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Achieving a negative pressure with a syringe

  1. May 7, 2007 #1
    I am a medical doctor needing some physics help. Quite often in patients we insert thin tubes into blood vessels for various purposes. These occasionally get blocked and we use syringes to try and clear these blockages. We attach a syringe and create a "negative pressure" by drawing back the plunger. Some of my colleagues claim that one can generate a more negative pressure with a syringe (piston) with a smaller diameter. Others claim that a piston with a larger diameter can generate a stronger negative pressure. So here is my question: For a given "pulling force" on the plunger of a piston, does the diameter of the piston influence the degree of negative pressure generated by the piston. I do know that according to the equation pressure = force/area, smaller diameters generate higher pressures. But does this hold true even when trying to generate a negative pressure with a syringe? Your help will be much appreciated and may help save lives.

    Following this posting i did a small experiment. I hung an one kilo weight on a 5 ml and 10 ml syringe and connected it to a Digitron electronic manometer. I got the following neg pressure readings (cm Hg).
    The 5 ml syringe: 57.3;55.8;56.7;56.6;55.9;56.6;57.4;56.1;56.1;56.6 The 10 ml syringe gave the following: 32.8;32.3;30.7;31.9;31.9;32.6;30.6;33.3;32.1;30.5
    I would like to"scientifically" say that the smaller syringe creates a bigger negative pressure. My statistical knowledge is near zero. Would a "Students t" test be an appropriate one to use ?
    Last edited: May 7, 2007
  2. jcsd
  3. May 7, 2007 #2


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    Welcome to the forums,

    More important than the force in this case is the change in volume inside the syringe. If we assume that we have an isothermal process then we can say that PV=P'V'. Since in a syringe it is only the length(l) of the cavity is changing we can rewrite this thus;

    [tex]Pkl = P'kl' \hspace{1cm}\text{where } k = \pi r^2[/tex]

    [tex]Pl = P'l' \Rightarrow \Delta P = \Delta l[/tex]

    So the change in pressure is dependant on how far back you pull the plunger, but you probably knew that anyway. However, we can also show that the amount of work you do pulling the plunger back is;

    [tex]w = c\ln\left(\frac{V}{V'}\right)[/tex]

    Again, since only the legnth of the cavity is changing;

    [tex]w = c\ln\left(\frac{kl}{kl'}\right)[/tex]

    [tex]w = c\ln\left(\frac{l}{l'}\right)[/tex]

    This shows that the amount of work you do in changing the pressure only depends on the distance you pull back the plunger and is independant of cross sectional area of the plunger.

    P.S. I'd be a little concered if my doctor liked to be called madpsychic :wink:
    Last edited: May 7, 2007
  4. May 7, 2007 #3
    In short, for a same force the depression ("negative pressure") is stronger with a small piston than for a large piston.
    But anyway, the best you can do, with any piston in optimum circumstances is make a null pressure. That is, complete vacuum, or a "negative pressure" equal to atmospheric pressure.
    For practical purposes, you must start with the piston full pushed (empty syringe) and then pull.
    That is really important.
  5. May 7, 2007 #4


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    Simple answer: yes.

    Caveat: is the "pulling force" really the same?
    Last edited: May 7, 2007
  6. Oct 26, 2007 #5
    Negative Pressure With a Syringe

    Large syringe barrels are harder to pull back and create suction/negative pressure than smaller barrels. All things equal, we want to know if a larger barrel is better for removing fluid, or viscous fluid and solids, through a catheter.

    Take two syringes. Each have identical 40mm plunger stroke lengths and are connected to identical catheters with a 2.0mm inner diameter that are 100cm long. Syringe #1 can hold 25ml with a barrel diameter of approximately 28.5mm. Syringe #2 can hold 50ml with a barrel diameter of approximately 40mm.

    1. To start we push the plungers completely forward to drain all air. Then, we put the catheter tip in water and pull back the plungers of each 40mm. Will one syringe provide better suction, flow, or negative pressure than the other? We know the larger syringe is much harder to pull back and can minimize this problem to make the pulling back of plungers near equal to the user.

    2. Now, imagine we can put a three way valve on the end of the barrel in between the syringe and the catheter. The valve allows you to suck anything into the syringe from a catheter, but, when the plunger is advanced in injection mode, the valve seals the connection to the catheter and the fluid can be pushed out the barrel into a drain bag. So, now with each syringe, we can quickly pull back the barrel, apply suction for a couple of seconds, and then quickly expel and suck again...a pulsated suction. Does pulsing suction create any advantage to improving drainage speed or the ability to suck larger items through the catheter? Will a 28.5mm or 40mm barrel drain faster or pull larger items better?

    Doctors are using syringes to pull out blood clots and other viscous fluids from the body. There are ways to make the pulling back of the plunger easier. I would like to quantify the benefits of larger barrels vs. smaller barrels and/or pulsing suction if all other things being equal.

    Syringe User
  7. Oct 26, 2007 #6

    How did you hang the weight in your experiment? To pull the plungers out or push them in?

    I think based on your test and the other responses, you can say smaller syringes provide bigger negative pressure only for equal pull force on the plunger.

    I would like to know if we assumed the plungers were pulled back mechanically, what is the benefit of a larger barrel vs. small barrel?
  8. Oct 26, 2007 #7


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    I believe you mean equal to 0 psi, not atmospheric (1 ATM = 14.69 psi).


