Medical calculations, part 2, Veterinary Pharmacology

Hi this is dr. Herndon and this is part
two of our review of medical

calculations. In this post i will be
explaining and demonstrating how to

calculate a drug dose and how to
calculate an amount of a drug that we're

going to administer to a patient. In
order to be able to do these

calculations we need to know three
things: you need to know the animal's

body weight,

the drug dosage rate, and the drug
concentration. A word about the terms

dose and dosage: these two terms are
often used interchangeably.

It's important to know the difference,
and for our purposes a dose of a drug is

what we're going to be calculating. It's
the quantity of drug active ingredient

that's delivered to a patient at one
particular time.

So you'll often see a drug dose written
as 20 milligrams or 20 units. A

dosage rate, however this is something we
don't have to calculate. A dosage rate is

going to be given either in the
veterinarian's orders, or it may be found

on the drug label itself, or we can look
it up in a drug formulary such as Plumb's.

This is the amount of drug per unit of
body weight and you'll often see it

expressed as mgs per kilogram, this is
probably the most common way to express.

Sometimes you'll see it as mgs per
pound,

other times as units per kilogram, and
again this is not something that we have

to calculate. Our dose is equal to the
animal's body weight times the dosage

rate of the drug, which is going to be
given. Now we can weigh our patient and

get body weight, sometimes we weigh in
pounds, oftentimes we have to convert to

kilograms; times the dosage rate. Again
the dosage rate is a given. It's either

going to be given in the veterinarian's
orders, we'll find it and Plumb's, or find it

on the drug label occasionally.

The thing to remember is that if you're
working with a dosage rate, let's say in

mgs/kg,  your body weight needs to
be in kilograms as well, so we could

cancel out these units, we need to work
with the same units.

By the same token, if you're working with
a drug dose in mgs per pound,

you need to make sure that your weight
is in pounds as well so those will

cancel out.  Alright let's look at this
next formula here, so we have calculated

drug dose, right there,

if we want to know how much of a drug
we're going to give a patient,

let's say how many tablets, or how many
mls, or how many ounces, or something

like, that we use our dose that we've
just calculated, and then we have to find

the drug concentration.  Dose divided by drug concentration equals the amount that

we're going to administer. So we can put
it all into one formula here,

the amount of drugs that we're going to
give to a patient is the body weight

times the dosage rate, that's your dose
formula, divided by the concentration of

the drug.

So, you may be asking yourself "where do I
find the concentration of the drug?" . Well

you need to go directly to the
medication itself and look at the label.

If you pick up this one on the left, this
is propofol, this is an injectable

anesthetic drug,

you can see in big blue letters right on
the front it gives you the concentration

in a percentage.

We're going to talk about solutions
later on. One percent, this is a one

percent solution of propofol, but they've
kindly given you the concentration in

mgs per ml - on the label you can see
it down there - 10 mgs per ml.  In the

middle here

this is amoxicillin capsules, they show
you a picture of the capsule in the

front, and over here in this red circle
is a concentration 500 mgs. What this

is telling you is that in each capsule
there are 500 mgs, so it's 500 mgs/

capsule. Over on the right here, these are
two different concentrations of the same

drug, midazolam, it's a benzodiazepine

these are injectable anesthetic drugs.
You can see that there they have

different color caps, there's orange and
there's green, and these signify that

these are two different formulations of
the same drug. This is important, if you

see two different color bottles there's
probably a reason for that.

On the orange cap battle on the left
you can see the concentration is one

mg per ml, on the green top bottle on
the right is a stronger one, this is five

mgs per ml.  So you need to be careful
when you're picking up something off the

shelf that you're grabbing the correct
concentration of drug.

Alright let's look at these two drugs
right here, these are going to be used in

our next two examples that i'm going to
talk about. On the left is Antirobe Aquadrops

drops, this is clindamycin hydrochloride
liquid oral solution, it's an antibiotic.

The concentration is right on the front of
the box, and it's also in the front of

the label, that's 25 mg per ml.

Over on the right here, these amoxicillin
tablets, amoxi tabs, if you look down

on this blue bar down here, says a
hundred mgs and then there's a little

picture of a tablet right down here, what
this means is that there are hundred

milligrams per tablet.

Ok let's try a dose calculation
example. We're going to use our

clindamycin over here on the right that
I just showed you in the previous slide.

We have all our information on the left

our patient is a 10 kilogram dog, that's
the weight. The orders given are for ten

mgs per kg of clindamycin, this is the
dosage rate. We know that the available

drug we have, because it's sitting over
here in my picture, is at a concentration

of 25 mgs per ml. The question asks
first "what's the dog's dose?" in

milligrams and then second "how much (or
what amount) of the drug are we going to

give?" in mls.

So we're going to set up our dose
formula: dose equals body weight times

dosage rate. Our kilograms cancel out, our
answer is going to be in milligrams: 10 x

dose. So it's a hundred milligram dose.

Now we have to get a hundred milligrams
from this 25 mg per ml solution.

