IV fluids, Veterinary Pharmacology
Hi pharmacology students, this is Dr.
Today I'm going to be talking about IV fluid calculations.
As aveterinary technician, it's often up to you to both place an IV catheter, and afluid administration set, and hook a patient up to fluids, and possiblycalculate a fluid drip rate or maybe calculate a constant rate infusion rate, and then, it will be up to you to maintain that patient's catheter, and make sure that thepatient is receiving the proper amount of IV fluids or medications.
So we'll betalking about formulas that we can use to calculate the amount of a medicationor the amount of fluid that's going to be administered either continuously orover a defined period of time.
In order to do this we have a selection ofvarious drip sets that we can use that are calibrated to deliver a precisenumber of drops per ml.
If you look in this picture and you see this littledrop coming out, the number of drops per ml differs based on the type of dripset.
The one that we most commonly use is called the standard macro drip set, andthat delivers 15 drops per ml.
This is called the “drop factor” and we use thedrop factor in our IV fluid calculations.
You will also see what's called a minidrip, or micro drip set used.
This delivers smaller drops, so there's moredrops per ml, 60 drops for ml.
There are other sizes of IV drip sets thatgive you 10 drops per ml and 20 drops per ml, so you need to know what you'reworking with in order to calculate fluid drip rates.
Let's look at the formula here.
This is abasic fluid rate formula.
So fluid rate, or commonly called a drip rate, so thiswill give you the number of drops per minute that you need to administer to apatient given a particular volume of fluid infusion, and a particular definedtime, and you also need to know the drop factor of the fluid administration setthat you're using.
So the fluid rate are in this case isgoing to be drops per minute we need to know the volume of the infusion in mls, we also need to know the timeperiod over which this patient is going to receive the fluids in minutes.
Nowthis is important right here has to be in minutes and then we need to know thedrop factor, going to do DF for drop factor, which is going to be in drops perml.
Now the answer is going to be in drops per minute, but we can alwaysconvert this to drops per second after we're done.
Alright, let's try a problem.
I've givenyou the fluid drip rate formula down at the bottom of this page, and up at thetop is a problem.
It reads: give 480 mls of lactated ringers solution over a fourhour period using a standard 15 drop per ml administration set.
What is the fluid drip rate? So we'relooking for the fluid rate or the fluid drip rate.
So the first thing I'd like to do inthese sorts of problems is find all my values.
So let's start with the volume.
What's our volume in mls? Our volumein mls is 480 mls.
Next a look for the time.
How long isthis infusion going to be administered? Well, in the problem it reads four hours.
So the time is foor hours, but we need to get that in minutes in order to plug itin.
So that's an easy conversion, because Iknow that one hour is equal to 60 minutes, and I can easily do thiscalculation.
That equals two hundred and forty minutes.
That's our time.
And the third factordown here is the drop factor, which is given.
In the problem we're using astandard 15 drop per ml administration set.
So now it's a matter of plugging inthe values.
So our fluid drip rate, which I'm going to write as R, equals thevolume of infusion, which is 480 mls, divided by the time of infusion, which is240 minutes, multiplied times the drop factor, which is 15 drops per ml.
I canlook at this real fast and see I'll be able to cross out mls, and I can see that my units are going tobe in drops per minute, so I'm going to do the calculations here.
480 divided by 240 x 15 equals 30 drops per minute.
Now this is great if you have the time tostand and adjust the fluid rate over the course of a minute, doesn't seem like a long time, but ifyou're counting 30 drops and then you have to readjust a couple times, that'sgoing to take you while.
You have something else to do, no doubt.
So we usually convert a fluidrate in drops per minute to either drops per second or drops for two seconds or fiveseconds or something that gives us a whole number so this is easy enough todo.
If I have 30 drops per minute I can easily convert two seconds 1minute is equivalent to 60 seconds I can see the minutes cancel out, 30 divided by 60equals a half, so 0.
5 drops per second.
Now, these administration sets aren'tmeant for you to count a half a drop so what we're going to do is to multiply ittimes something that's going to give us a whole number, and this is easy because0.
