The top 5 drug calculations every vet nurse needs to know
Calculations are one of the most important skills for us to learn as veterinary nurses… but they can also be one of the most confusing.
We use so many calculations daily in all of our patients, from calculating drug dosages to calculating resting energy requirements and making feeding plans, to calculating and administering fluid therapy, and much more.
But I’m going to (perhaps controversially) say that our medical (and critical care) patients are arguably the ones who use this skill the most. Why? Well…
They’re often on a cocktail of medications (either given as intermittent doses or constant infusions)
They’re usually inappetent or anorexic, requiring calorie calculating and nutritional planning
They’re also often dehydrated, due to anorexia and/or increased fluid losses, requiring careful rehydration
Some are also hypovolaemic, requiring emergency fluid resuscitation… meaning our fluid therapy calculations suddenly need to get a lot quicker!
To help you calculate and administer treatment to your medical patients easily (when you’re juggling a busy ward, incoming emergencies and a ‘to-do’ list that’s growing by the second) here are my top 5 drug calculations I think every vet nurse needs to have in their back pocket.
Free on July 24th? I have something for you…
Want to feel more confident with calculations?
I have a workshop coming up that will really help you with this!
Join me on July 24th at 7 pm BST for a deep dive into calculations. We’ll be chatting through all the calculations we commonly encounter in practice, simplifying them by teaching you my tips, tricks and hacks to calculate them easily, and we’ll work through a ton of practical examples together on the call!
On top of that, everyone who attends gets:
A free copy of the VIMN digital calculations guide
A workbook of example questions to accompany the call
A certificate for 90 minutes of CPD
A recording of the live call to watch back whenever you’d like!
To save your spot, click here, or DM me the word calculations on Instagram to get the link sent automatically to your inbox!
Calculation #1: Drug dosages (and expressing them in mg/kg NOT ml)
Yes, this one seems simple, but bear with me, because there’s a really important point to keep in mind with these.
First up, how do we calculate our drug dosages?
Simply, we take the dose (in mg/kg, for example) and multiply this by the body weight (in kg), then divide by the concentration of the drug (mg/ml) to get the total volume required.
But we also need to think about how we express our drug doses…
As well as actually calculating the volume of drug our patient needs, it’s also really important to think about how we express our drug dosages.
Let’s look at an example to dive into this further. Say I have a 5kg cat with cholangiohepatitis, and I’m giving them a dose of buprenorphine. I can express that one of two ways:
The dose they’ve received (0.02mg/kg)
The total volume they’ve received (in this case 0.02mg/kg x 5kg / 0.3mg/ml = 0.33ml)
Which of these gives me more information about what my patient is receiving? Their dose does.
Try and get into the habit if possible of expressing your patient’s drugs on your hospital sheets, at handovers, etc in mg/kg, rather than total ml.
It gives us an idea of how much of the drug they are actually receiving, so for things like analgesia, for example, we know how uncomfortable they are, and how much wiggle room we have to adjust their dose.
For example, the dosage range for buprenorphine is 0.01-0.03mg/kg, so if a patient is on 0.01mg/kg and they still seem painful, we quickly know that we can top them up or increase their subsequent doses.
When we work in total volumes, we can’t do this - as the volume is dictated by the patient’s body weight, so we have no idea what that volume actually means for that patient.
Calculation #2: Converting units
I used to HATE it when I had to calculate things that weren’t in mg/kg. It hurt my brain, and I could never remember how to adjust the calculation to make the maths easy.
But it actually is pretty simple, you just need to remember 2 rules:
To convert micrograms/kg to mg/kg you need to divide the dose by 1000
To convert grams/kg to mg/kg you need to multiply the dose by 1000
Knowing how to do this is important because if our units are not the same on either side of our calculation, we won’t be able to do the maths!
So before you do your calculation, look at the dose and the units. If they’re the same, you’re doing a simple calculation as we outlined above - but if they’re different, you’ll need to convert them so they match.
