IV fluids: How to easily calculate what your really giving.




I give this mini lecture quite often to the residents and medical students during the course of the rotations.  I thought I would pass this on to you all as well.

Question for you. How many grams of Salt are you giving your patient when you give a liter bag of 'normal saline?'

So let's start with the basic concept of 0.9% Saline.

What does the 0.9% actually mean?

Well 0.9% is a standard measure, in this case indicating 900 mg/dL.  

So that's 900 mg of sodium chloride per 100 cc or 9000 mg (9 grams) per 1 liter bag of normal saline! 

My canned joke here is your giving NS at 100 cc/hr=2.4 Liters/day x 9 grams/liter is over 20 grams of salt but....you have him on a 2 gram sodium diet.  Good thinking :)  🤦‍♂️

Ok so how many grams of sodium are in half normal saline? 

Well of course you would say 4.5 grams and you would be correct but let's look at the math.

0.45%=450 mg/dL x10 dL=4500 mg of salt/L of 1/2 or 0.45% saline.

Now the deeper question.  So how many MEQ per liter of normal saline, or 1/2 Normal saline.  🤔

It's easier than you think to calculate that.  

Sodium chloride has a molar mass of 58.44 g/mol (google search <1 second) 

If you have a bag of 0.45% normal saline and, as above, you have 4.5 grams/Liter of Normal saline then to figure out the meq/L...

Take 4.5 g/L and divide by 58.44 g/mol-->0.077 mol/L convert to milimoles by x 1000 and you get 77 meq/L of Sodium and 77 meq/l
of Chloride so grand total=154 meq/L.  

Let's take this one step deeper.  Say you have your 1 liter bag of D5W.  You’ve made your patient NPO after midnight.  

Question. How many CALORIES are in this bag?  

D5W is D5% 

5% is 5 grams/dL and so 50 grams/L of dextrose. 

A Carbohydrate is 4 kcal/gram so 200 calories in one bag of D5W. 

It turns out due to the nature of Dextrose, a gram of Dextrose is actually 3.4 Kcal so it is 3.4 x50 which is 170 calories but you get the idea.

Last one.  Let's say you want to make a bag of 0.2% saline with 40 meq KCL.  

How many grams of Sodium chloride?  

Correct 0.2= 200 mg/dL x10=2 grams 

We know already there is also 40 meq KCL 

So how many meq of solute is this bag total ?

2 grams of NaCl/L divided by 58.44 g/mol is 0.034 mol/L of Na and 0.034 mol/L of Chloride 

Multiply by 1000 for meq and you get...68 meq NaCl + 40 meq KCL=108 meq/L quite a hypotonic solution.  

Hope that helps explain what’s in fluids and how to easily figure out what your giving or getting! 

Please check out our website for a look at UreaAide, a better tasting lower cost Urea for SIADH. 

https://www.KidneyAide.com 







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