5 times 2 equals one, but we have to multiply both top and bottom by two sothat we're multiplying by 1, right? So 0.
5 times 2 equals one, and on the bottom, twotimes one second equals two seconds.
So this is a much a easier thing to workwith here, one drop every two seconds, you can easily adjust and count in twoseconds and adjust your drip rate and then go on and do something else.
Ok, now here's an example for you to try.
Go ahead and pause the video, try thisproblem, and when you're finished to go ahead and start the video again and I'llgo over the answer.
Alright let's find all our values forthis problem.
What drip rate would you use to delivera hundred and twenty mls, (that's our volume, ) of point nine percent sodium chloride? (We don't need to use this percentagethis just tells us what type of solution it is) over a two hour period, so we know the time is two hours, which comes out two hundred and twenty minutes, using a micro drip, or60 drops per ml administration set.
Alright so let's set it up.
Drip rateequals our volume, divided by the time in minutes times our drop factor.
I'm going tocancel out our mls, I can cancel out a hundred twenty over a hundred twentythat equals 1, so it's 60 times 1, this comes out to 60drops for a minute.
Okay, 60 drops per minute.
This is easy to convert to drops persecond.
60 drops per minute (remember one minute is equivalent to 60seconds) minutes cancel, 60 divided by 60 equals one drop per second.
Let's go over the next type ofcalculation you might run across, these are called constant rate infusions, orCRIs, and I'm going to tell you that these are universally hated by bothveterinarians and veterinary technicians alike.
However, I believe it's because most ofthe time the technicians or the vets don't have a really good formula on handto use.
That being said, nowadays there's different types of drug calculators orspreadsheets or whatever that you simply plug numbers in and it'll do thecalculations for you.
However I do think it's important for you to have at leastone tool to be able to use if you're asked to calculate a constant rateinfusion.
So what we're talking about here aresome sort of medications that are given via an IV infusion and typically they'redrugs that either have a really short half-life or they have to be given invery, very, small amounts and they're often given along with fluids at aconstant rate over a particular period of time.
Sometimes we'll give constant rateinfusion of let's say an analgesic like fentanyl or something like that duringan especially painful surgery.
Maybe we have to control an arrhythmia in apatient so we're giving lidocaine or something like that in an IV infusion.
You will see when working withconstant rate infusions that the actual dosage rate of a constant rate infusion, it looks sort of similar to a drugdosage rate, it's typically in micrograms per kilogram per minute.
So if you look carefully at this here's the”looking like a dosage rate” part here, micrograms per kilogram, except you'll usually see in micrograms because we're working with much smalleramounts.
So we're working with a small amount of a drug in micrograms perkilogram of body weight.
But the only difference here is that it's over a course of a period oftime, so I added time in.
So micrograms per kilogram per minute is going to bethe dosage rate of the constant rate infusion.
Now here I've given you a formula, there are many ways to do CRIcalculations.
I think for your purposes if you haveone formula or like one tool that you can use your going to be better off.
Theproblem is having to memorize it.
Well, I put this formula on our formula sheetfor you to use when you need it.
It may be something that you want tokeep somewhere and make sure you know what all these letters mean so this isimportant M equals D times W times V over R timesthis weird number.
Alright what are these letters? “M” isthe number of milligrams of drug to add to a base solution.
“D” is a dosage rate of the drug that'sgoing to be given at the constant rate infusion and again I mentioned that thedosage rate for the CRI drugs is usually in micrograms per kg per minute.
“W”, asbefore, is the body weight of the patient in kilograms, and “V” is the volume inmls of the base solution.
so the base solution could be like asmall bag of IV fluids or lactated ringers solution or something like that.
And “R” is the rate of delivery in mlsper hour.
I'm not going to go into detail now butwe have automated infusion pumps which make it much more easy to deliverconstant rate infusion drugs, where you can set this up and you can plug in themls per hour, but this is how fast we're going to give this infusion anddown here is a conversion factor that's key to this whole formula right here.