Let’s look at an example…
Say we’re pre-medicating our 5kg cat prior to their abdominal ultrasound and oesophagostomy tube placement, and you’re going to give them 2mcg/kg of 0.5mg/ml dexmedetomidine.
You’ll need to convert your dexmedetomidine dose to mg/kg, so it matches your concentration.
2mcg/kg / 1000 = 0.002mg/kg
0.002 mg/kg x 5kg = 0.01mg
0.01mg / 0.5mg/ml = 0.02ml
They’ll need 0.02ml of dexmedetomidine to go alongside their 0.33ml buprenorphine.
Calculation #3: Diluting medications that are too small to draw up or administer
We’ll often end up in a situation where our drug volumes are too small and need to be diluted.
This is especially true for very small patients, very concentrated medications, or very small doses.
But how do we do this?
We first need to calculate a dilution factor - ie. the volume we’ll need to dilute our drug by to reach a set concentration. Once we’ve got this, we can then easily work out how much neat (undiluted) drug, and how much saline/water for injection we’ll need to dilute it.
Here’s how to do it:
Divide the neat concentration by your new desired concentration. This gives you your dilution factor.
Take the total volume of your diluted drug that you’ll need, and divide this by the dilution factor. This gives you the volume of neat (undiluted) drug you’ll need to add to your syringe.
Take the total volume of your diluted drug that you’ll need, and subtract the volume of the neat/undiluted drug you just calculated. This tells you how much diluent to add.
Mix your diluent and undiluted drug together and you’re done!
Calculation #4: How to convert a percentage solution to mg/ml
Does anyone else’s brain hurt with percentage solutions?
I honestly used to hate them so much - in fact, I remember sitting in college as a first-year SVN trying to read them and just deciding that I never used percentage solutions in practice, so I didn’t need to learn them!
I wish I could go back and tell SVN Laura that they are not as complicated as she thinks (and also give her a kick up the bum for ‘just not learning them’).
Because the truth is, there’s a really easy way to convert a percentage solution to mg/ml… and that’s to add one decimal place to the end.
A 2% solution becomes 20mg/ml,
A 5% solution becomes 50mg/ml,
A 15% solution becomes 150mg/ml, etc!
Then, once you’ve got your percentage solution converted, you can use it in any other drug calculation!
Calculation #5: How to administer medications as constant rate infusions
There will be times when we need to administer our drugs as continuous infusions, not intermittently.
This has a few advantages:
It eliminates the peaks and troughs associated with intermittent dosing
It allows us to control our doses more closely and adjust our doses more rapidly
There are a couple of ways we can administer CRIs: either in a separate line via a syringe driver, or in a patient’s IV fluids.
Personally, I prefer the syringe driver method, since you can adjust the rate independently of the patient’s fluid therapy - whereas, with the other method, any changes to the patient’s fluid rate will adjust their CRI dose, too!
Here’s how to do it:
If you’re using a syringe driver:
Dose (mg/kg/hour) x weight (kg) / concentration (mg/ml) = rate in ml/hour
Don’t forget, though, that you’ll need to convert your units if they don’t match… And with CRIs, you also need to make sure your time measurement units match, too! If your dose is per minute, but you need a rate per hour, you’ll need to convert that too.
If you’re using a fluid bag:
Once you’ve got your rate of the drug in ml/hour, you’ll need to multiply this by the number of hours the patient’s fluid bag will last - this tells you how much of the drug needs to be added to the bag.
Bag size (ml) / fluid rate (ml/hour) = number of hours the bag will last
Drug rate (ml/hour) x number of hours = volume of drug to add to the bag
Then you set the pump at your patient’s desired fluid rate, knowing it is also delivering their CRI at the same time!
So that’s it, my 5 top drug calculations I think every VN needs to know! Whether you’re calculating a simple drug dose, diluting a medication, converting units or converting a percentage solution… or even giving your drug via a CRI, you’ve now got the tools to do it quickly and easily.
I hope this has helped you - let me know on Instagram which one you’ll be using with your next medical case!
PS. Don’t forget to join me on July 24th if you want more examples just like these! There are still a few tickets left, so RSVP to save your spot here!
