Back Pressure in IV lines, why it happens, how to fix them

[OVeractiveBrain]

Hello all:

I have recently run into a complicated problem in Labor and Delivery, which I am sure will come up again in my career in medicine.

Thes setup: 20G in the left forearm with a dual port hub (one for the fluid line one for instant access to medications). The fluid line then flows back to a three way device. From there, two lines are connected, in one, a 1000cc NS bag, which is turned off. The other line is to a 500cc NS bag we were to bolus into a patient.

The problem: When we opened the line wide open, nothing ran. At first, the nurse failed to remove the plastic lock that had pinched off the line. Fixed that. Still wont run.

What we checked:

THe line flushed well, a 10cc bolus injected through the medication port flushed well, no disruptions, skin raises, etc. Basically, the line is patent.

All the blocks are wide open. That is, the roller-wheel was wide open, all blocks were open, no kinks, or pinches.

The infusion bag was closed to allow the bolus to flow in.

OPening the infusion bag caused the fluid to back up in the bolus bag.

Changed the heights of the infusion and the bolus bag to level, above and below one another.

When we put positive pressure on teh bag (the bolus bag), it flowed well, dripping at what I expected a wide open line to do.

Opening the infusion bag at this point caused fluid to flow back into the infusion chamber.

What im thinking:

Either we misused the three-way, and only one of the lines should have been running, or there was some serious back pressure from the veins. Granted she was a pregnant woman in contractions, but her pressure was 118/52 at a HR of 72. In addition, contracting muscles serve only to push fluid towards the heart, not back against the flow. No, there was no BP cuff on that arm, nor was there ever any blood backing up into the line.

I ask this question because i want to know other solutions. We ended using a pressure bag (like the ones hospitals use for heparin infusions in MI transports) around trhe bolus in order to get the fluid to flow. I dont have these, nor do i know of any in any service in my area. Should the occasion arise in teh field i would like to have a better solution other than "just squeeze the bag" as I might have other things to do at the same time.

- OVeractiveBrain

[becksdad]

Ok, I'm going to stick my neck out on this one, 'cause I haven't run into this, but some things just seem to make sense to me here. I'm sure I will at least learn something!

If the 1000 ml bag ran first, wouldn't the pressure from that be greater than that of the 500 ml bag? And then when you applied a pressure infuser to the smaller bag, that then created greater pressure than the 1000 ml, causing it to back up? I wonder if this problem would have occured using 2 bags of equal volume? Or would opening the port on the drip set of the smaller bag (like you do for a glass bottle such as Tridal or Diprivan) have resolved the problem?

Also, it seems to me that BP would not have affected the flow of the NS, since the vein is cannulated in the direction of blood flow. Any of this make sense? Educate me.

[JPINFV]

First things first, this is not a blood pressure issue. Veins have a very low blood pressure and the measured blood pressure is the pressure in the arteries.

Think of using a garden hose. Kink the hose a good distance from the nozzle. Between the wall and the kink is an artery, the kink is a capillary, and the hose from the kink to the opening is a vein. The pressure from the wall (i.e. heart) pushes the water through the kink (capillary). After the kink you have a large space with little fluid volume (veins are called volume reservoirs because they are able to stretch to aaccommodatea higher volume at a lower pressure. Systemic arteries are called pressure reservoirs because they aren't as elastic. Thus you end up with less volume, but a higher pressure to allow perfusion) so you have a lower pressure.

Just wondering, did you move the catheter any? I'm wondering if you ended up getting stuck in a thrombus or a valve.

[becksdad]

Yeah, I didn't think it had anything to do with BP, although I wasn't thinking of it in terms of veins being capacitance vessels. Duh.

But Overactive kind of explained how the line was patent, when the 1000 ml bag ran alone, it was fine; flush went fine; no evidence of infiltration. The only other thing I could think of was difference in volume. I'm pretty much just excercising my brain here, but it could come in really handy sometime.

[ptemt]

Hard to say what happened having not been there but it sounds like a balance of forces is occurring. The 20 gauge catheter is a very small orifice relative to the inner diameter of the connective tubing. Force being equal to pressure times area you have greater head pressure in the 1000 ml bag than the 500 ml bag. With a common connection to the two bags I would expect the one with greater pressure to back flow to the one with less, until the pressure is equalized. At the same time though I would expect some flow through the catheter. However, as the big bag back flows to the small bag a venturi effect may occur at the junction that leads to the catheter. The venturi would reduce pressure at the catheter junction and this may reduce or stop flow to the catheter until the balance of pressure is completed in the two bags.

An illustration of the venturi effect can be observed watching a smoker driving with the window cracked open. As the air rushes by the car (or car rushes by the air actually), the pressure in the car is reduced and the smoke is drawn out the crack.

[OVeractiveBrain]

The suggestion about volume size might explain it. THere is more pressure on a column of water 1000mL tall (however tall a 1000mL bag is) over a 500mL bag. The frustrating thing is why wouldnt the fluid just flow into the vein. Since veins are capacitors, youd thing any excess pressure would be distributed into teh vein, not up the bag applying its own pressure.

However, that was not something I had thought about.

The real trouble Im having with that explanation is purelu that if you have a 1000 bag running for fluids and attach a 250 bag with drug in it, you can run both, either piggy-backed or attached at a 3-way (a 3-way that allows flow from all directions) which I myself have done.

This was just an irritating problem I havent seen, and i appreciate everyone's input

-OveractiveBrain

[Ridryder 911]

It has to do with volume and the pressures... thus is why when you do piggyback drips, it is essential to adjust it if you want the primary to run after the initial infusion or start another infusion.

Second, why would anyone have such a configuration? Establish another IV line, and run it appropriately.

R/r911

[ptemt]

The suggestion about volume size might explain it. THere is more pressure on a column of water 1000mL tall (however tall a 1000mL bag is) over a 500mL bag. The frustrating thing is why wouldnt the fluid just flow into the vein. Since veins are capacitors, youd thing any excess pressure would be distributed into teh vein, not up the bag applying its own pressure.

-OveractiveBrain

It's all physics, the catheters inner diameter determines the coefficient of flow. A 14 or 16 gauge catheter may provide more favorable results. I'll go with Ridryder's experience and recommendation on this one!

[fiznat]

I like to try and fix the simple things first if possible. Take the tape off of the IV and fiddle with it just a little bit-- you could have been up against a valve or something silly like that. Equally, triple check all of the IV tubing for kinks/locks. A trick I also like to do is take the line and wrap it tightly around my hand so that some fluid is forced through the line into the vein. Often that helps to clear out a line that has slowed down after sitting locked for a while. Also try rearranging your bag setup. Seems like a pretty weird valve/lock system you guys had goin there: high potential for screwups haha.

Seems to me that if the 1000cc bag was shut off, it shouldnt be exerting any pressure on the line and therefore shouldnt be effecting the flow out of your 500cc bag at all.

Screw it. I would probably just keep the #20 for meds, and start another line for fluid. Should probably have 2 lines anyways for someone about to give birth or undergo surgery whatever the case may be.

Given that all of that fails, we *do* have a way of applying constant pressure to fluid bags in the ambulance-- a BP cuff! Wrap it around the bag and tighten it up, it works sweet. Also good for keeping fluid boluses going while moving the patient on a stretcher that doesnt have an IV pole. Just gotta remember to keep inflating the cuff as the volume of fluid in the bag gets depleted.

[Off Label]

Sounds like a problem using the 3 way stop cock. But I'm still trying to figure out, all these years later, why they needed to bolus from a 500 cc bag plugged into a line where they had a 1000 cc bag already hanging.