## Drug Math Tutorial Part I

Becoming an educator was one of the most eye-opening experiences I have had since I first started in EMS. Remembering what it was like being a student, and stripping away all my experience was a pretty hard task. Since I have started teaching I have noticed some trends. I am hoping to provide another resource of education for the prehospital clinician. Where better to start than the areas I see the most people struggle.

This series is aimed towards the paramedic student, but it could be a refresher for the rest of us. I will be reviewing drug administration and calculations. Enjoy the tutorial, and provide comments if I mess up!

In this section I am going to review some simple conversions and units of measure.

Weight:
Unfortunately when we are doing drug calculations, we have to use kilograms instead of pounds. This can be a pain in the butt for some people, but it isn't as hard as you might think.

lb = pound
kg = kilogram

1 kg = 2.2 lbs
5 kg = 11 lbs
10 kg = 22 lbs

If you can remember that for every 5 kilograms you have 11 pounds and 10 kilograms you have 22 pounds you can make your life a little easier by just memorizing a few weight conversions.

20 kg = 44 lb ____ 60 kg = 132 lb
30 kg = 66 lb ____ 70 kg = 154 lb
40 kg = 88 lb ____ 80 kg = 176 lb
50 kg = 110 lb ____ 90 kg - 198 lb

So if you have someone that is 121 lbs, you know that 110 lbs is equal to 50 kg and every 11 pounds is 5 kilograms, so 121 lbs = 55 kg.

If you memorize those weights, you can double them for heavier patients.

Since 50 kg = 110 lbs, you know 100 kg = 220 lbs

You will encounter many patients where you have to guess their weight due to them being unresponsive or aphasic anyhow, so being within a few kilograms is usually acceptable. With the very elderly or very young, you want to get as exact as you can. Always double check your math!

The exact way to do your weight conversion is weight in pounds divided by 2.2.

132 lbs/2.2 = 60
answer: 60 kg

Some find it easier to subtract 10% then divide by 2. I will explain..

To find 10 percent of a number, move the decimal to the left by a single integer. Since the weight in pounds will be a whole number, the decimal will be invisible, and all the way to the right. ex. 164 lbs = 164.0 lbs.

You will be rounding numbers in this example. If your number is greater than .5 round up to the nearest whole number. Less than .5 round down. If you accidently round the wrong way, your answer will still be pretty close.
you have a 156 lb patient
.....
10% of 156 = 15.6
.....
Round that to 16 and subtract from weight
.....
156 - 16 = 140
.....
now divide by 2
.....
140/2 = 70
answer: 70 kg
Since we know that 70 kg is actually equal to 154 lbs, we know this method isn't exact. It is pretty close though, and many find this method easier. Just dividing by 2 can leave you further off.

If you are within 5 lbs with a patient > 40 kg, you should have no problem. This should not have an adverse effect based on the dosage change of any prehospital medication. You could probably be even further off with most meds, but why risk it. Take the medications as serious as they truly are.

Another great method that I had forgotten, from medic65:
Take your weight in pounds lets say 250lbs and divide that by 2. 125. Now, 125 is 3 digits, take the first TWO of that number (12)5 and subtract (12) from 125. That gives you 113. That is your weight in kg.

Now, if you have a smaller weight, lets say 160lbs. Do the same thing, 160/2 is 80. Since 80 is only 2 digits, you only subtract the first number from this (8)0 - 8 gives you 72.
Practice all the methods and see what works for you. Remember, when treating adults, the patient's weight is usually a guess from the get-go. Precision is more important with the very young and/or very old.

In part II we will review drug concentrations.

#### 3 comments:

medic65 said...

A much easier way to get weight in kg from pounds is more simple than that.

Take your weight in pounds lets say 250lbs and divide that by 2. 125. Now, 125 is 3 digits, take the first TWO of that number (12)5 and subtract (12) from 125. That gives you 113. That is your weight in kg.

Now, if you have a smaller weight, lets say 160lbs. Do the same thing, 160/2 is 80. Since 80 is only 2 digits, you only subtract the first number from this (8)0 - 8 gives you 72.

Easier? :)

Adam Thompson, EMT-P said...

Thanks, I was trying to remember that method. I have been trying to remember it for my students, but couldn't. I updated my post, and gave ya the credit.

Rogue Medic said...

Adam Thompson, EMT-P,

The numbers do not have to be precise. The patients do not know what is written in the text book. We should generally be cautious with unfamiliar medications and use the lower end of the dose range, since almost all doses are in a range, rather than a specific dose.

There are some medications that change their behavior based on dose, such as atropine.

Sometimes more important than dose is the rate of administration. Side effects are often related to the dose or the rate of administration.

You provide some handy methods for calculating weights.