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Medical Math - Titratable CRI

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Megan Brashear, CVT, VTS(ECC), walks through the calculations necessary to create a titratable medication CRI. These calculations will allow you to create a CRI where 1ml/hr = 1mcg/kg/hr (or 1mcg/kg/min) so the dose to the patient can be changed quickly and easily. Common examples of this are fentanyl and dopamine.

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Megan Brashear's picture
Megan Brashear

CVT VTS(ECC)

Enrolled: 07/2011

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Heather B's picture

Would you take the same amount of saline out first before you add that amount of drug, so the combination of the two is still at 100ml in the bag?

Megan Brashear's picture

Hi Heather, you are correct. Since you are doing all of your math for an end volume of 100ml, you want to remove an equal amount of saline from the bag before adding your drug. This is what "QS" means on the label of these CRIs.

Gary  Anderson's picture

I try to teach my students to maintain units through their calculations. It would be great if units could be added throughout yours as well.

Leigh Mulvey's picture

Where can we find the answers to the questions on the worksheet? I want to make sure I made the correct calculations.

Beth Ann Fretz's picture

Hi Leigh,

Thank you for bringing this to our attention. The answers to this worksheet are now on the last page.

Morgan Fleming's picture

Would you be able to elaborate on similar dilutions (specifically for Fentanyl) for smaller patients, when you are wanting your end volume to be much less than 100 mL? Thank you!

Christy Michael's picture

Hi Morgan, what you ask is an excellent question! Often we make up fentanyl CRIs into syringes instead of 100ml bags as well. The only change in the calculation is to change the 100 to the final volume that you desired. In Megan's calculation for fentanyl, if she wanted to create a CRI for a 20kg patient in a 60ml syringe instead of a 100ml bag, she would have multiplied the 20 times 60 instead of 100 and the rest of the math is the same.

Math with written words is a lot harder so I'll show you a less-words version as well:
- 20kg dog so if 1ml/hr = 1mcg/kg/hr we want our CRI concentration to be 20mcg/hr (also 20mcg/ml final concentration, this part doesn't change)
- If we want the total volume to be 60ml then we multiply 60 * 20 = 1200 mcg / 60ml
- 1200 / 50 = 24ml fentanyl qs 60ml of 0.9% NaCl

Please let me know if that doesn't make sense!
Christy

Jenny Sanders's picture

At the hospital I work at, we often only pull up a certain amount of drug (fentanyl, dopamine, etc.) into a syringe without adding any saline or fluids and load the syringe into a CRI pump that will go into one of the ports of the regular fluid line that is running. Most of the techs just use an automated CRI calculator to figure out their numbers, but I don't always trust it so I am wondering what would be the difference in the calculation if you don't add any fluids to the drug and then what you would set the rate at on the CRI pump?

Sarah Harris's picture

Hi Jenny. That's a great question because medications can also be delivered undiluted as a CRI. I'll give you an example of an order from a doctor and the math required to figure out mL/hr needed.

DVM orders: 1mcg/kg/hr of Fentanyl(0.05mg/mL) to be administered to a 40kg dog

We can solve this with three steps:
1. Figure out how many mcg/hr the patient needs:
1 mcgx40 kg=40mcg/hr
2. Convert mcg to mg:
40mcg/1000=0.04mg/hr
3. Convert mg/hr to mL/hr (Want/Have):
0.04mg/0.05mg=0.8mL/hr

Answer= 0.8mL/hr of undiluted Fentanyl is the rate to be entered into your CRI pump

Hope this helps!

