity of a animal body(s). What is more important I need to measure it on several bodies and compare the results. The cavity is small and contains liquid. That liquid can be extracted and that sets the volume of cavity to 0.

    It's important to note that the tissue surrounding the cavity is elastic. It streches as liquid (e.g. water) is being injected.

    I need to do one of the following:

    Inject water into the cavity until the tissue surrounding it stretches to a certain point, kept constant for all the bodies, measure the injected volume and calcualte from there.

    OR

    Inject a constant volume of water into cavities and then measure how much the tissue strecthed for each one.

    Also important thing to note that other structures may be pressing to the elastic tissue with force that is not constant for every specimen. Would that affect the results; should I extract the tissue surrounding the cavity and measure it in specific conditions?
     
    Last edited: Nov 27, 2013
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  3. Nov 27, 2013 #2

    mfb

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    Both look like reasonable approaches. Which one is better will depend on biology and the purpose of the measurement.
    An alternative is a measurement of the volume<->pressure-relation at several points.

    It can influence the results.
     
  4. Nov 27, 2013 #3
    I know both are valid approaches, however, I'm interested in the techincal aspect of it. It's easy to measure the volume I inject into the tissue, but how do I measure how much did the tissue strecth?

    So the tissue we are talking about is pericardial sack. It wraps around the heart and attaches itself where great blood vessels enter the heart. The pericardial cavity (cavity I'm talking about), is the space between the heart and the sack, and does not communicate with any other space (it's completly enclosed).

    Now, on one side we have a hollow heart, and on the other, thoracic organs that are all stacked around the pericardial sack.

    Would cutting out the heart along with the pericardial sack and placing it into a water container (the water would then fill the heart and the area around the pericardial sack) solve the problem? We would have the same water inside the heart and around the sack.

    Also, how would I do it to get consistent results? What should I keep constant? Water volume, should it be related to the volume of the actual heart? Or should I watch for something else? Would the position of the heart affect the results?


    My final goal is to get volumes of pericardial sacks that would be comparable to each other (by keeping any other aspects that would influence the measurement constant).
     
  5. Nov 27, 2013 #4

    mfb

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    I thought you planned to measure pressure.
    If you are interested in the total volume of something: put it in water, measure how much water it displaces.

    It would probably lead to a constant outer pressure.

    I guess that is a biology question.


     
    Last edited by a moderator: Nov 27, 2013
  6. Nov 27, 2013 #5

    berkeman

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    Is there a reason that you don't just use the volume of the pericardial fluid as your measure of the volume? I guess the fluid is more of a lubricant, though, and may not fill the full pericardial volume...?
     
  7. Nov 27, 2013 #6
    But the tissue is elastic, so I can't get constant results...

    Yep.
     
  8. Nov 27, 2013 #7

    berkeman

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    It sounds like you should make several measurements to see if you can figure out the best way to proceed.

    When the heart is dead, do the valves open up? That is, are all 4 chambers open to the same pressure if you fill the heart with fluid? If so, I'd fill it and add a pressure sensor and tie off the veins and arteries to close the heart volume.

    Then, put the heart in a water bath, and inject fluid into the pericardial volume. Measure the pressure of the liquid in the pericardial volume versus the pressure of the liquid in the heart as you put more liquid into the pericardial volume. That will give you some idea of how the pressures vary as you get close to "filling" the pericardial volume. From all of that, you may be able to choose a pressure that you use for the liquid in the pericardial volume to call it "full".

    OOTOH, since liquid is incompressible, and the myocardium probably doesn't compress much either, the main stretching that will happen is in the size of the pericardial sac itself. So maybe it won't be necessary to monitor the pressure inside the heart itself...
     
  9. Nov 27, 2013 #8

    That seems like it would all work. However, there are several thing I'd like you to tell me:

    • Where can I get a pressure sensor that would fit in the heart and how much would it cost?

    • Would hidrostatic pressure of the water in bath have any effect on the results(I'm a bit rusty on physics, I just want to make sure I cover all the corners)? Since hearts are of all shapes and sizes, I would have to heart on a certain depth, but efect of the pressure might be greater on bigger hearts, no?

    No, filling the heart would give more reliable results. I don't think myocardium can compress much but still... I'd at least have to check...

    And also, how can I measure the pressure inside the sack? I'd have to get the sensor into it without damaging the sack itself...
     
    Last edited: Nov 27, 2013
  10. Nov 27, 2013 #9

    mfb

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    It does not have to be inside, a connection to the heart (via a pipe or similar) is sufficient.

    It does, it acts as an external pressure on the heart.
     
  11. Nov 27, 2013 #10
    How would that effect the results? Hidrostatic pressure would be depending only on the height of submerged heart, right?

    p=ρ*g*(surfaceToHeartBottom-surfaceToHeartTop)

    So if I were to slant each heart so that their effective height would be the same, I'd get consistent results, right?

    But on the other hand, if I don't seal the heart, it would only affect the height of pericardial sack. Given it's shaped unregularely, how would that work?

