The Science Behind Medical IV's and Flow

[wolf!]

Hi, I'm a nurse, so my physics background is about that of a lay person. Please explain on that level. Ordinarily an Intravenous (IV) fluid bag is one liter. An IV piggyback is usually 100ml. usually the fluid in each is the same 0.45% NaCl. The piggyback may also get medication mixed in with it. such as a Gm of Rocephin.
The main one has a small gauge tube that is spiked into the port at the bottom of the bag that continues to the patient's arm, infused into the vein with, commonly a 18 Ga catheter. That tube will have a port, there the same gauge tube can be connected to the piggy back bag. The Main bag is on a pole, about 1.5 meters above the infusion site. the piggyback is placed just below the main bag. The accepted explanation for why the piggyback flows and runs out before the main one does (intentional) is gravity. To say the lower one has higher gravity. I tend to disbelieve this explanation. Does anyone know what the real reason for this effect is?
Thanks,
al

 

[Studiot]

Hello wolf and welcome to Physics Forums.

Interesting name for a nurse. Was this anything to do with the legend of Romulus and Remus?

You mention gravity so I assume your IV is not pumped?
However should your arrangement not include a non return valve (I believe you call them anti reflux valves) in each line?

As to why the smaller bag empties first, that is simple - it is smaller. It does not have 'higher gravity', in fact it has slightly lower if mounted below the main bag. The correct term is not gravity but 'pressure head'.

The pressure head does not depend in any way on the size of the bag or volume of its contents. It depends only on the height above the patient.
Since both bags are at similar height the pressure head will be similar so you can expect the smaller bag to empty first, given equal flow rates.

The natural flow rate depends only upon the pressure head and thus the height. The actual flow rate may, of course, be different since the natural rate may be modified by a controlling valve.

 

[wolf!]

Hi, my log in is a carry over from motorcycle stuff.
What is pressur head, and why does it make the lower bag empty first?
thanks

 

[wolf!]

In my thinking, the pressure head, in the pressure of the fluid at the patients arm. this is dependent on height. the higher the fluid, the more pressure? the more pressure the faster the movement of fluid? so the higher bag should empty faster until the fluids surface is the same as the lower bag? so the top should empty first.
gravity. the greater height allows more force due to gravity?
so my thinking of this topic, it should be the opposite of what is occurring?
i

 

[wolf!]

BTW there is no pump. the flow rate is regulated by a thumb wheel device that squeezes on the tube, after they join. the smaller bag usually has a connector that is a smaller gauge than the tube at the connection. the connection is off the main line and is not in it's line of flow.

 

[wolf!]

I'm sorry, I meant the piggyback is higher than the main bag.
so is the pressure head a gravity thing?

 

[Studiot]

Wow that's quite some information.

I'm just trying to say that the pressure in the line does indeed depend upon the height, as you say.

However since you are hanging both bags within a few cm of each other in height this is a red herring as regards to emptying time. The difference of a few cm in 1.5m is small compared to the difference in volume each bag holds. They will both offer a similar natural flow rate but the bag with much more fluid will take longer to empty.

I also commented that the flow is normally controlled (set) by a valve, which you call a roller clamp.

 

[Integral]

Pressure head is given by ρgh, where ρ is density of the fluid, h is the height. The blood pressure in the arm will be a subtraction from the pressure head, but since both bags will be effected equally it does not need to be known.

If the density of the fluid in the smaller bag is greater then the saline solution it will have even a greater pressure head. Very likely the flow rates out of the 2 bags is very similar, the smaller bag empties first simply because it is smaller.

 

[wolf!]

The smaller bag tends to empty before the bigger bag even starts, to any mesurable degree. it happens when the fluid is identical. fully unclamped the small bag will empty in minutes then the larger bag starts to empty, at pretty much the same rate.the height of the fluid in the bottom bag is usually around ten cm of the bottom of the top bag.
I know it sounds funny, and there should only be a tiny difference in flow, if any, but this is the common practice, and it drives me nuts figuring it out. I've gon so far as to count the drop rate and empty time for a single bag alone at different heights, with the same tube, dripping into a glass.just to definitely rule out simply the argument of height alone increases flow rate by weight of gravity. my other theory on this is I weigh the same in the basement as i do on the 20th floor

 

[wolf!]

example, see iv medications on this sample page:
http://books.google.com/books?id=od...zgK#v=onepage&q=i v piggyback gravity&f=false

 

[Studiot]

you have already told us that the larger bag contains 10 times as much fluid as the smaller one.

So would you notice if the larger bag emptied less than 10% of its contents in the same time as the smaller one emptied 100% of its contents?

 

[mfb]

Do you have a sketch of the setup?
Can the pressure in bag 1 influence the pressure in bag 2? If yes, there is an easy solution: The pressure in the lower bag is determined by the (higher) pressure coming from the upper bag. It is higher than the environmental pressure, therefore (nearly) nothing flows out.

