Isotonic, Hypotonic and Hypertonic IV solutions

Nursing Students Student Assist

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I was told that .9% saline solution is "normal" or isotonic for our cells.

However, if 5% dextrose is put into an IV and combined with 1/4 of .9% saline (therefore around .22% saline) then this is still considered "isotonic."

I know that 5% dextrose seems to be the magic number; meaning that's about the max amt. of sugar you can add to an IV bag for it still to be considered "isotonic."

However, in reading, I see that if you add 2.5% Dextrose to .45% saline then you're still ok......I pretty much "get" all that.

My big question is.........how is the 1/4 of .9% saline quantity derived? Is there a magical equation or allowed amount??

Ex...if saline is less than______ this then Dextrose can be ______ or vise versa. etc. etc.

Just curious how 5% Dextrose with just water is ok, but 5% Dextrose with 1/4 of .9 is also ok.....how is that possible? It must be some type of equation, chemical balance thing, or something else for it all to work out OR....is it as simple as 1/4 of .9 is such a small amount that it can still be considered Isotonic if combined with 5% Dextrose.

I hope that makes sense as to what I am asking. Please advise.

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Specializes in Neuro, Telemetry.

Are you talking about 1/4 NS or 1/4 of a bag of NS? 0.9% saline is just NS. If it is not 0.9% then it is not NS. It would be 1/4 NS or 0.5 NS. I have not heard of a solution called 1/4 0.9% NS. Only 1/4 NS, meaning the solution is 1/4 that of NS. I have also seen it labeled 0.25 NS or 0.225 NS. All the same thing. And the key here is tonicity. It's not necessarily an equation. It's what the tonicity of the solution is once infused into the body. Tonicity is determined by the electrolyte content of the solution. I'm sure you've read up on how the body's normal serum has about 300mEq/L electrolyte content. So an isotonic solution would match or be close to the body's normal level once infused. Too much higher or too much lower is what determines if a solution is hypo or hyper. I don't think we learn how to "equate" the serum electrolyte level of the fluids since a that's not our job. Out job is just to know the different fluids, what osmolality they are, and what they are used for.

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Specializes in Medical-Surgical/Float Pool/Stepdown.

I would suggest looking into buying a resource like Fluid and Electrolytes made easy, the book helps explain the concepts in different ways and then you'll always have it around for a quick review...

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Specializes in Vascular Access.
Are you talking about 1/4 NS or 1/4 of a bag of NS? 0.9% saline is just NS. If it is not 0.9% then it is not NS. It would be 1/4 NS or 0.5 NS. I have not heard of a solution called 1/4 0.9% NS. Only 1/4 NS, meaning the solution is 1/4 that of NS. I have also seen it labeled 0.25 NS or 0.225 NS. All the same thing. And the key here is tonicity. It's not necessarily an equation. It's what the tonicity of the solution is once infused into the body. Tonicity is determined by the electrolyte content of the solution. I'm sure you've read up on how the body's normal serum has about 300mEq/L electrolyte content. So an isotonic solution would match or be close to the body's normal level once infused. Too much higher or too much lower is what determines if a solution is hypo or hyper. I don't think we learn how to "equate" the serum electrolyte level of the fluids since a that's not our job. Out job is just to know the different fluids, what osmolality they are, and what they are used for.

Normal Saline (NS, 0.9% Sodium Chloride) is Isotonic. This means that its milli osmo's are around 290, which is the same as normal serum osmolarity/osmolality (you'll see it addressed by some both ways). Anything solution which has its mOsm/L between 240-340 is considered isotonic, or just like blood plasma. Below 240 = Hypotonic. Above 340 = Hypertonic. 1/4th Sodium chloride is NOT .25.

1/4th Sodium Chloride is .225%. Think of it this way. If Normal Saline = 0.9%, then 1/2 NS = 0.45% and 1/2 of that = 0.225%.

Now if you combine two osmolarities ( each being hypotonic by themselves ), but now they are combined, then you may very well have an isotonic solution, with its final osmolarity being between 240-340. However, remember that Dextrose in water will act in the body as a Hypotonic solution. Your brain quickly metabolizes the sugar, leaving only free water, which is hypotonic.

There are many resources online for you to goggle for you to make sense of this topic.

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Specializes in Critical Care, Education.

AH SO! Turns out that all that basic Chemistry stuff is important after all.

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Now if you combine two osmolarities ( each being hypotonic by themselves ), but now they are combined, then you may very well have an isotonic solution, with its final osmolarity being between 240-340.

Uh-oh. I know you probably didn't mean to imply that a student can mix two bags of 250cc of 0.45% and have 500cc of 0.9%, but that's what this sounds like. The total amount of water has something to do with osmolarity, and the concentration doesn't change. :)

Ah, yes, HouTx. This is why I flinch when I read about nursing students who aren't required to take chemistry in their programs. OMG.

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Specializes in Vascular Access.
Uh-oh. I know you probably didn't mean to imply that a student can mix two bags of 250cc of 0.45% and have 500cc of 0.9%, but that's what this sounds like. The total amount of water has something to do with osmolarity, and the concentration doesn't change. :)

Ah, yes, HouTx. This is why I flinch when I read about nursing students who aren't required to take chemistry in their programs. OMG.

Not sure how you came to the conclusion that I was implying that one could take 0.45% Sodium Chloride and add an additional bag of equal ingredients and it becomes NS??? But, if so, that wasn't my intent.

What was being referenced here is that there are different types of Sodium Chloride solutions and that 1/4 wasn't .25 sodium chloride. ... Also what was being said was that if One HAD a liter bag with two ingredients, ie. 1 L D5/0.225 Sodium Chloride, then that bag has an osmolarity that will be isotonic in nature, or hypertonic in nature becuase you are combining osmolarites. Now, a solution may be isotonic in the bag only, and act as a hypotonic in the body, but what was being referred to here is mOsm/L.

NS is isotonic, but NS with 5%Dextrose included in the bag is not. Again, when you have two additives, the osmolarity changes. Remember that the pH doesn't change, but mOsm do, with greater diluents.

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Not sure how you came to the conclusion that I was implying that one could take 0.45% Sodium Chloride and add an additional bag of equal ingredients and it becomes NS??? But, if so, that wasn't my intent.

It was when you said this:

Now if you combine two osmolarities ( each being hypotonic by themselves ), but now they are combined, then you may very well have an isotonic solution, with its final osmolarity being between 240-340.

It is not possible to mix two hypotonic solutions and come up with an isotonic one. Just ... not.

There is no such thing as an osmolarity as an entity. Osmolarity is a quality. You could say, "Two solutions of different osmolarities" but an individual ingredient, like NaCl or dextrose, doesn't have osmolarity unless it's in solution, and it's the solution that has the osmolarity as a result of the concentration of the ingredient in the diluent.

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