The top 3 calculations to help you nurse your medical patients
I thought I was pretty confident with calculations until I made the leap into referral practice - and all of a sudden had to calculate things other than IV fluid rates, simple drug calculations and the occasional tube feeding plan!
The truth is, our medical patients provide us with SO many opportunities to use our brains - as well as planning their nursing care, there are a lot of calculations we commonly perform when caring for them.
In this post, I’m talking you through the essential calculations you need to know about when it comes to caring for medical patients. By the end of this post, you’ll be a pro at calculating medications, fluid therapy, and nutritional requirements!
1: Medications
When it comes to drug calculations, there is a lot we need to think about. We need to know not just how to calculate basic drug volumes, but also how to:
Convert units,
Convert percentage solutions to mg/ml concentrations,
Reconstitute powdered medications for administration, and
Dilute medications
To start, let’s take a look at our fundamental drug dosage calculation:
To calculate a dose of liquid medication (either via injection or an oral solution), simply multiply the dose (in mg/kg) by the patient's body weight (in kg), then divide it by the concentration of the drug (in mg/ml).
When calculating tablets, the maths looks a little different. Here we multiply the dose (in mg/kg) by the patient's weight (in kg), then divide by the tablet size (in mg). This gives us the number of tablets required for the patient's dose (in reality, this often needs a little rounding to accommodate for tablet size, and how easy it is to divide the tablets).
These calculations only work if the concentration and the dosage of the drug are both in the same units. If one-half of your calculation is in grams or micrograms, and the other half is in milligrams, you’ll need to convert it before going ahead and calculating your dose.
Another problem we can encounter when calculating drugs is if the concentration is expressed as a percentage, not in mg/ml.
If you have a percentage solution, you’ll need to convert this before calculating the amount your patient needs - this is usually a case of moving the decimal place. There are exceptions to this rule, though, so be careful if using this shortcut!
You may also find yourself in a situation where the ‘standard’ medication is too concentrated to safely administer - meaning your dose will be too small to draw up! If this is the case, you’ll need to dilute your medication to make a new, less concentrated formulation.
We have two methods of giving injectable medications - as single doses (or boluses as they may also be called) or as constant rate infusions.
CRIs are a great option for certain medications as they eliminate the peak and trough effects of intermittent dosing, and allow us to closely control the dose our patient receives.
There are two ways of administering CRIs - and the maths differs between them. We can either administer a CRI via a syringe driver (in a separate line to our patient’s intravenous fluids) OR within their fluid therapy.
Bear in mind, though, that any CRIs you give as part of your patient’s IVFT also need to consider your patient’s fluid rate. If you turn your patient’s fluid therapy down, you’ll also reduce their CRI rate.
2. Intravenous Fluid Therapy
The way we give intravenous fluids to our patients depends on whether they are dehydrated (where fluid losses come from the interstitial space) or hypovolaemic (where fluid losses come from the intravascular space).
Dehydrated patients require their fluid deficit to be corrected at lower rates, over longer periods - usually around 12-24 hours.
To calculate fluid rates for dehydration, we first need to calculate their fluid deficit. We do this by estimating their percentage dehydration and then multiplying this by 10.
On top of this we add the patient’s daily maintenance requirements, and measure and account for any ongoing losses, through things like diarrhoea, vomiting, polyuria or chest/wound drains.
Remember - fluids are not a benign treatment! They have complications and contraindications just like any other medication, and this means we need to monitor their use carefully.
Reassessing our patients’ fluid balance is a vital nursing consideration for our medical patients - especially in patients who have things like renal disease or cardiac disease, and are less tolerant of fluid therapy.
So keep a close eye on those hydration parameters, and monitor for signs of fluid overload!
Fluid rates for hypovolaemic patients are very different. These patients require high fluid rates to be administered rapidly, for a short space of time. It’s essentially the opposite of how we correct dehydration - and calculating these rates carefully is very important, as we risk overloading the patient if we run the bolus for too long.
To calculate a fluid bolus:
Calculate the total bolus volume the patient needs
Calculate how many of those boluses fit into 1 hour
Multiply the bolus volume by the number of boluses that ‘fit’ into 1 hour to get your ml/hour rate for your infusion pump
3: Nutritional Requirements
Nutrition is an absolutely vital consideration for our medical patients! So many of them present with inappetence, anorexia and weight loss, requiring careful monitoring of their calorie intake as they recover.
Nutritional planning is also an area where we really get to use our nursing skills - from determining what to feed, to how much to feed, to monitoring response to feeding and much more!
Firstly, we need to know our patient’s resting energy requirement (RER). This is the number of calories required to maintain the body in a state of rest, in a thermoneutral environment.
There are several different calculations which can be used, but the allometric formula RER = 70 x (bodyweight^0.75) is considered the most accurate, especially in very small or very large patients.
There may also be times we need to include life stage or lifestyle factors in our patient’s nutritional plan. We’re not going to do this in all of our patients, especially those who are unwell or at an increased risk of refeeding syndrome - but we frequently do this in puppies and kittens, for example.
Once you’ve got the number of calories your patient needs, it’s just a case of dividing that by the calorie density of your food, and then determining how many meals your patient needs each day.
So that’s it - my top calculations to make nursing your medical patients easier!
If you want the opportunity to put these into practice and learn even more calculations to help nurse your patients, make sure you join me on November 14th at 7.30 pm GMT for the free medical nursing workshop! Sign up for your free place here!
References
Olson, M. 2022. Energy calculations: gauging the proper caloric intake for patients. Today’s Veterinary Nurse [Online]. Available from: https://todaysveterinarynurse.com/nutrition/veterinary-energy-calculations-and-proper-caloric-intake/
