Can someone explaine dimensional analysis to me?

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I mean, I understand what it is, and how all the cancelling out works and what not, but my confusion comes in when deciding what goes where. How do you know what to put in the first equation, middle, and so forth? For some reason it is not clicking in my head, especially when you need to do conversions. Is there any good youtube videos that dumb it down? Someone please help me out here? Thanks!

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Specializes in ICU Surgical Trauma.

At the beginning you have to write down what is give in the problem. Then after that you can put whatever conversions you need to do anywhere in the problem as long as they all cancel out. When you cancel everything out then you should be left with what is needed. If you need you need ml/hr then you should be left with ml at the top and hr at the bottom.

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Try these links below. They are kind of long, but they are very in depth. They helped me to nail down dimensional analysis.

http://atsvid.uthscsa.edu/mediasite/Viewer/?peid=e911a310c9b04308b0272a353a187d5f

http://atsvid.uthscsa.edu/mediasite/Viewer/?peid=2333668ab7ad41ae99f35d8e14b73acc

Hope this helps!

-HN

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It'll be easier if I give you an example that I made up and we can work it from there.

A MD orders 250 mg of blahblahblah. The bottle of blahblahblah states that 1 tablet is 1000mg. How many tablets should the nurse give?

The easiest way that I was taught was that when you first write out the problem you start from the right of the equation so you want to find out what units the answer is going to be in the answer and you work your way left. In this example we are trying to find how many tablets. So I first write out:

=______Tab

Now here is where it gets super easy. Since you know the answer is going to be a tab, you know the top part of the fraction on the other side of the = sign is going to be a tab too. So I look for any "tab" information they give me in the question. Ahhh - there's one. So far my equation looks like:

1tab/1000mg = ________tab

Now since the bottom part of the fraction is in "mg" you know the next top part of the fraction to the left is going to be in "mg" so that the mg's can cancel each other. So our example looks like:

250mg/1 * 1tab/1000mg = ______tab

I hope I explained it well enough. No matter what problem that you get with medical dosage, when you write the question, start from the answer on the right and work your way left - you will alway write it in the right way if you understand what the question is asking you.

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Dimensional analysis is. . . when you think you got something right, then you DON'T. Because you don't, you have to withdraw from your nursing program. That's what dimensional analysis is! Then, you end up more confused than ever, lol.

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good threAd. I dont get dimensional analysis either.... YET> ill try those links yall posted. any other pharm advice?

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Specializes in Cardiac, Rehab.

Ratios. Thats what it's all about. If that is a bit hard to grasp, then think about it this way.

You start the problem with what it is you want to solve for. It can be drops/hr or ml/hr or number of tabs to administer. Once you have that, you work your way to the right putting in the known values which are, wait for it, Ratios. So if you are solving for ML, and you have a solution that is 1000mg/500ml, you write it as ML = and then you can flip the 1000mg/500ml over to be 500ml/1000mg. You want the term on the top to remain and the ones on the bottom to go away. Then you add in the other "ratios" that you are give so you cancel out the unwanted terms till you are left with what it is you are trying to solve for.

If it doesnt make sense now, give it time, you have to practice it quite a bit till it becomes second nature. Most of what trips people up isn't setting up the problems, its making stupid math errors. Good Luck.

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Specializes in Trauma, Teaching.

Order (dose): give 500 mg IM

Start with what you need to end up with, for instance, mL.

__mL =

next comes what is available, put the unit on top that matches your unknown (whatever is closest to mL) on top, avail drub is 2 g/mL _mL = mL/g

next is the dose, put the units so they will start to cancel mg, ml, g, etc.

_ mL = available (mL/g) x dose (mg)

now if you need conversions (gram to mg) g on top, mg on bottom

_mL = mL/g x mg x g/mg

unk = avail x dose x conversion

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Our school recommended this book for Dimensional Analysis practice:

The Nurse, The Math, The Meds: Drug Calculation Using Dimensional Analysis by Joyce L. Mulholland

http://www.amazon.com/Joyce-Mulholland-APRN-Nurse-Paperback/dp/B003ZXZPNQ/ref=sr_1_2?ie=UTF8&qid=1315600246&sr=8-2

It is an excellent tool to grasp DA. I passed my DA exam on the first attempt with a 100%.

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