Now let's try to do a constant rateinfusion problem the problem.
On the top over on the left-hand side are thedifferent values that we need to figure out and over on the right-hand side isthe formula we are going to use and an explanation of all the differentletters.
Alright let me read the problem first.
A 44 pound dog with CHF, that means congestive heart failure, is ordered to receive a dopamineconstant rate infusion at five micrograms per kilogram per minute.
You will add 200 milligrams of dopamineto a one liter bag of LRS solution, that's lactated ringers solution.
This here is just telling you what yourconcentration is, that solution you just made up at what rate in mls per hourwill you administer the solution to deliver the correct dosage.
So the firstthing we're going to do here is to figure out what all our values are andI'm going to use this down here to help me remember what my letters mean.
Let'sstart at the beginning.
We have a patient that's 44 pounds, we need weight in kilograms.
I did thecalculation for you, 44 pounds divided by 2.
2 equals 20 kilograms.
The drugthat's going to be administered as a CRI is dopamine at a rate of five microgramsper kilogram per minute, so that is “D”.
The dosage rate of the drug in micrograms perkilogram per minute so “D” equals 5 micrograms per kilogram per minute.
Don'tever leave off your units when you're writing out formulas, that gets you introuble then you'll forget what you end up withwhen you get your answer.
You will add 200 mgs of dopamine.
Well that's our “M” right here the numberof milligrams of drug to add to the base solution, all right we're going to put that up here, 200 milligrams of dopamine to a one liter bag of lactated ringers solution, here's this, “V”, 1 liter, let's just changethat to milliliters, 1000 mls.
At what rate in mls per hour will youadminister the solution to deliver the correct dosage? So “R” is going to be arate, so this is what we don't know this is what we want to know right here.
“R” isthe rate of delivery of your drug in mlls per hour.
So we can plug in all ournumbers here.
The interesting thing here is that we are solving for R, so ifwe want to rework our formula we can look at it this way, we're going to move this up and we're going to movethis “M” down here.
We're going to have “R” equals “D” times “W” times “V” divided by “M” times sixteen point six seven.
I just moved things around a little bit, it'sthe same formula.
So what we're going to do is plug in allour numbers into this formula.
So I'm going to go to another page and do thatso I have more space to work.
Ok let's go ahead and plug in all ourvalues in our constant rate infusion here.
So we're looking for “R” in this case.
“R” is the rate of administration of ourdrug that we're giving.
“R” equals “D” which is our drug dosage rate, which isfive micrograms per kilogram per minute.
“W” is the weight of the patient, 20kilograms.
“V” is a volume of the total amount ofsolution we're putting our drug into, which is a thousand mls, “M” is theamount of dopamine that we are adding to the fluid, which was 200 milligrams, sixteen point six seven is ourconversion factor.
Take a look at the unit's here, see ifyou can cancel anything out.
I can see I can cancel kilograms here, minutes andml stays.
I should be able to cancel out myweights but one of the problems is that the units are different so we're goingto have to get them the same.
I think what i'm going to do is convert 200milligrams to micrograms, so then we can cancel our micrograms.
So I know that ifI move my decimal point three places to the right, so our value instead of 200milligrams comes out to be 200, 000 micrograms.
So we're going to use this inour formula instead.
I'm going to move this up into my formula right here.
Ok, now let's do the math, well we cancancel out micrograms, now let's do the math, so 5 times 20 times a thousanddivided by two hundred thousand and sixteen point six seven equals zeropoint zero three, and our answer is going to be in mls per minute.
Now, if we needed to convert from mlsto minute to mls per hour, let's say our infusion pump is settingis in mls per hour, that's easily changed.
03 mls perminute I know that 60 minutes is equivalent toone hour, cancel out our units, 0.
03 times 60 equals1.
8 mls per hour.
I've spoken for too long in this video, so instead of giving you a practice problem in the video, I think we should move on to the IVfluids quiz on canvas.
There are a number of different fluidrate and CRI problems and we will go over that class.