Drug Calculations – problems involving patient weight  105

Drug Calculations – problems involving patient weight 105


This video is provided as supplementary
material for courses taught at Howard Community
College and in this video I want to show how to do drug
calculations when you have to consider the weight of a patient. Let’s start with a fairly simple one
and we’ll work our way up through harder and harder ones.
This says the order is for 2 milligrams per kilogram of weight and the patient ways 20 kilogramsl We want to know how many milligrams are
to be given. As we usually do, we’ll start out with
the order. That’s gonna be a fraction,
2 milligrams over 1 kilogram. And let’s fill in the unit we want
to end up with. So I’ll need an equal sign,
I’ll leave a place for the answer, and write down the milligrams, since that’s what we need to end up with.
And then what we’re going to do is multiply the order amount by the
patient’s weight. So that’s going to be 20 kilograms. And we’ll just put that over 1 so we can
keep everything as a fraction. And now we’ll cancel whatever units we can. We can cancel out the kilograms.
We’re left with milligrams, which is fine — that’s what we want. We’ve got only 1’s
in the denominator, so we can just multiply across to get the answer. That’s 2 milligrams times 20 is 40 milligrams. So the basic approach is going to
be take your order, multiply it by the
patient’s weight, and that will give you the answer.
Let’s make it a little more complicated. This one once again says the order reads
2 milligrams per kilogram of weight. The patient once again weighs 20
kilograms. But now we see that the label reads 160
milligrams per 1000 milliliters. We want to know how
many milliliters are to be given. We’ll start with that order — that’s
2 milligrams over 1 kilogram. Let’s fill in the units we want to end
up with, so I’ll put in an equal sign, a place for the answer,
and the units — milliliters. We’ll multiply the order by the patient’s
weight. The patient weighs 20 kilograms,
so that’s 20 kilograms over 1. And now we’ve got the information for what’s on hand —
it’s a 160 milligrams per 1000 milliliters. I want to end
up with milliliters in my answer, so I’ll take that and write
if as a fraction with the milliliters in the
numerator. 1000 milliliters over 160 milligrams.
Now let’s cancel all the units we can. We’ve got milligrams of the here and
milligrams in the denominator, so they’ll cancel. We’ve got kilograms in the
denominator and we’ve got
kilograms kilograms in the numerator.
They’ll cancel. Let’s see if we can simplify some
factions. I’ve got a 2 in the numerator and 160 in the
denominator, so let’s divide both of those by 2.
160 divided by 2 is 80. I’ve got 20 in the numerator and 80 in the denominator —
we can divide both of those by 20. 80 divided by 20 will just
become 4 And now what we have….
let’s mulitply across and see. I’ve got 1 times 1 times 1000 millilters, so that’s 1000,
the ‘milliliters’ is written, and for the denominators I’ve got
1 times 1 times 4, so that’s 1000 over 4 milliliters, and that fraction will simple
down to 250 milliliters So for this one we multiplied the order by the patient’s
weight by the on-hand information and
that gave us our answer. Let’s get a little more complicated. This says the order reads
4 milligrams per kilogram of weight, the patient weighs 20 kilograms, the label reads 2 grams
per 1000 milliliters. We want to know how many milliliters are
to be given. So I’ll write in the order — that’s 4 milligrams over 1 kilogram. let’s put in the information we need to
tell us what we want to end up with — we want to end up with the number of milliliters.
So I’ll have the equal sign, a space for the answer, and milliliters.
We’ll take the order and multiply it by the patient’s weight.
The patient once again was 20 kilograms. So that’s 20 kilograms over 1. We’ve got this information for what we have on hand.
The label reads 2 grams per 1,000 milliliters and since we want milliliters
in our answer, I’ll put the 1000 milliliters in the numerator. And the denominator
will be the 2 grams. And the only thing left to do is to deal
with the fact that we’ve got milligrams in the order and grams in the ‘on-hand.’ So we’ll use the conversion that we have, 1 gram — I’ll put that in the numerator — equals 1000 milligrams. So this should allow me to cancel out
all units except for the milliliters. Let’s see. I’ve got milligrams over here and milligrams over here,
so they’ll cancel out. Kilograms and kilograms, so they’ll
cancel. And I’ve got grams and grams,
so they cancel. I can also get rid of the 1000’s that
I have, since I have 1000 in the numerator. I’ll divide that by 1000 and there’s
1000 in the denominator — I’ll divide that by 1000. Let’s keep simplifying. I’ve got 4
here in the numerator and the 2 in the denominator, so I’ll divide both of those by 2. So it’s going to be 4 divided by 2
is 2 and 2 divided by 2 is just 1. At this point there are only 1’s in the
denominator, so all we have to do is multiply across
the numerators to get the answer. That’s 2 times 20 is 40 times 1 milliliter is
40 milliliters times 1 is still 40 milliliters. So the answer here is going to be
40 milliliters. We got that by multiplying the order by the patient’s weight by the on-hand information by the
conversion we needed because we had both grams and milligrams. And here’s the last one. This one says order reads 4 milligram
per kilogram of weight. The patient now weighs
66 pounds, the label reads 2 grams
per 1000 milliliters. How many milliliters are to be given? Let’s start out with the order —
that’s 4 milligrams per kilogram.
We want the answer to be in milliliters. We’ll take the order, multiply it by the
patient’s weight — that was given in pounds,
so that’s as 66 pounds over 1. The label reads 2 grams per 1000 milliliters.
So we’ll take that information and make traction with 1000 milliliters
in the numerator and the 2 grams in the denominator. Now there’s two more things we have to do. We’ve got milligrams and grams,
so let’s put in the conversion for that — 1 gram is the same as 1,000 milligrams. So I’ll put in the fraction 1 gram over 1000 milligrams. And I’ve got to deal with that 66 pounds. Well, 1 kilogram is 2.2 pounds, so I’ll add one more fraction — I’ll have 1 kilogram as the numerator and 2.2 poinds as the denominator.
And this is all going to clear up very nicely.
Let’s start getting rid of the units. I’ve got milligrams in the numerator
and milligrams in the denominator, so they’ll cancel,
kilograms as a denominator and kilograms as a numerator —
that will cancel. I can cancel out the
pounds that I’ve got and I can cancel out the grams. The only
unit I’m left with is milliliters. and that’s how I want to end up.
I’ve also got 1000 the numerator 1000 in the denominator,
so let’s divide both of those by 1000. I’ve got 4 in the numerator and 2 in the denominator —
let’s divide both of those by 2. So the 4 become a 2, and the 2just
becomes 1. I’ve got this 2 in the numerator and 2.2 in
the denominator — I can divide both of those by 2. So the 2.2
divided by 2 is is 1.1. And at this point let’s multiply across.
I’ve got 1 time s 66 times 1 times 1 times kilograms… Ooops, that milliliters. 66 milliliters over 1.1. And I can simplify this fraction and
turn it into just 60 milliliters. So for as complicated as this seemed,
it was just a series of little steps. We wrote down the order,
multiplied it by the patient’s weight, multiplied by the fraction we got from the
on-hand information. And then did whatever conversions we
needed: we had to convert between grams and milligrams
and between kilograms and pounds. We cleaned up everything we could
and we ended up with our answer, 60 milliliters.
So I hope this all helped. Take care, I’ll see you next time.

