Peds med math help - making a bag 1/4 NS?

[mmeezyy]

Hi everyone. I just started my fourth semester and we're about to take another med math test on Friday for our peds and L&D rotation. I'm usually pretty good with med math, but this question has stumped me and I was wondering if any one could help? My instructor went over it in class, so I've got how to work it out, I just have no idea where some of the numbers come from!!

1. You have an IV order for D5 1/4NS with 20meq KCL/L to run 1L in 12 hours. You have a 500ml bag of D5W.

a. You need to add concentrated NACL to make ¼ NS. NS has 154 mEq/L Concentrated NaCl comes in 4mEq/ml. How many ml will you add to the bag to make ¼ NS?

answer: 4.8 mL

b. KCl comes in 40meq/20ml. How many ml will you add to the bag?

answer: 5 mL

c. How many ml/hr will the IV run?

83 mL/hr

Thank you!!!

[medic978]

you need to change the numbers they are giving you so everything is the same L to mL and that sort of stuff I would post the work but don't know if id get in trouble

[medic978]

My other question is if you have the answers what did you use to get them?

[JustBeachyNurse, LPN]

What did you do to get answers and what were your answers?

We can guide you if you show your work , even if wrong we can point you in the right direction.

Make certain all your units match (use all mL not L & mL)

[mmeezyy]

We went through it briefly in class so I wrote down how she worked it out, I'm just not sure where some of the numbers(bolded in the problems) come from? This is the work:

a) 154/4 = 38.5 mEq/L

38.5/2 = 19.23/4 = 4.8 mL

b) 10/40 x 20 = 5 mL

c) 1000/12 = 83.3 mL/hr - this one I understand.

Thank you so much for your time!

[medic978]

ok so you showed me your work here is mine

lets see if I can do this in the post box

the concentration of NS is 154 mEq/L times 0.25 for 1/4 concentration that gives you 38.5 mEq/L

1ml x 38.5 mEq/L x 1L x 500ml

4 mEq 1L 1000ml =4.8125ml = 4.8

you have to make sure you change over to ml instead of liters

2.) 40 mEq/20 ml 20mEq KCL/L same as 20 mEq KCL/ 1000ml divided by 2 because you are using a 500 ml bag =10 mEq/500ml

20 ml x 10 mEq = 5ml

40 mEq

3 your wanting to give 1L in 12 hrs or 500ml in 6 hrs 500ml/6hrs = 83ml/hr

[medic978]

hope that helps

[KelRN215, BSN, RN]

We went through it briefly in class so I wrote down how she worked it out, I'm just not sure where some of the numbers(bolded in the problems) come from? This is the work:

a) 154/4 = 38.5 mEq/L

38.5/2 = 19.23/4 = 4.8 mL

b) 10/40 x 20 = 5 mL

c) 1000/12 = 83.3 mL/hr - this one I understand.

Thank you so much for your time!

For the first one, for straight NS you have 154 mEq/L. How many mEq in your 500 mL bag? Half of 154, right? So 77 mEq per 500. BUT that is straight normal saline and you want 1/4 normal saline so you divide by 4 and you get 19.25 mEq that you need to add. Your normal saline is available to you as 4 mEq/mL so you divide 19.25 by 4 and you get your 4.8 mL.

The work you present is a little different than how I did it but the answer works out to the same. Your instructor divided by 4 in the first step to figure out how many mEq/L in 1/4 normal saline. She then divided by 2 because your bag is 500 mL and the concentration given is per liter. Then divide by 4 again because you have 4 mEq/mL of normal saline.

For the second one, you need to add 10 mEq to your 500 mL bag because the order is 20 mEq/L. That's where the 10 comes from. 40 mEq/20 mL = 2 mEq/mL so you need to add 5 mL to make 10 mEq.

[mmeezyy]

Thank you both so much!!! It makes a lot more sense now. KelRN215, the way you worked through part A is how I like to work through my problems, so thank you for explaining it that way. :)