Does this math make sense to you?

CuriousMe · Jan 27, 2011

I need some really good math minds. I am really good at math, have the answer and the way the person figured this out and still can't make it make sense. Here's the problem:
Order: Some drug 1000mg in 250mL of solution infusing at 13 mL/hr. Client's weight: 72kg
Figure out the medication dosage in mcg/kg/min: the answer is 12 mcg/kg/min. Which makes sense when I work it backwards. I have the DA that the answer came in and it doesn't make sense at all. Can anyone figure out how to work this problem using Dimensional Analysis that makes sense?

Here's how I set it up:

1000mg/250 mL X 13 mL/60min X 1000mcg/1 mg = 866.66667mcg/min

Then divide that by the 72 kg and you get 12 mcg/kg/min

JSTARZ · Jan 27, 2011

This is an advanced dosage calculation and I have never been a fan of that "dimensional analysis" fellow. I have been at this RN thing a while and ITS TOO LATE TO TEACH AN OLD DOG NEW TRICKS! :) I use a simple method called "FACTOR 15"...It is kind of magical and quick yet perfect for codes...here goes

Amount of drug (in 250 mL of fluid) / "15" / weight (kgs) X Flow rate= mcg/kg/min

1000 mg/"15"/72k g X 13 mL/hr=12.03 mcg/kg/min

FYI: The "15" is a constant, it does not change. It only works using the amount of a drug in 250 mLs of fluid. However, it does work for multiples of 250 yet you would have to convert and use the amount of drug in 250 mLs. For example if this drip of 1000 mg was in 500mL of fluid as opposed to 250 mL you would need to divide and use 500 mg in 250 mL. This can be determined using simple ratio & proportion.

500 mg /"15"/72 kg X 13 mL/hr=6.01 mcg/kg/min....which makes sense, considering the fact that you have a concentration of 1/2 of the original drug.

Hope this help. If not, take it and stick it in your back pocket!! Best of luck!

CuriousMe · Jan 27, 2011

This is an advanced dosage calculation and I have never been a fan of that "dimensional analysis" fellow. I have been at this RN thing a while and ITS TOO LATE TO TEACH AN OLD DOG NEW TRICKS! :) I use a simple method called "FACTOR 15"...It is kind of magical and quick yet perfect for codes...here goes
Amount of drug (in 250 mL of fluid) / "15" / weight (kgs) X Flow rate= mcg/kg/min
1000 mg/"15"/72k g X 13 mL/hr=12.03 mcg/kg/min
FYI: The "15" is a constant, it does not change. It only works using the amount of a drug in 250 mLs of fluid. However, it does work for multiples of 250 yet you would have to convert and use the amount of drug in 250 mLs. For example if this drip of 1000 mg was in 500mL of fluid as opposed to 250 mL you would need to divide and use 500 mg in 250 mL. This can be determined using simple ratio & proportion.
500 mg /"15"/72 kg X 13 mL/hr=6.01 mcg/kg/min....which makes sense, considering the fact that you have a concentration of 1/2 of the original drug.
Hope this help. If not, take it and stick it in your back pocket!! Best of luck!

wow, it hurt my head just to read that.....I'll stick to DA :-)

CBsMommy · Jan 27, 2011

Here's how I set it up:
1000mg/250 mL X 13 mL/60min X 1000mcg/1 mg = 866.66667mcg/min
Then divide that by the 72 kg and you get 12 mcg/kg/min

This makes sense and does come to the right answer. Maybe I just need to step away from it to have it absorb into my thick head! Thanks!

CuriousMe · Jan 27, 2011

This makes sense and does come to the right answer. Maybe I just need to step away from it to have it absorb into my thick head! Thanks!

Where are you getting hung up?

CBsMommy · Jan 27, 2011

Where are you getting hung up?

For me, it's the cross multiplying and the way this question doesn't seem to jive. For instance, we usually set up the answers with the same figures in DA, like: mL/gtt * gtt/min * min/hr

See how each leads into the next? I guess I'm having a hard time with reasoning out how to set up the different factors. Like in the solution above we went from 60 mins and crossed that with mcg. It's just not logical in my brain! LOL

CuriousMe · Jan 27, 2011

For me, it's the cross multiplying and the way this question doesn't seem to jive. For instance, we usually set up the answers with the same figures in DA, like: mL/gtt * gtt/min * min/hr
See how each leads into the next? I guess I'm having a hard time with reasoning out how to set up the different factors. Like in the solution above we went from 60 mins and crossed that with mcg. It's just not logical in my brain! LOL

I see what you mean. This happens when you need to get rid of both the top and the bottom of the first equivalency ( in this case we're converting the units of both mg and mL). Although of course none of this is really cross multiplied...you cross out the units diagonally....and you're not crossing out mins and mcg, you're being left with mins and crossing mcg with the mcg in the first equivalency

When it all just leads from one to the other, we're just converting one of the first two numbers.

So, if the answer was supposed to be in mg instead of mcg....it would have looked as you describe. To set odd ones like this up, I do the leading (as you described) but then look again at what I need to end up with and then add on what I need.

Hope this helps!