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Medical IV's and flow

  1. Aug 5, 2012 #1
    Hi, I'm a nurse, so my physics background is about that of a lay person. Please explain on that level. Ordinarily an Intravenous (IV) fluid bag is one liter. An IV piggyback is usually 100ml. usually the fluid in each is the same 0.45% NaCl. The piggyback may also get medication mixed in with it. such as a Gm of Rocephin.
    The main one has a small gauge tube that is spiked into the port at the bottom of the bag that continues to the patient's arm, infused into the vein with, commonly a 18 Ga catheter. That tube will have a port, there the same gauge tube can be connected to the piggy back bag. The Main bag is on a pole, about 1.5 meters above the infusion site. the piggyback is placed just below the main bag. The accepted explanation for why the piggyback flows and runs out before the main one does (intentional) is gravity. To say the lower one has higher gravity. I tend to disbelieve this explanation. Does anyone know what the real reason for this effect is?
    Last edited: Aug 5, 2012
  2. jcsd
  3. Aug 5, 2012 #2
    Hello wolf and welcome to Physics Forums.

    Interesting name for a nurse. Was this anything to do with the legend of Romulus and Remus?

    You mention gravity so I assume your IV is not pumped?
    However should your arrangement not include a non return valve (I believe you call them anti reflux valves) in each line?

    As to why the smaller bag empties first, that is simple - it is smaller. It does not have 'higher gravity', in fact it has slightly lower if mounted below the main bag. The correct term is not gravity but 'pressure head'.

    The pressure head does not depend in any way on the size of the bag or volume of its contents. It depends only on the height above the patient.
    Since both bags are at similar height the pressure head will be similar so you can expect the smaller bag to empty first, given equal flow rates.

    The natural flow rate depends only upon the pressure head and thus the height. The actual flow rate may, of course, be different since the natural rate may be modified by a controlling valve.
    Last edited: Aug 5, 2012
  4. Aug 5, 2012 #3
    Hi, my log in is a carry over from motorcycle stuff.
    What is pressur head, and why does it make the lower bag empty first?
  5. Aug 5, 2012 #4
    In my thinking, the pressure head, in the pressure of the fluid at the patients arm. this is dependent on height. the higher the fluid, the more pressure? the more pressure the faster the movement of fluid? so the higher bag should empty faster until the fluids surface is the same as the lower bag? so the top should empty first.
    gravity. the greater height allows more force due to gravity?
    so my thinking of this topic, it should be the opposite of what is occurring?
  6. Aug 5, 2012 #5
    BTW there is no pump. the flow rate is regulated by a thumb wheel device that squeezes on the tube, after they join. the smaller bag usually has a connector that is a smaller gauge than the tube at the connection. the connection is off the main line and is not in it's line of flow.
  7. Aug 5, 2012 #6
    I'm sorry, I meant the piggyback is higher than the main bag.
    so is the pressure head a gravity thing?
  8. Aug 5, 2012 #7
    Wow that's quite some information.

    I'm just trying to say that the pressure in the line does indeed depend upon the height, as you say.

    However since you are hanging both bags within a few cm of each other in height this is a red herring as regards to emptying time. The difference of a few cm in 1.5m is small compared to the difference in volume each bag holds. They will both offer a similar natural flow rate but the bag with much more fluid will take longer to empty.

    I also commented that the flow is normally controlled (set) by a valve, which you call a roller clamp.
  9. Aug 5, 2012 #8


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    Pressure head is given by ρgh, where ρ is density of the fluid, h is the height. The blood pressure in the arm will be a subtraction from the pressure head, but since both bags will be effected equally it does not need to be known.

    If the density of the fluid in the smaller bag is greater then the saline solution it will have even a greater pressure head. Very likely the flow rates out of the 2 bags is very similar, the smaller bag empties first simply because it is smaller.
  10. Aug 5, 2012 #9
    The smaller bag tends to empty before the bigger bag even starts, to any mesurable degree. it happens when the fluid is identical. fully unclamped the small bag will empty in minutes then the larger bag starts to empty, at pretty much the same rate.the height of the fluid in the bottom bag is usually around ten cm of the bottom of the top bag.
    I know it sounds funny, and there should only be a tiny difference in flow, if any, but this is the common practice, and it drives me nuts figuring it out. I've gon so far as to count the drop rate and empty time for a single bag alone at different heights, with the same tube, dripping into a glass.just to definitely rule out simply the argument of height alone increases flow rate by weight of gravity. my other theory on this is I weigh the same in the basement as i do on the 20th floor
  11. Aug 5, 2012 #10
  12. Aug 5, 2012 #11
    you have already told us that the larger bag contains 10 times as much fluid as the smaller one.

    So would you notice if the larger bag emptied less than 10% of its contents in the same time as the smaller one emptied 100% of its contents?
  13. Aug 5, 2012 #12


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    Do you have a sketch of the setup?
    Can the pressure in bag 1 influence the pressure in bag 2? If yes, there is an easy solution: The pressure in the lower bag is determined by the (higher) pressure coming from the upper bag. It is higher than the environmental pressure, therefore (nearly) nothing flows out.
  14. Aug 5, 2012 #13

    But we have been told fluid does flow out??
  15. Aug 5, 2012 #14


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    And here is a sign of the possible pressure regulation I described:
  16. Aug 5, 2012 #15
    Yes, nurses call it a roller clamp. (post#7)
  17. Aug 5, 2012 #16
  18. Aug 5, 2012 #17
  19. Aug 6, 2012 #18

    Andy Resnick

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    The "three reservoir problem" is exactly what you describe: two reservoirs are the two bags, and the third is the vein:


    Depending on the resistances of the various tubes, it's possible to have one bag drain faster than the other, or have both drain at the same rate.
  20. Aug 6, 2012 #19
    OK, I understand that:
    the top bag yields the highest pressure in the line
    the bottom bag yields the medium pressure in the line
    the vein has the lowest pressure in the line.
    the highest pressure will dominate the flow to the lowest pressure.
    the high pressure will not backflow into the medium because it has a lower pressure than that to release to.
    what i do not understand is: the top bag has less weight than the bottom one does, so there is less weight of water pressing on the bag outlet than the lower bag.
    how does the top bag have higher pressure in the tube?
  21. Aug 6, 2012 #20


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    The mass in the bags does not matter, just the height of the water column is relevant. This is a bit unintuitive, but can be calculated easily. In terms of a big bag with a small hole at the bottom: Most of the weight of the water is kept by the surface of the bag, which is connected to the pole.
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