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Simple question about fluid pressure (medical injection)

  1. Nov 29, 2012 #1
    Hello, and thank you kindly for considering my question.

    Background: Medical needles come in gauges such that a smaller gauge represents a larger needle.

    Scenario: Doctor Jack says "18 gauge needles are best for this procedure", but Doctor Jane says "27 gauge needles are best because they are smaller".

    Parameters: 10 mL fluid being injected over 100 seconds. Inner diameter of 18 and 27 gauge needles respectively: 0.838 mm, 0.210mm. For simplicity, assume that the needle is in a vein.


    1) How much faster (velocity) does the fluid exit a 27 vs. 18 gauge needle?
    2) How much 'harder' does the fluid hit the wall of the vein with 27 vs. 18 gauge?

    I have answered question #1 using basic mathematics (volume of a cylinder): 18.131 cm/s vs. 288.716 cm/s, or roughly 16 times faster.

    I "dumbed down" question 2 because I'm not sure if I should be asking it in terms of pressure, kinetic energy, or something else. My gut instinct is that it isn't too much different from a car hitting a brick wall (in the sense that 2x velocity = 4x energy: 1/2 mv^2, if memory serves), so at about 16 times the velocity, the force(energy) of the fluid hitting the wall should be roughly 256 times more in the 27 versus 18 gauge (...I think?)

    Thanks so much for your time and consideration!
  2. jcsd
  3. Nov 30, 2012 #2


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    Staff: Mentor

    I understand now that you are a nurse looking at the characteristics of injections. If you don't get good responses in this Physics sub-forum, let me know if you'd like your thread moved to the Mechanical Engineering sub-forum, where fluid dynamics gets discussed more. You may still get some good responses here in the Physics sub-forum, though. :smile:
  4. Nov 30, 2012 #3


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    Under usual conditions the idea that "fluid hits a vein" and that this is harmful, is fallacious. So smaller needles are better, because they do less damage to the vein (when piercing the vein).
  5. Nov 30, 2012 #4
    I haven't heard much on this topic, really, so I'm not basing my inquiry on any preconceived notions about which is better or worse. I am merely trying to understand a physical concept. Clearly, if you shove a given volume of fluid through a smaller hole, the jet generated is going to move faster, and collide with a stationary object 'harder'. The meaning of 'harder' in this concept is what I am attempting to understand, and the quantification of how much 'harder' is one objective.

    The actual context is joint injections such as a knee, or shoulder. From the little that I know at this point, I don't think that either gauge is particularly bad for this purpose, but I found it interesting that one particular practitioner felt that 27 gauge was 'better' for this purpose. I am really just trying to understand the forces in play during the procedure, and then I might be in a position to make some limited judgement about the notion that '27 gauge is better'...

    A related idea that is coming up on my list of things to understand is the practice of only using 10mL or larger syringes to flush central venous catheters (CVCs), and peripherally inserted central catheters (PICC lines). The idea is that the smaller syringes apply more pressure to the catheter wall. But... that is getting a bit (lot) off topic.

    Thanks for your reply.
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