We're going to go to our second part.
Amount equals dose divided by concentration,

so the amount equals a hundred eggs
we're going to use our dose here divided

by the concentration which is 25 mgs
per ml, cancel mgs, a hundred divided by 25 equals


So this patient is going to get four
mls of the Antirobe aquadrops

orally.

Here's another example using the
amoxicillin tablets. We have a 40 pound

dog.

the order is for five mg per pound of the
amoxicillin, which was a label dose,

the drug is availableat a hundred mgs
per tablet, and it asks "what's the dog's dose?"

and "how much drug will be given" , or how many tablets. why don't you try this yourself,

pause the video, then when you come back
I'll go over it.

Ok I've written out our dose formula
here,

dose equals weight times dosage rate, in
this case I didn't have to convert from

pounds to kilograms because my dosage
rate was in mgs per pound, that's

important. We can cancel our pound
units, 40 times 5 equals 200, and our

units in are in milligrams.

so here's our dose right here 200
milligrams. Amount is dose, which is 200

milligrams divided by the concentration
of the drug, which is a hundred mgs per

tablet, cancel our milligrams, 200 divided by 100 equals 2, and our units are in tablets.

Two tabs, so this animal will be getting
two amoxicillin tablets for each dose.

Sometimes drug dose rates use body
surface area instead of weight.

Oftentimes chemotherapy drugs have
dosage rates that are in mgs per meter

squared instead of mgs per kilogram.
Sometimes you'll see other drugs that

use body weight in meters squared
instead of kilograms as well, so meter

square refers to the body surface area
and to be able to figure this out what

we need to do first is we determine the
weight in kilograms of an animal and

then we go to a chart, a conversion chart,
and look up the body surface area for

that particular weight in kilograms.
There's one in Plumb's that you can use

it's called "Conversion tables for weight
in kilograms to body surface area" and

that can be found in the appendix.

Let's try this example here. We have our
patient, a 45 pound dog, that's going to

be treated for lymphosarcoma, which is a
type of cancer, with a drug called

vincristine sulfate down here, which is a
chemotherapy drug, and they give you the

concentration-- one mg per ml, which you
can also find on the label.  The dose, or

the dosage rate, of this drug is point five
mgs per meter squared, and this is an

example where the terms are used
differently, it says dose is point five mgs per

meter squared, but you can see that this
is a rate this is your dosage rate here.

So the first thing we need to do is
convert 45 pounds to kilograms, and I

have done that, and it comes out

to go to the chart in Plumb's and i'm

going to look up 20 kilograms, and it
will give you the body surface area,

which in this case is . 7 4 meter
squared, so if we're gonna go to our

amount which equals, instead of body weight,

we're going to use body surface area, so
that's point 74 meter squared, body

weight, or body surface area, times dosage rate up here, which is point five mgs per

meter squared, divided by the
concentration of the drug, one mgs per

ml, we can cancel out body surface area,
we can cancel out our milligrams, our

answer is going to be in mls, and I use
my calculator to do this, and our answer

is 0.37 mls of vincristine solution.

It's important to note that when you're
recording information in the patient's

medical record that you always record
the drug dose of the active ingredient

in the medical records, such as the
number of milligrams of drug, or units of

drug, or it could be milliequivalents
if it's an electrolyte or something like

that. Don't just record the number of
tablets or milliliters. If you simply

write down this animal got 20 mls of
some drug that gives you absolutely no

information whatsoever. Drugs come in
many different concentrations. Here's an

example here,

Baytril,  you can see there's five
different types of tablets

three of them are these flavored tablets
down here which come in three different

concentrations, 136, 68, and 22.7 and
there's two types of capsules over here

in two different concentrations, then
there's even an injectable form. So, as

you can see, if you simply wrote patient
got "two tablets of Baytril" we have

absolutely no idea what the dose of the
drug is that this patient received.

Alright let's use this example here. This
is a very simplified medical treatment

chart, over on the left column here you
have the date and time the medication

was given, in the middle is the treatment
itself, and over on the right are the

initials of the person who gave the
medication. So on the left over here

you're going to be writing the date that
the medication was given,  the

particular time that it was given in the
middle here you're going to write the

treatment itself. So in this case, we're
using the first example of clindamycin

will be writing the dose amount of the
drug hundred million

Sam's clindamycin HCL, was given orally,
so I'm writing PO, and then my initials

(whoever did it), so this gives you the
dose, the drug, and the form that it was

given in, at whatever time, let's say it
was 7:30am on the tenth of September. It

would be incorrect to write in the
treatment section four mls of

clindamycin. This gives you no
information as to how much active

ingredient is in this drug. We know that
we used a 25 mg per ml solution, but

the next person reading this is going to
have no idea what the concentration of

the drug is this, so this gives you no idea of
how much of the active ingredient that

patient actually received. If you wrote it
this way four mls of a 25 mg per ml

clindamycin solution, that would be okay,
but then the person still has to go

figure out what the dose was.

Ok so this concludes part two of our
review of medical calculations

now go ahead and take the quiz that's on
canvas it's called calculations quiz

number two