I need intracatheter pressure explained to me

[kemansha1]

I am a new grad working about 7 months on a med-surge floor and last night I found out that I have been using a carpuject incorrectly on PICC lines. If I need to deliver 1 ml of a medication I thought it was ok to use the carpuject and then flush with a 10ml syringe. (I always use a 10 ml syringe to flush before and after the medication first to ensure patency and then to ensure the medication is delivered.) My understanding of this was that a tiny 1 ml syringe would deliver the medication at pretty low pressure basically into the lumen of the catheter and then the 10 ml syringe would raise the pressure by raising the volume and flush the medication through the line and into patient's blood vessel. But then I was told last night that this is wrong and that the carpuject RAISES the intralumenal pressure too high putting the patient at risk. I checked the policy and sure enough it does say never to use any syringe smaller than a 10 ml syringe and I guess that also includes the carpuject vials. I feel terrible that I have been doing this incorrectly all this time but I also just don't understand the physics of this. I have been trying to visualize the mechanics of this all morning and I can't understand how a carpuject can possibly be a higher pressure mechanism than the 10 ml syringe because the small volume of medication is being delivered from a small vial into the catheter via the same tubing that 10 ml volume is being delivered into. Can anybody shed some light on this subject for me?

[blondy2061h, MSN, RN]

Think of a hose nozzle that you can adjust how wide the spray comes out on. If you adjust the nozzle for a wide spread spray, the spray has far less force, right? Because that same amount of water is being spread over a wider surface area.

It's the same with syringes. The 10ml syringe has a wider diameter than a smaller syringe. It's that hose with the wide nozzle on it. You can't change how much fluid the PICC can accept at one time, but if you're spreading out the surface area with the wider syringe barrel, you're causing weaker flow and thus less pressure.

[blondy2061h, MSN, RN]

And this, folks, is how you ask a question and get a good answer on AllNurses.

Well done, OP, and thank you, blondy, for an answer that anyone could understand.

:)

While that answer makes sense, I do have a follow up question: if the adapters or luer locks are all the same size, does that mean the "spray spread" is the same?

Once the fluid reaches the luer lock, the amount of pressure has already been determined by the syringe. I may be visualizing the wrong, so hopefully someone who's more of an IV access expert will chime in, but I believe it becomes the least common denominator setting the pace, sort of thing.

[Asystole RN]

https://www.bd.com/posiflush/flash/syringe_pressure.html

Keep in mind that the PICC internal lumen can contain 1-2mL of fluid. If you inject 1mL slowly all you are doing is priming the PICC. The brisk flush of saline will bolus the medication...I assume you have been very popular with your patients?

[Asystole RN]

And this, folks, is how you ask a question and get a good answer on AllNurses.

Well done, OP, and thank you, blondy, for an answer that anyone could understand.

:)

While that answer makes sense, I do have a follow up question: if the adapters or luer locks are all the same size, does that mean the "spray spread" is the same?

The smallest part of any catheter is the internal lumen of the catheter itself.

[kemansha1]

Thank you! This video is exactly the explanation I was looking for. I was thinking that the pressure would be determined by the change from one diameter to the other ie: if you hook up a garden hose to a fire hose, the pressure caused by the change from high volume to lower volume capacity would raise the pressure. And really I thought that higher pressure was what we were aiming for since there is always the possibility of occlusion. Also since the length of the PICC line is longer than a PIV I figured there is more need to flush more fluid and at higher pressure.

[kemansha1]

Yes. Good point.

[MunoRN, RN]

So long as the lumen is patent then you aren't going to develop high intraluminal pressure no matter what size syringe you use, this is why a common recommendation is to assess patency with a 10ml syringe and if the lumen is patent a smaller bore syringe may be used. If the line is occluded then the weight applied to the plunger will translate to a higher PSI (1 pound over a 1 square inch plunger = 1 psi, 1 pound over a 0.25 square inch plunger =4 psi), but only if the pressure is unable to escape.

[kemansha1]

https://www.bd.com/posiflush/flash/s..._pressure.html

Keep in mind that the PICC internal lumen can contain 1-2mL of fluid. If you inject 1mL slowly all you are doing is priming the PICC. The brisk flush of saline will bolus the medication...I assume you have been very popular with your patients?

Yes. Good point.

[SummitRN, BSN, RN]

This is applied fluid mechanics/dynamics.

The equations are actually pretty complex, but I'll make it pretty simple.

(We will consider the IV fluids noncompressible. We can ignore hydrostatic head (hydraulic head))

Think back to your middle school science class with a hydrualic piston setup:

F1 = F2 * (A1 / A2)

As applied to this problem:

Force exerted inside the catheter is directly related to force you put on the syringe plunger

Push harder = more force = more pressure and flow

Force exerted inside the catheter is inversely related to (pi * syringe plunger radius squared)

So a wide syringe plunger = less force on the catheter.

That is vastly oversimplified because you also have the Hagen–Poiseuille equation.

Translated:

1. You push harder (or raise the gravity bag) and some of the pressure is relieved by increased flow (Q). Flow rate increases.

2. However, if your fluid is viscous, more of your force converts to intracatheter pressure. That is why D50 comes in a very wide plunger syringe (you are more likely to break the plunger base before you generate pressure sufficient to burst even a partially occluded catheter) and we gently flush with NS after!

3. If you make your catheter longer (like a PICC/CVC) it increases pressure!

4. Double the diameter of your catheter and flow decreases 16X for the same pressure on your plunger. (r^4).

You could make a lot more relational generalizations, but I think you get the point

The last point is that there are published intracatheter pressure limits, and and the catheter will undergo elastic deformation before failure, thus pushing 1mL of nonviscous IV fluid pushed hard even with a very small syringe would be extraordinarily unlikely if not impossible to burst a PATENT catheter. However, an occluded catheter could be burst!

[NICU Guy, BSN, RN]

I am having a hard time wrapping my head around this. In my mind, the size of the syringe is irrelevant. If I have a 1 mL syringe and administer it over one minute and a 10 mL syringe and administer 1 ml over one minute, wouldn't you be administering at the same psi since the Luer lock/ gauge of the needle is the same on both syringes? You are administering the same amount of fluid over the same amount of time through the same diameter leur lock/needle.

[SummitRN, BSN, RN]

If I have a 1 mL syringe and administer it over one minute and a 10 mL syringe and administer 1 ml over one minute, wouldn't you be administering at the same psi since the Luer lock/ gauge of the needle is the same on both syringes? You are administering the same amount of fluid over the same amount of time through the same diameter leur lock/needle.

Yes you would have the same intracatheter pressure because you have a constant volumetric flow rate (Q).

However, you would have to push with more force on the plunger of the 10mL syringe to achieve it.

The wider the plunger, the less pressure you will generate in the catheter for the same amount of force you apply to the syringe plunger.

Force exerted inside the catheter is inversely related to (pi * syringe plunger radius squared)