IVPB flow rates

0 I'm having problems determining the correct volume to use when calculating IVPB flow rates.
I'm using the V/T x C = R equation.
Example 1 from my text:
cimitidine 250mg q6h in 50 mL D5W IVPB to be infused in 20 minutes
available 300 mg/2 mL of cimitidine
drop factor 10 drops/mL
I calculated that you would give 1.7mL of cimidine
I would think you would add the 1.7mL to the 50mL then calculate V/T x C = R.
The book has us calculating 50mL, not 51.7mL as V.
Using 50 mL you get 25 gtt/min, using 51.7 mL, you get 26 gtt/min.
Which one is right?
Example 2:
theophylline 100 mg in 50 mL of D5W x 30 minutes q6h IVPB
theophylline available 80 mg/15mL
drop factor is 60 gtt/mL
I calculated to give 18.8 mL of theophylline.
The book adds the 18.8mL to the 50mL to give 68.8 mL for the Volume.
Why is this one added? Why would you use 68.8 and not 50mL for Volume.
Using 50 mL for Volume, you get 100 gtt/min. Using 68.8 for Volume, you get 138 gtt/min.
The correct answer in the book is 138 gtt/min.
When do you add them together? It doesn't say in the book? Is it a certain mL? If you have less than 5mL do you not add? 

1Apr 29, '10 by RNTutorI remember discussing that same question with my classmates when I was in nursing school! You are actually exactly right.
If the medication volume is LESS than 5ml, then you DO NOT add it to the total volume. If the medication volume is MORE than 5ml, then you DO add it to the total volume. This helps make sure that the pt is getting the full dose of their medication, at the proper dilution.
It's also a good idea to check with your nursing dept, just to see if they have their own guidelines for testing purposes. But that is the rule I have always used, and it is confirmed in the text that I use for a math reference ("Clinical Calculations", by Kee & Marshall, 5th edition, pg 212).scifihippie likes this. 
0Apr 29, '10 by scifihippie, BSN, RNTHANK YOU!!
I have my final tonight and the dosages HESI next week. I have emailed my instructor many times about this and her response is a bunch of ambiguous nonanswers. 
0Apr 29, '10 by RNTutorHaha, it's so funny because that's what happened to me in nursing school too!! My classmates and I kept asking our profs, and I guess they weren't sure either so they would just refer us back to the book, or give an ambiguous nonanswer. Once we found it specifically said that in our book, we figured they couldn't mark us wrong if they used that rule.
Does your school use HESI as an exit exam (i.e. you have to pass it to graduate/pass the class), or just as a way to check how you're doing? 
0Apr 29, '10 by scifihippie, BSN, RNEach semster you have to take a HESI and it's 5% of your grade. I just took Foundations on Tuesday night and then Dosages in next week. So, for now, it's just a check on how you are doing. In the final semster it's different, not only does it count as 5% of your grade, you must make a 950 on it to graduate.

0May 2, '10 by goopsyscifihippie,
do you have any advice on the foundations hesi? I am taking that soon, and have been answering all the questions I can find online and in the back of my foundations book. Do you feel like you were well prepared or do you think there was something else you could have done to prepare yourself? Any advice at all helps. thanks.