# Piggyback Calculation Question

1. Hi, all!

My clinical group and I are having a hard time figuring this calculation out. Actually, even my instructor is having a rough time. She's telling us one set of numbers and we're getting another. I just don't understand her rationale. If some of you will have a go at it, I'll post our answers in a while. Here's the problem:

The order is for Iron via IVPB. It is a 250mL bag that needs to be run by pump over 90 minutes. What do you set the pump to?

We have two different answers with two different theories. I'm really interested to know what all of you think!

Ionafey
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3. TV=total volume to be infused in mls divided by TT=Total time

250mls/90 min.
=2.7mls/min times 60mins =166.6 rounded off to 167mls per hour.

Hope this helps

250ml over 90min you're just making the fraction 250/90 which gives you mls per minute. Then multiply by 60 to get mls per hr
5. Actually, that is the answer we all got: 166ml/hr. It is the clinical instructor who said that pharmacy told her that is wrong. She said that it is actually supposed to be 187ml/hr. Her reasoning is that you calculate it for 2hrs and then subtract 30 minutes . . . well, I'm not exactly sure what her take on it is. I don't understand it and was hoping someone here could help me figure it out. It's something about the pump being set for an hour or something. I'm really at a loss. However, if this esteemed community says that it is 166 and provided a rationale, I may be able to tactfully talk to her about it. There are ten of us in this clinical group and we should not be told how to calculate them the wrong way!

Anyone with insight into this?

Ionafey
6. Sounds as if we all learned it the same way, and not the way the pharmacy is telling her. I would have done it 250mL divided by 90 times 60 as well.
7. Quote from Ionafey
Actually, that is the answer we all got: 166ml/hr. It is the clinical instructor who said that pharmacy told her that is wrong. She said that it is actually supposed to be 187ml/hr. Her reasoning is that you calculate it for 2hrs and then subtract 30 minutes . . . well, I'm not exactly sure what her take on it is. I don't understand it and was hoping someone here could help me figure it out. It's something about the pump being set for an hour or something. I'm really at a loss. However, if this esteemed community says that it is 166 and provided a rationale, I may be able to tactfully talk to her about it. There are ten of us in this clinical group and we should not be told how to calculate them the wrong way!

Anyone with insight into this?

Ionafey
God help me - forgive me here, but I'm confused as to how pharmacy is subtracting a time (30 minutes) from a RATE in mL/hr. You can't subtract unlike units! Having worked as a pharmacy tech, I can tell you THEY are also very wrong!

And the instructor is also frightening me.

Infusing 250 mLs over 90 minutes gives you 166 - 167 mL/hr. And if you have any doubts about your own sanity in this (I know it's easy to get yourself confused, God knows I've done it enough to this point - this threw me so for a loop I found myself recalculating 250 mL/hr over 2 hours - regardless of what planet this pharmacy thinks it's on, that's 125 mL/hr, and you can't take time off of a rate), program an automatic infusion pump and let it tell you the answer.

Someone should hang that tech - or even the RPh - by their toes with IV tubing. For HOURS.

Edited to add - OMG - they've calculated 125 mL/hr, HALVED that to get THAT rate over 30 minutes, and then ADDED 62.5 mL/hr to 125 mL/hr. That's the 187.5.

No one doing that calculation on the pharmacy/instructor's end has BOTHERED to reverse it, and see if you do that, you'd be INFUSING MORE FLUIDS THAN WHAT YOU HAVE ON HAND - you'd be infusing 375 mL over a two-hour period - mathematically impossible if you only have 250 mLs in the bag!!!

YIKES YIKES YIKES!!!!
Last edit by carolinapooh on Feb 14, '07
8. Using the pharmacy/instructor formula, you would actually infuse 280.5 ml in the hour and a half the IVPB was hanging. Hope you have an adequate amount of flush and a patient who is not on fluid restriction.
T.