Nathan asked
Phenytoin (Dilantin) comes from the pharmacy as an IV solution of 1000mg/250mL. A loading dose of 1000mg has been ordered at the conservative rate of 25mg/min. At what rate will the pump be set in milliliters per hour?
Answer
This type of question is very commonly seen in pharmacy school, the pharmacy technician course and nursing programs. It is very easy to become overwhelmed and confused when approaching these problems. The truth is however that while many of these problems appear complex, with practice, they are all solved in a similar manner.
It can be tough to write out equations in an email and go through calculations, but I will try my best!
For this specific problem,
we are going to be using a basic principle known as dimentional analysis, or a
conversion of units.
I always like to break
away from the wording and just write down what numbers I am working with.
Phenytoin: 1000mg/250 ml
Dose Rate: 25mg/min
Unknown Dose Rate: ?
ml/hour
This problem requires two
conversions of units.
Firstly, according to the
problem, we know that the drug is being dosed at 25 mg/minute. Let's find out
how many mls of phenytoin are in 25mg. We can find this because the problem
gave us the concentration of phenytoin at 1000mg/250ml. We will set up a ratio.
1000mg
25 mg
-------- = ---------
250 ml
x ml
This equation is stated
as, if 1000mg is to 250ml then 25mg is to x. We can set these equal to
each other since they are directly related.
Working this equation out,
we cross multiply and get 1000x = 6250. Divide both sides by 1000.
This comes out to x= 6.25
ml
So we know that in every
6.25 ml, there is 25 mg of phenytoin. This is our first conversion. We just
converted our dose rate from 25mg/min to 6.25 ml/minute.
Our second conversion will
the the time component of the dose rate. Currently we have 6.25ml/minute and we
want ml per hour.
We know that there is 60
minutes in an hour, so we can set up this equation:
6.25 ml
60 minutes
------- (*)
------------ = Dose Rate
1 minute
1 hour
When we are multiplying
across, we can cancel out units that are on the top and bottom. Therefore, we
can cancel out minutes. After multiplying, we come out with:
375 mL/Hour. This is our
answer.
Please don't hesitate to
follow up with me! I would be happy to write this out in an easier to
understand way. I can also help with any other problems you are having!