    Never mind, I think that your point was that the "negative pressure" IS equal to 0 psi (i.e -14.69 psig = 0 psia), or was it?
    Last edited: Oct 26, 2007
  9. Oct 26, 2007 #8


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    Sounds like you have "scientifically" anwered your own question. Nothing like empirical evidence to prove your theory!
  10. Oct 27, 2007 #9

    First of all, madpsychic, if you are a doctor in fact, this sounds like a malpractice case waiting to happen. A minimal amount of air will be contained within the needle of an empty syringe. When you stick an empty syringe into the human body, you risk allowing that air to diffuse out or be sucked out. And that will kill someone.

    Second of all, you need to forget about your "Pressure = Force/Area" equation. In this case, the correct math would be to consider that the pressure is a function of the volume generated inside the syringe. To say that pressure = force/area doesn't give you anything in this case, and it is incorrect to simply look at this equation and say "a smaller diameter generates a bigger pressure". Only greater force will generate a greater pressure. If the diameter is smaller, it will simultaneously take less force to pull out the syringe. But this is a simple problem, and math isn't really needed; it may be given if you desire.

    To answer your question, allow me to explain something. Assume you have some syringe, with the needle plugged, such that you create a negative pressure when you retract the stopper. Everyone agrees on this.

    The magnitude of the pressure generated in the syringe depends ONLY on the force applied to the stopper. This is the punchline. The amount of negative pressure generated within ANY kind of syringe (10mm diameter, 20mm diameter, etc.) will be the same, if you pull on each one with the same force. Your question assumes that using different syringes, you may somehow be able to generate a higher magnitude of pressure. In fact, this only applies for a special case discussed in the next paragraph.

    With a syringe-type device, there exists the special case of "maximum negative pressure", call it MNP, where the stopper is retracted as far as it can go, before it will simply slip out of the back of the tube, thereby preventing you from generating any higher MNP. If you want to talk about this MNP, you will find that the larger the syringe, the larger the MNP. The reason is the following. The magnitude of the pressure depends of the amount of volume which has been created, and a larger syringe can simply tolerate the creation of a larger volume (e.g., it can hold more cc's) before the MNP is reached.

    Hence, if you are talking about pulling the stopper as far back as possible, a larger syringe will allow you to generate a larger negative pressure. Otherwise, it won't matter which syringe you use. It's no easier to generate pressure in one syringe or another; the same amount of pressure takes the same amount of force, in every case.

    I am not sure why your experiment failed to indicate this, but I suspect that it is because of friction issues preventing the stopper of the larger syringe to freely retract.
  11. Oct 30, 2007 #10
    Syringe Question

    Dear Mordechai9,
    Thanks for the help. My question still remains. Lets ignore that it takes more force to pull back a larger plunger.
    1. What will the MNP difference be between a barrel with a 29mm and a 40mm diameter if each have an identical 40mm stroke?
    2. Will the larger barrel pull liquids or solids faster through a catheter if connected to a catheter?
    3. Consider that we can pull negative, hold for a second, then push the plunger back in and drain all the previous negative pressure and pull negative again from the catheter. Can this help us suck pathology?

    We know of a unique syringe design that allows physicians to pull negative pressure easier than before and are tying to hypothesis the benefits of it's use in aspiration biopsy and thrombus removal.
  12. Sep 13, 2008 #11
    well i am a doctor myself and such a debate did arise with me as well...
    Question was, if I wanted to drain some fluid out of a cavity, whether a 20 cc syringe be better or 10 cc.....
    to prove that 20 cc would be better, i took equal amount of water and timed it using the 2 different volume syringes..... it proved that 20 cc was better as it was quicker to drain the same amount of fluid...
    remember guys that the question was which one is quicker...as in medical emergencies time is more important.... it might be tiring though to use a 20 or a 50 cc syringe.
    any comments?
  13. Sep 13, 2008 #12
    Re: Syringe Question

    I think you need to re-read his post more carefully.

    The 40mm diameter will have a larger maximum, obviously, but this was just an example to show how the larger volume has more, ummn, volume. If the needle is not blocked, there is no MNP. You just get to draw more before the plunger runs out of space. The pressure during the pull depends upon the speed the plunger is pulled with. The speed depends on the force applied.

    The speed depends on the force applied, not the diameter of the barrel. Don't confuse speed with force. If you have a constant speed, then the larger diameter will pull more, but then a larger force is required.

    I don't understand this question sorry. Others may not either. If the above doesn't help you then try re-phrasing that one.

    What is the volume of fluid you must pull? I assume not enough to fill the smallest syringe.

    As you imply, the larger syringes are maybe a bit more tiring. From a physics perspective the work done is the same, but with the smaller syringe you get a bit of leverage - less tiring, but with the bigger syringe more effort can be brought to bear.
  14. Sep 13, 2008 #13


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    Re: Syringe Question

    Perhaps the difference between "pressure" and "force" would be a usefull discussion at this point. You see, the force of the needle will be the pressure multiplied by the area. This means that a small needle can give you more pressure over a smaller area, or a large needle can give you less pressure over a larger area, but the overall force will be the same for both, and it will be the same as the force with which the plunger is pulled, minus a bit for friction and the like.

    Now this friction will have two main components; the friction in the syringe parts, and the drag caused by the fluid itself. This second component can be significant if the fluid is viscous. A larger needle will suffer less drag from the fluid than a small needle, and the amount of difference is greater with more viscous fluids.

    Overall, I would say that the large-bore needle works "better" (depending on how you measure that) for drawing highly viscous fluids. Especially if you take your fluids "chunky-style"; the larger needle will not clog as easily.
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