Drug Calculations Made Easy
A tried and true method of performing drug calculations.
Many times, in the critical care transport world, we must perform multiple drug calculations in order to care for our critically ill or injured patients. This method will help to make you more efficient and accurate when performing this task.
By Dan Turner, Retired UMMC Flight Nurse
Do drug calculations give you a headache? Do you and your partner argue about the proper pump rate? Do you ever just take someone else’s word that a pump rate is correct or do you just “wing it” and hope that everything turns out alright? If you answered yes to any of the above questions, this is the article for you!
The technique that I’m going to share with you will work for single IV push volumes or for finding how many ml/hr to set your IV pump for. Once you get this down, you’ll be able to calculate a drug dosage simply by entering it into a calculator as you say the dose aloud. Before we get started, I want to set a few ground rules:
1. Your formula will require you to multiply all numbers of your desired dose and then divide it by the available concentration.
2. Units in the available concentration must be converted into the same units as your desired dose.
3. Don’t forget to multiply the volume of the drug you’re using with the desired dose line, whether it’s in a vial, premixed bag, or in a bag you mixed yourself.
Alright—let’s get started.
The format that I will be using for the scenarios and their calculations will look like this:
Ordered: Drug 3 mcg/kg/min
Patient weighs 70 kg
Available: 500 mg in 250 ml
I know we mostly work off of protocols and do not require orders for each drug that we administer during a transport, but let’s just pretend that you called your medical control physician to give an update prior to transport and before you could hang up, he ordered these drug dosages for a particular scenario. Let’s start with a simple, single IV push calculation.
1. Ordered: Versed 0.1 mg/kg
Patient’s weight: 17 kg
Available: 10 mg in 2 ml vial
Ok, let’s say the desired dose as we enter it into our calculator.
0.1 mg x 17kg x 2 ml vial= 3.4 divided by 10 mg =0.34 ml
0.1 x 17 x 2
—————— = 0.34 ml IVP
See what we did there? It’s that simple. Say it as you enter it then divide by the concentration available.
Now let’s do one for mg/min.
Ordered: Lidocaine 3 mg/min
Patient weighs: 75 kg
Available: 2 Gm in 250 ml
3 mg x 60 minutes in an hour x 250 ml, divided by 2,000 mg = 22.5 ml/hr
3 x 60 x 250
—————— = 22.5
Anytime “per minute” is used in a desired dose, you have to multiply x 60, as in sixty minutes in an hour. Also notice that since the desired dose was in mg, I converted the available concentration from Grams to mg—that’s very important for you to remember!
A Quick Word on Converting Units
To convert mg to mcg, simply add three zeros. 1 mg = 1,000 mcg. To convert Grams to mg, simply add three zeros. 2 Grams = 2,000 mg. Now, let’s convert Grams to mcg. What is that you said? Yes—it’s that easy: 2 Grams = 2,000 mg = 2,000,000 mcg. Easy stuff, huh? And we thought this stuff was difficult. Lucky for us—it’s not difficult at all. We just have to do the calculations in a structured fashion.
Ok, next one:
Ordered: Dobutamine 3 mcg/kg/min
Patient weighs 80 kg
Available: 500 mg in 250 ml
Let’s do it!
3 mg x 80 kg x 60 minutes x 250 ml divided by 500,000 mcg.
3 x 80 x 60 x 250
————————= 7.2 ml/hr pump rate
That’s all there is to it! Just remember to simultaneously say and enter your desired dose, multiply x volume of available drug last on the top line, then divide it all by the concentration that has been converted to the same units as your ordered dose. Well done! You should now appear to be a genius to your colleagues. Please let me know what you think about the article and thank you for reading.