Critical care math using dimensional analysis

RetrieverGirl, BSN, RN · Jul 18, 2013

It is hard to give an example because it seems as though the test questions are always much harder than the practice problems that we get. However, my downfall is with mcg/kg/min. If I remember correctly it was dopamine 800 mg/500 mL with a flow rate of 15 ml/hr. Patient weight 154 lbs. How many mcg/kg/min is the patient receiving? These are the problems that I use dimensional analysis because I see everything laid out when there are so many steps with the conversions. RLtinker I appreciate your help, but that method for these kinds of problems thoroughly confuse me

RetrieverGirl, BSN, RN · Jul 18, 2013

I don't have to use dimensional analysis, but I find for these mcg/kg/min questions that it seems to help me see everything better.

RetrieverGirl, BSN, RN · Jul 18, 2013

Mrs. Brown, a 78 kg patient has Diprivan running at 10 cc /hr. The physician asks you what dose she is receiving in mcg/kg/min. The concentration of Diprivan is 10 mg/mL. What dose is she receiving?

bluedove1 · Jul 18, 2013

So you need to calculate from mL/hr to mcg/min?

bluedove1 · Jul 18, 2013

If you use the formula : concentration(mcg/mL) x rate(mL/hr)/ 60 min/h x weight(kg) you will have to use proportion formula first to covert mg in IV to mcg then calculate concentration, then calculate mcg/kg/min so you have to memorize the formula which isn't hard if you can keep formula straight .....formula remembering isn't my thing so DA is for me so this is how I would set this up

X mcg/kg/min = 10 mL/hr x 10mg/ mL x 1 hr/60 min x 1000 mcg/ 1 mg x 1/78 kg =

Try both ways to see which way works for you....remember that mL, mg, hr will cancel out and you will be left with mcg/kg/min

Hope this helps a little bit

nurseprnRN, BSN, RN · Jul 18, 2013

Mrs. Brown, a 78 kg patient has Diprivan running at 10 cc /hr. The physician asks you what dose she is receiving in mcg/kg/min. The concentration of Diprivan is 10 mg/mL. What dose is she receiving?

So... there are 10mg in every cc, and she's getting 10cc/hr, so that's 100mg/hr. So far, so good.

How many mcg is that, in the hour?

What do you need to do to find out how many mcg that is in a minute?

Then how do you know how many mcg/kg that is in that minute? That's your thought process, and that's your answer.

RLtinker, LPN · Jul 18, 2013

Mrs. Brown, a 78 kg patient has Diprivan running at 10 cc /hr. The physician asks you what dose she is receiving in mcg/kg/min. The concentration of Diprivan is 10 mg/mL. What dose is she receiving?

(((10mcg/1000)*154)/2.2/800)*500)*60 is similar to the way it is done in DA, just put everything you multiply on top and everything you divided by on the bottom, add your units and your 1's and you got the DA version.

I am kinda courteous as to why they are having you go backwards. Anyways, you just do all the steps in reverse. Using DA you know that you need to go from 10ml/hr ----> mcg/min----> and whatever this values is you divide it by the Kg.Looking at the concentration you know there are 10mg per mL, and you are giving 10ml/hr, so you are giving 100mg/hr. There are 60min per hour and 1000 mcg per mg.

AN doesn't make showing this easy in DA, but here it is.

10ml x 10mg x 1hr x 1000mcg x 1

---------------------------------------

1hr x 1ml x 60 min x 1 mg x 78 kg

When you cancel the units and multiply all the top values and then multiple all the bottom values you should be left with

100,000 mcg

------------

4680 min kg

divide the last 2 values

21.36 mcg/kg/min

21.4 mcg/kg/min

Guests · Jul 19, 2013

I see many students and new nurses using it to "solve" math problems without really understanding what is going on with the situation. Taking care of a patient requires more than just solving an equation, it requires an understanding of the relationships between volumes, rates, dosages, etc. When young nurses just plug numbers into an equation and get an answer (as if for a test question), they often miss what is actually happening with the fluid/medication/etc. and the patient. The proportion method promotes an understanding of the quantities of meds/fluids/etc. involved --

I could not agree more.

Many students are just trying to turn a crank or utilize equations without understanding the physical basis of what they're doing.

I teach "med math" using:

1) Tiny M&Ms (to help grasp the concept of quantity)

2) Kool-Aid, sugar, and food coloring (to help grasp the concept of concentration)

3) Balloons (to help grasp the concept of volume)

4) Straws and stirrers (to help grasp the concept of volume flow)

5) Pails, beakers, graduated cylinders, balances/scales, stopwatches and rulers/micrometers/calipers (to help grasp the concepts of volume, mass, density, and flow rate)

Once they get comfortable with the 'real world,' then we delve into the world of maths and equations.

One would hope that students would come out of chemistry (actually, out of 6th grade) with these very fundamental concepts solidified in their minds but it doesn't seem to be the case.

llg, PhD, RN · Jul 19, 2013

Once they get comfortable with the 'real world,' then we delve into the world of maths and equations.
One would hope that students would come out of chemistry (actually, out of 6th grade) with these very fundamental concepts solidified in their minds but it doesn't seem to be the case.

I have spent much of my career working with new grads and student externs, etc. It always amazes me how some people fail to grasp those basic concepts -- and therefore struggle trying to be nurses in the real world, where an understanding of those concepts becomes important. They can answer the questions on their schools specially-prepared exams, but they struggle with real life situations because of that failure to grasp the basic concepts.

RetrieverGirl, BSN, RN · Jul 21, 2013

tinker: I see where I went wrong. I made the mistake of flipping some of my units:banghead: . It's going to take some practice for sure... Any other pointers or tips? Thank you!!!