Simple question about fluid pressure (medical injection)

[Egregious]

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.

Questions:

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!

 

[berkeman]

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.

Questions:

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!

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.

 

[htg]

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).

 

[Egregious]

Under usual conditions the idea that "fluid hits a vein" and that this is harmful, is fallacious.

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.

 

[PJC01]

Hello,

Thank you for your question regarding fluid pressure in medical injections. I can provide some information and insights on this topic.

Firstly, the difference in gauge size between 18 and 27 is significant. As you mentioned, the inner diameter of an 18 gauge needle is 0.838 mm, while the inner diameter of a 27 gauge needle is only 0.210 mm. This means that the 27 gauge needle has a much smaller opening for the fluid to pass through, resulting in a higher velocity.

To answer your first question, the fluid exiting a 27 gauge needle will be approximately 16 times faster than the fluid exiting an 18 gauge needle. This is due to the smaller opening and therefore, higher pressure in the 27 gauge needle.

As for your second question, you are correct in thinking that the energy of the fluid hitting the vein wall will be significantly higher with the 27 gauge needle. This is because of the relationship between velocity and kinetic energy, as you mentioned. The force of the fluid hitting the vein wall will be roughly 256 times more in the 27 gauge needle compared to the 18 gauge needle.

However, it is important to note that in medical procedures, the force of the fluid hitting the vein wall may not be the only factor to consider. Other factors such as the type of medication being injected, the location of the injection, and the patient's individual needs may also play a role in determining the best needle size for a specific procedure.

I hope this helps to answer your questions. Thank you for your interest in fluid pressure in medical injections and for taking the time to ask such thoughtful questions. Best of luck in your studies!

Sincerely,