Laura Waller's picture

Hi, great video! Thanks. My hospital uses a syringe pump to deliver CRI's separately from fluids. Is there a reason it would be better to add it to fluids? For example, we will make up FLK CRI's, put it in a syringe pump and that's it. I can see how it would be beneficial to have them separate, so you can change each drug individually, easily knowing your dosage. I find that we tend to back off by percentages - for example, back off on the FLK by 20%, so all drugs are being decreased vs just one. Would that be the best reason to mix them with fluids? I'm a little worried about fluid intake as well, it seems like it would overly complicate how much fluids the patient is getting. Thanks in advance. Laura

Sarah Harris's picture

Hi Laura. Just like you, we frequently deliver our CRI's using syringe pumps for exactly the reasons you stated. However, some hospitals don't have the same resources and may not have access to syringe pumps, making administration through a fluid bag easier. Even with syringe pumps available there are a couple of reasons that make it beneficial to add medications to fluid bags. One reason is if the drug being administered is a very small total volume. It can be helpful to dilute it down to a volume larger than you are able to in a syringe, to help facilitate it getting to the patient in a reasonable amount of time than is possible using a lesser volume in a syringe. Another reason that we choose a fluid bag over a pump is related to the cost of refilling a CRI. Some patients we know, right off the bat, that they will be in the hospital for multiple days. Refilling a CRI requires nurse/staff time and the use of additional supplies. We account for this in our hospital by having a "CRI refill" charge. Lastly, you brought up a really valid point about fluid intake. With the growing use of polypharmacy in our treatment therapies we always need to consider the total volume of fluids the patient is taking in. We individualize this therapy based on the cardiovascular and hydration status of our patients. Frequently, in patients that are most susceptible to fluid overload we will decrease the volume of the maintenance fluids as we increase the fluid rate of our CRIs. Thanks for the great questions!

Emily Chinn's picture

Hypothetically, say I have a 100 lbs dog, and my doctor wants me to make my Fentanyl CRI at 5 mcg/kg/hr at 25 mls/hr and they want it in a 500 ml bag of LRS so that it lasts approx 20 hours if we for some reason have to maintain at 5 mcgs/hr or 25 mls/hr in the case that the patient is uncomfortable, ect. How would that math look?

Carolyn Tran's picture

Hi Emily! Great question. I had one of our criticalists, Dr. O'Marra, help me with this one. There's a few ways to get what you need. It's helpful to start by clarifying what your goal is:
5mcg/kg/hr of fentanyl diluted in 25mL/hr LRS

We are starting with a 100lb dog, which is 45.5kg.
That means we want 45.5kg*5mcg/kg/hr=227.5mcg of fentanyl delivered per hour.
Now all that is left to figure out is how to dilute that fentanyl. There are multiple ways to arrive at an answer, you can pick whichever is most intuitive to you.
Option 1:
You have a 500mL bag of LRS that you want to run at 25mL/hr. That means that you have 20 hours worth (500mL divided by 25mL/hr) of fluid in your bag.
We know that we want to give 227.5mcg/hr of fentanyl, and that our bag will last 20 hours, so by multiplying 227.5mcg/hr by 20 hours, we find that we want our bag to contain 4550mcg total of fentanyl (20 hours worth of our hourly fentanyl dose)
4550mcg fentanyl divided by the concentration of fentanyl (50mcg/mL)= 91mL fentanyl
We then remove 91mL LRS from our 500mL bag and add 91mL fentanyl, so our total volume remains 500mL.
Option 2:
We know we want to deliver 227.5mcg fentanyl per hour in a total volume of 25mL/hr.
By dividing our mg/hr dose by our mL/hr, you will be left with the concentration of your solution in mg/mL. (227.5mcg/hr)/25ml/hr)= 227.5mcg/25ml (the per hr cancel each other out)
227.5mcg/hr fentanyl divided by 25mL/hr = 9.1mcg/mL fentanyl. This represents the concentration of drug that you want to have in order to deliver 5mcg/kg/hr of fentanyl to this patient at 25mL/hr. Once you have the concentration, you can multiply it by any volume in order to determine how much fentanyl to add to the bag.
For a 500mL bag, we simply multiply 500mL * 9.1mcg/mL fentanyl = 4550mcg fentanyl in a 500mL bag.
4550mcg fentanyl divided by the concentration of fentanyl (50mcg/mL)= 91mL fentanyl
We then remove 91mL LRS from our 500mL bag and add 91mL fentanyl, so our total volume remains 500mL.

Please let me know if this is too confusing. I'd be happy to show my work in a diagram if that would help. Thanks!
-Carolyn