Another med calculation question

I don't know why im having such a hard time with this, all of a sudden.
I'm pretty sure my problem is that I never really learned dimensional analysis.
Here's the question. I don't understand what goes where, but more important, I don't understand WHY they go where they do. Any explanation would be greatly appreciated.
6. Order: A heparin drip is infusing at 20 ml/hr.
Drug Label: Heparin 25,000 units in 500 mL of D5W
At this rate, how many units of heparin per hour are being delivered? 

Jan 24, '07Specialty: 2 year(s) of experience in Oncology, Medical ; From: AU ; Joined: May '06; Posts: 421; Likes: 126What's your answer? I got 25 units per hour. I am not quite sure if I am correct.

Jan 24, '07Specialty: 3 year(s) of experience in Geriatric/Psych ; Joined: Jan '07; Posts: 35; Likes: 2i believe its 1000 units/hr

Jan 24, '07Specialty: 3 year(s) of experience in Geriatric/Psych ; Joined: Jan '07; Posts: 35; Likes: 225,000 units per 500ml = 50 u/ml x 20 units=1000 units

Jan 24, '07Specialty: 2 year(s) of experience in Oncology, Medical ; From: AU ; Joined: May '06; Posts: 421; Likes: 126You're certainly correct. Geez, I suck.

Jan 24, '07Specialty: 3 year(s) of experience in Geriatric/Psych ; Joined: Jan '07; Posts: 35; Likes: 2we all have those bad days

Jan 24, '07Specialty: med/surg, telemetry, IV therapy, mgmt ; Joined: May '05; Posts: 15,027; Likes: 8,983Quote from bonny619a heparin drip is infusing at 20 ml/hr. the label reads heparin 25,000 units in 500 ml of d5w. at this rate, how many units of heparin per hour are being delivered?
you want to end up with units/hour.25,000 units/500 ml (dose on hand) x 20 ml/1 hour (dose to give) = 1,000 units/1 hour (after canceling out the label, "ml" and performing the math)another way you can figure this out is to determine how many units you have in each ml of d5w by dividing 25,000units by 500ml to learn that there are 50units of heparin per each ml of solution. if the patient is getting 20ml and hour, that is 20ml x 50 units, or 1000 units of heparin an hour.Last edit by Daytonite on Jan 24, '07 : Reason: Typo! Typed the wrong answer into the computer. Sorry. 
Jan 24, '07Joined: Jan '04; Posts: 80; Likes: 17Daytonite,
I need to take drug math this summer. I am going to take it online thru my cc here in Texas. I just want you to know that everytime I have seen a post about Drug math I look at it and study it, and then I look for your explanation. I have just about conquered my DA fear thanks to you. Now I purposely seek out more problems and try to solve them using DA so I will not be surprised when I have to know it cold for our quizzes coming this summer.
Thanks for your explanations. You are one of the few people that can explain math in an internet forum so that I actually understand most of it ! Thanks again!
Laura 
Jan 24, '07Specialty: med/surg, telemetry, IV therapy, mgmt ; Joined: May '05; Posts: 15,027; Likes: 8,983Thank you, Laura. I'm glad if my explanations are helping you. It's very hard to present math problems on this software. I can't show fractions the way I would like. And I can't show how labels are being factored out. It helps to be current in understanding how ratios work and how to do some basic manipulations with fractions. That really is all the math that is involved other than basic adding, subtracting, multiplying and dividing. Good luck with your class. You can always post a question on the forum or PM me if you are stuck on a problem.

Jan 24, '07Joined: May '06; Posts: 528; Likes: 34Yep, it was 1000 units/hr.
I'm still gettins stuck on some though. Can you use dimensional analysis on ALL med questions? It just doesnt seem to work on them all. Let me find an example.
4. Order: Ancef 1 gram over 30 minutes. The drip factor is 20gtts/mL
Drug label: Ancef 1 gram in 50 mls of NS.
The nurse would set the pump rate at how many mL/hour? 
Jan 24, '07Specialty: med/surg, telemetry, IV therapy, mgmt ; Joined: May '05; Posts: 15,027; Likes: 8,983ancef 1 gram over 30 minutes. the drip factor is 20gtts/ml. drug label: ancef 1 gram in 50 mls of ns.
the nurse would set the pump rate at how many ml/hour?50 ml/30 minutes (amount to be given in the time to be given) = x ml/60 minutes (amount to set the pump at per one hour of time) 
Jan 24, '07Specialty: med/surg, telemetry, IV therapy, mgmt ; Joined: May '05; Posts: 15,027; Likes: 8,983It might be easier for you to think of this last problem another way.
If I'm infusing 50mL over 30 minutes, how much would I be infusing over 60 minutes? That is really what the question is asking.