 

[Studiot]

The pressure in the lower bag is determined by the (higher) pressure coming from the upper bag. It is higher than the environmental pressure, therefore (nearly) nothing flows out.

But we have been told fluid does flow out??

 

[mfb]

Where?

why the piggyback flows and runs out before the main one does

The smaller bag tends to empty before the bigger bag even starts, to any mesurable degree. [...] fully unclamped the small bag will empty in minutes then the larger bag starts to empty

And here is a sign of the possible pressure regulation I described:

the flow rate is regulated by a thumb wheel device that squeezes on the tube, after they join

 

[Studiot]

And here is a sign of the possible pressure regulation I described:

the flow rate is regulated by a thumb wheel device that squeezes on the tube, after they join

Yes, nurses call it a roller clamp. (post#7)

 

[wolf!]

pic of setup.
http://3.bp.blogspot.com/-1-s8FBqGOR4/TnunSmhB5XI/AAAAAAAAArA/nzbXjsZTDoE/s1600/piggy2.jpg

 

[Studiot]

Here are two vids of this procedure.

http://www.youtube.com/watch?feature=endscreen&NR=1&v=N0rCuC2XE1U

The last two minutes of this one are particularly informative.

http://www.youtube.com/watch?feature=player_detailpage&v=DPqpPD8bwVo

They show that to get the various effects there is an interaction between the roller clamps settings and the bag and piping system.

 

[Andy Resnick]

<snip> Does anyone know what the real reason for this effect is?
Thanks,
al

The "three reservoir problem" is exactly what you describe: two reservoirs are the two bags, and the third is the vein:

http://excelcalculations.blogspot.com/2011/05/three-reservoir-problem.html

Depending on the resistances of the various tubes, it's possible to have one bag drain faster than the other, or have both drain at the same rate.

 

[wolf!]

OK, I understand that:
the top bag yields the highest pressure in the line
the bottom bag yields the medium pressure in the line
the vein has the lowest pressure in the line.
the highest pressure will dominate the flow to the lowest pressure.
the high pressure will not backflow into the medium because it has a lower pressure than that to release to.
what i do not understand is: the top bag has less weight than the bottom one does, so there is less weight of water pressing on the bag outlet than the lower bag.
how does the top bag have higher pressure in the tube?

 

[mfb]

The mass in the bags does not matter, just the height of the water column is relevant. This is a bit unintuitive, but can be calculated easily. In terms of a big bag with a small hole at the bottom: Most of the weight of the water is kept by the surface of the bag, which is connected to the pole.

 

[wolf!]

I tried that online calculator, and it is indead cool, and does show an output of higher pressure. from the get go I knew the outcome was higher pressure. What I'm trying to find out is the logic why?

 

[wolf!]

I'm beginning to think the further the distance to fall creates increased velocity, which increases pressure?

 

[Studiot]

Careful with Andy's calculator. (But thanks to Andy for posting it)

Your mainline and piggyback lines contain flow restrictors which alter and complicate the pressure relationships.

There are two types of flow restrictor

The (connection) nozzles which introduce fixed pressure drops and the roller clamps which introduce variable ones.

Neither of these appear in the spreadsheet.

 

[wolf!]

r the sake of answering the question, we'll assume both have identical variables, except one is higher than the other, and absolutely nothing else is different.
I'm beginning to think the answer has not yet been found, and it is considered an accepted truth, without logic, because this thread in going in 10 different directions, except the question.

 

[Studiot]

What's the problem?

The piggyback bag produces a slighty higher pressure at the connection point due to its slighty higher elevation.

This will make the fluid flow faster in the auxiliary line.

However the fluid in either the mainline or the auxiliary can be slowed by the use of the roller clamps.

 

[wolf!]

yes, I understand that, what it does. my question is why.
Why does The piggyback bag produce a slighty higher pressure at the connection point due to its slighty higher elevation.

 

[Studiot]

It is well known that water tries to flow downhill.

So it is down hill from the upper bag to anywhere in the lower bag

The underlying driving force is gravity and we measure this tendency as pressure head which is the difference of elevation.
Downhill is the direction that makes this difference positive.

 

[wolf!]

so the reason for the higher bag draining first is not known at preset?
all that is known is that it happens?

 

[mfb]

The reason is known and was explained ~5 times here in this thread.
The higher bag leads to a higher potential pressure at the connection, this leads to a higher outflow.

 

[Egregious]

so the reason for the higher bag draining first is not known at preset?
all that is known is that it happens?

Hi Wolf!, I'm a nurse, too. From reading this thread, I think I understand the answer. Think of it this way: Imagine a really long tube extending vertically that contains an entire liter of fluid. Now, imagine a shorter tube (same diameter) extending vertically that contains only 100mL of fluid. For all practical purposes, you may ignore the volume of the bags that the tubes are connected to... Think about the very tall column of water in the 1L tube with its higher weight, pressure, and 'desire' :-) to flow first. The fluid (column) in the lower, and thus 'less tall' tube will simply have to wait...

...does that help?