Medication Calculation Help

riverguy · Jun 16, 2006

first you figure how many ml=.06units by using the equation dose desired divided by dose on hand times mls of solution on hand. That is .06units/15units x 500ml=2ml so then .06units/min = 2ml/min of the 500ml solution bag. You then multiply the 2ml/min x 60, because there are 60 min in 1 hour and you get 120ml/hr. Hope that wasn't to confusing.

mimibear · Jun 16, 2006

Okay, I have not done this for nursing school yet, but I am good at math. I am sure there are many correct ways to do this - here is mine. You know you have 15units in 500 ml, so each 1 ml has .03 u (15/500). To infuse .06 units a minute you need to infuse 2ml a minute (.03 x 2 = .06) so 2ml/min x 60 minutes an hour = 120 ml.

Hope you were able to follow that - good luck!

EricJRN, MSN, RN · Jun 16, 2006

If there are 15 units in 500 mL, then there is 1 unit per 33.3 mL. (500 mL divided by 15 units = 33.3 mL/unit)

In one minute, we want to give 0.06 units. How many mL is that? Since we know that there are 33.3 mL in one unit and we want to give 0.06 of a unit, we can multiply 33.3 * 0.06, which equals 2 mL (or really close to it).

So we know we need to give 2 mL/min to get the 0.06 units. But the question asks for mL/hr. We know that there are 60 minutes in an hour, so we'll multiply 2mL/min * 60 min to equal 120 mL/hr.

If it doesn't make sense for you to talk it out like this, there's another technique called dimensional analysis that you can use. Look up dimensional analysis on a Google search for more information. Because it involves placing one value over another and cancelling, it can be a little tricky to illustrate on a forum like this.

Daytonite, BSN, RN · Jun 16, 2006

By dimensional analysis (or factor label method where you end up factoring out the labels on your numbers so you end up with the correct labels in the right ratio):

0.06 units / 1 minute (dose desired) X 60 minutes / 1 hour (conversion factor) X 500 mL / 15 units (dose on hand) = 1800 mL / 15 hr (after multiplying all numbers and factoring out labels) = 120 mL / 1 hr. (final answer)

buddhababy2monster · Jun 16, 2006

Thanks for all your help, I understand how you get to the answer now. It's great seeing the different ways to come up with the right one. Wish me luck on my quiz!

Dratz · Jun 16, 2006

Can anyone help me figure this one out?

Order: Neostigmine 0.5 mg IM t.i.d.

Supply: neostigmine 1:2000

Answer is 1 ml but how the heck does a person arrive at that?

Thanks

MJJFan1, BSN, RN · Jun 16, 2006

You probably ended up with 1000mg but remember:

1ml of 1:1 = 1000mg

Daytonite, BSN, RN · Jun 16, 2006

Neostsigmine 1:2000 refers to it's dilution. 1:2000 means there is 1 gram of neostigmine in each 2000 mL of solution. By dimensional analysis (factor label method) the solution to the problem is obtained this way:

0.5mg / 1 (dose desired) X 1 gram / 1000 mg (conversion factor) X 2000 mL / 1 gram (dose on hand) = 1000 mL / 1000 (unreduced answer) = 1 mL (final answer, one dose)

You will give 1 mL t.i.d.

Dratz · Jun 16, 2006

I still don't get it. Back to the drawing board for me. I am Canadian so do not understand Dimensional Analysis.

Thanks,

Daytonite, BSN, RN · Jun 17, 2006

ok, lets try another way. since neostigmine 1:2000 is 1 gram in 2000 ml do a little math to change that into milligrams. the ratio 1:2000 can also be written as a fraction with 1 in the numerator and 2000 in the denominator. divide the denominator into the numerator to obtain a decimal. doing the division and keeping the labels in the numerator and denominator you'll get 0.0005 grams/1 ml. since the dose you need is in milligrams you need to convert 0.0005 grams to milligrams. simply multiply it by 1000 to get 0.5 mg. keep in mind that now that you've reduced this fraction it now represents 0.5 mg per 1 ml. because in dividing the numerator into the denominator you, in effect, made the denominator the number "1". now, using the formula of dose desired divided by the dose you have just determined that you have on hand multiplied by the amount the dose on hand comes in you get 0.5 mg neostigmine (dose desired) divided by the dose on hand 0.5 mg neostigmine (dose on hand) multiplied by 1 ml (that's the amount i determined in the calculations above that the dose on hand comes in). both the 0.5 mg in the numerator and the 0.5 mg in the denominator factor out to give you the number 1 and all you are left with is 1 times 1 ml. the answer is 1 ml. if you compared what was just done here with the way i worked the problem by dimensional analysis you will see that everything was done with the exception that with demensional analysis you set up an equation of multiplication where you deliberately manipulate the fractions (ratios) so that you factor out labels, include conversion factors so you can get from grams to mg and are ultimately left with an answer with the label of ml, an amount, which is what the problem is asking for.

dimensional analysis is not a u.s. thing. it is mostly used by scientists in chemistry and physics (there are a few of them in canada, i presume) to calculate amounts of chemicals and elements that are needed in reactions. it is a method that adapts very well to working medication problems. you can see an interactive video for chemistry students on how dimensional analysis (or, factor labeling) works at this website

http://www.wwnorton.com/chemistry/tutorials/ch1.htm - click on "1.4 dimensional analysis". it is an interactive program that explains da (dimentional analysis) using animation rather than video in what i thought were very simple terms. also includes several problems you can try your hand at which are not chemistry related but utilize the concepts of da.

mayflower2000, RN · Jun 17, 2006

:) this is how i calculate.

DA- 0.5 MGS

SOH- 1GRAM OF NEOSTIGMINE IN 2000ML SOLUTION

1) convert SOH unit to have the same unit to DA. 1gram is equivalent to 1000mgs. now. it becomes 1000mgs in 2000ml

2) we have to find out how many mgs/ml in our SOH.

1000/2000 =0.5 mgs/ml

3) we want 0.5 mgs, our DA

0.5mgs DA/ 0.5mgs/ml SOH = 1ml

hope, i have made it easier to arrive the answer.