    If I use higher pressure in the sack itself, the effect of the hidrostatic preasure would be weaker, right? Would it be weak enought to be disregarded?
     
  12. Nov 27, 2013 #11

    berkeman

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    As long as you keep the whole thing near the surface of the water, the pressure shouldn't be much different from atmospheric. How big are the hearts? Are they human or animal?

    On your question a while back about how to measure the pressure as you insert fluid into the pericardial sac, you should be able to find a syringe system that has a pressure monitoring gauge associated with it.
     
  13. Nov 27, 2013 #12

    berkeman

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    BTW, are you going to put a link to this PF thread in your paper's bibliography? :biggrin:
     
  14. Nov 27, 2013 #13
    Your pressure sensor would not be a dial guage that you can tap on and curse is the dang thing is working.

    Here are my thoughts on the subject.

    All you need is to measure difference in liquid levels through some kind of manometer or modified version.
    If you submerge the heart and the manometer liquid level and the tank surface level is the same, the heart would be unstressed with equal fluid pressure inside and out.

    If you now fill your sack, also connected to a manometer, and all three levels are equal - heart, tank, sack - then equal pressures exist in all three. The heart should not be stresssed, and the sack should not be stretched.

    You can then play around by raising and lowering the pericardial pressure tube, and or the heart tube and see what effect that has.

    As you add water to the sack the level in the tank will rise a bit. If significant, you have to measure that rise, or the heart could be suspended a set distance below a floating platform.
     
  15. Nov 27, 2013 #14

    berkeman

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    More importantly, since we are discussing making these measurements outside the body, but your OP asked about making them in-body (or in-vivo?), will you want to circle back and make these volume measurements with the pericardial sac in-place in the body?
     
  16. Nov 28, 2013 #15
    Can I? :confused: Should I? :biggrin:

    Well, if we say that the height of a heart would be ~12cm (and would be as close to the surface as it can be), the hidrostatic pressure on the heart would be ~1 Pa, right? Given the fact that the atmspheric pressure is about 100 kPa, that would be difference of 0.001%, which is almost nothing... The problem of hearts having a different sizes would also be solved because the differenc between individual hearts would be measured in tenths of a Pascal.

    Correct me if I'm wrong... :D
    Well actualy, I guess I just paraphrased what you said... :biggrin:

    Human hearts. I should be able to find a container that would hold them. :D

    That is one way I could go, the plus is that it would not require use of instruments (digital manometers, to be exact)... The question is will I be able to close the heart and how precise it would be, but that's a whole new subject...

    Well, I intended to do it while everything was still in a cadaver, but since I wouldn't be able to control internal (meaning one from within the heart) and external (meaning one from the surroundings) pressure to the sack, I gave up on the idea...
     
    Last edited: Nov 28, 2013
  17. Nov 29, 2013 #16

    mfb

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    10cm of water correspond to 100Pa, or ~1% of the air pressure.
    I have no idea what pressure the sack can handle - much more than 100 Pa?

    As an initial test, I would put the heart in water (close to the surface, just to fill everything with water at the same pressure) and measure the volume/pressure relation within reasonable limits (it should not explode, of course). I have no idea how the curve will look like, but I think it will allow to think about a proper test setup.
     
  18. Nov 30, 2013 #17

    Baluncore

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    Has anyone suggested that something like a CAT scan would give you all the data you need.
     
  19. Dec 12, 2013 #18
    I don't think I'll be able to get access to a CAT scanner...

    Okay, new development...

    It seems that I'll have to do this whole thing in situ. So the sac will have to remain in the body.

    Here's the idea. I'd enter a plasic tube through the topmost part of the sac. Then I'd introduce a narrower tube to the one previously set. I'd navigate the tube to the lowest point in the sac (more or less).

    Pericardial fluid are several molecules (e.g. glucose, ions) and bodys (e.g. lymphocites) dissolved (or just contained) in water. I'd have to find a fluid that is more dense than the pericardial fluid and which is non-polar (since water is polar). Then I could push out the pericardial fluid and stop when the liquid I'm using reaches the tube. The pressure wouldn't be big, but I'll find i way to secure the tube to the sac so that there are no leaks around it.

    Only problem is whether the things dissolved in water would pass to the fluid I'd use.

    How does that sound?
     
  20. Dec 12, 2013 #19

    mfb

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    If you are interested in the amount of water inside before the experiment, just remove all that water (by pumping it out)?

    Things that are soluble in water are usually not soluble in hydrocarbons, and if you don't do it too slowly diffusion should not be an issue anyway.

    The two tube setup could leave some water inside, if the setup is not perfect.
     
  21. Dec 12, 2013 #20
    I'm interested in "standard" volume of the sack, which is not equal to amount of serose fluid (which is in it initially).

    The substitute fluid would be of greater density than water, forcing it to exit first through the top (the tube must be at the very top fot that to work). I can also try squezeeing the sack a bit to get everything in place and prevent any pockets from forming.
     
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