56 comments

  1. making it complicated. convert lbs to kgs first and g to mg then plug in the numbers. making it harder than it has to be.

  2. Please tell me these college courses you mentioned at the beginning of the video are "101" courses, because this math is so simple it's stupid. Why does it even need to be taught? OMG. For the sake of my already-minuscule faith in humanity, PLEASE tell me students don't seriously have a hard time figuring this out?!

  3. tyl..pero..nakakahiloyunglast,,mas-ok-po-kung-wag-maglagay-ng-additional-fraction–deretsong-convert-nalang…tyl-padin..tnx.-GOD–I-GOT-IT..TYL—AND-TY-TO-U-SIR..

  4. Wouldn't it be easier to make the conversions first and then calculate. We have to convert regardless in this field. But thank you, your calculations videos have helped tremendously.

  5. Can you help me with these two problems? A nurse has to administer a medication twice a day, at 10am and 4pm. The first dosage is 3/8 oz and the second dosage 2/7 oz, she only has one oz of the medication. Will she have enough for both doses?
    ย a nurse needs to administer 6 ml of X medicine in a solution of 150 ml. At what rate would you set transfusion of the medicine so the patient receives it at 1.5 ml/ min

  6. I did the 3rd calculation differently by calc and found it easier 4/1 x 20/1 x 1000ml/2000mg =40ml to do this I just converted the 2g to 2000mg by multiplying it by 1000 because when converting a bigger unit e.g (g) to (mg) you have to x by 1000 ๐Ÿ™‚

  7. Good explanation! ….. I've been teaching med-math for over a decade and can say that the VAST MAJORITY (~80%) of my students find dimensional analysis MUCH more complicated than any of the other methods that can be used to solve these. While I understand how to use DA, I find many students don't always include units when doing calculations despite me begging them to do so! If just one of the many "fractions" used in DA is inverted b/c units are wrong, the entire answer is el-wrongo! But kudos for those who learn how to be consistent with always including units after numbers, DA can be a great method to use. It just hasn't been in my personal experience.

  8. I finnaly got to solve these dosage calculations and i got them correct. For the last problem, I got 60,000,000 ml and I was like, duuude…you already killed your patient!!! and i redid the problem and i got it right!! Thanks for your videos

  9. Order to administee nitroglycerin 1/150 sublingually. You have nitroglycerin 0.4-mg tablets. How many tablets would you administer to your client.
    A 0.25
    B 0.5
    C 1 tablet
    D 2.5 tablets

  10. How did you get 1g/ 1000mg? I think it would be easier to convert the 2g to ml (2000ml)hence we are looking for that.

  11. Doing all that dividing at the end is a waste of time just multiply across the numerator and denominator and then divide at the end

  12. I hate this long hand explanation.
    Your making it confusing. Just do the conversion lbs to kg x mg. Grams to mg. Then do your basic dose over have x volume .

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