Medical calculations, part 2, Veterinary Pharmacology
Medical calculations, part 2, Veterinary Pharmacology
Hi this is dr. Herndon and this is part
two of our review of medical calculations. In this article 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.
Hi this is dr. Herndon and this is part
two of our review of medical calculations. In this article 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.
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