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In this lesson, we’re going to talk about hypertonic solutions. What are they, how do they affect the body, and why do we use them?
Again, let’s quickly review what we mean when we talk about tonicity. Tonicity compares the osmolarity of two solutions. In these cases, we’re comparing an IV fluid to blood plasma. If we have a solution that is less concentrated than blood plasma, or has a lower osmolarity, it’s considered hypotonic. If the solution has a similar concentration, or osmolarity, we call it an isotonic solution – iso meaning ‘same’. If the solution has a higher concentration or osmolarity, we call it a hypertonic solution.
So, when we’re looking at a hypertonic solution – that means it has an osmolarity that is HIGHER than the blood plasma, typically greater than 375 mOsm/L.
Some examples are really anything higher than 0.9% sodium chloride – so 1.5%, 3%, or 5% sodium chloride. To give you an idea of how powerful hypertonic these are – the osmolarity of 3% saline is 1026 mOsm/L. That’s literally over 3 times more concentrated than the blood plasma. Other ones would be adding 5% dextrose to an isotonic solution like NS or LR, or having more than 5% dextrose in water – specifically we can use 10% dextrose in water, or D10W. We also see that D5½NS is hypertonic in the bag. But something similar happens here that happens with D5W. The dextrose portion can get used up and sometimes cause this to be more isotonic than anything else. The osmolarity is just over 400 mOsm/L, so once you use up those sugar molecules, the osmolarity drops a lot closer to the actual osmolarity of the blood itself. We use this a lot in patients with DKA actually, once we’ve brought their sugars down we give them this to help balance their sugars and maintain the fluid in their vessels. So it’s kind of a tricky hypertonic one. The most common things you’ll see us give that are hypertonic are 3% Saline, D5NS and D10W.
As we already mentioned, the osmolarity of a hypertonic solution is greater than 375 mOsm/L, remember that blood is about 275 – 295 mOsm/L. So we’re introducing a solution that is much more concentrated into the blood vessels, and the blood plasma will now be more concentrated than it was before, compared to the cells. When you have a higher concentration on one side of a semipermeable membrane, which way is the fluid going to shift? The fluid will want to shift toward that side, right? So what we see is the fluid shifting out of the cells and into the blood plasma. That means the cells are going to shrink. Of course, if they shrink too much, they won’t work properly. But, as you see…sometimes we’re actually trying to get them to shrink.
So why would we use one of these super concentrated solutions? The two MAIN reasons you’ll see it used in the clinical setting are hyponatremia, or low sodium levels, and cerebral edema. Remember normal sodium levels are 135-145…but when I say low sodium, in this case, I don’t mean 132. We wouldn’t give a hypertonic solution for that. I’m talking in the 120’s or even 110’s – super dangerous range. We’re gonna give a 3% sodium chloride solution to try to get that sodium level back up. Now – we talk about this in more detail in the hyponatremia lesson, but it’s SO important that you know that we shouldn’t correct sodium too quickly because it can cause severe neurologic damage. Make sure you check out that lesson to learn more. The other main reason we use hypertonic solutions is for edema – usually cerebral edema, but it could also be other kinds of edema. Again, the goal is to shift fluid out of the cells and tissues and into the bloodstream. This can help alleviate the pressure in the brain or any other issues caused by this edema. Again, sometimes we actually want the cells to shrink a bit. Other precautions you need to know is that hypertonic saline – that’s 3% or higher, and at some facilities even the 1.5% saline, – MUST be given in a central line. It is way too caustic and hypertonic to be used peripherally, it can cause a lot of issues. Also, remember the whole point here is to shift fluid into the blood vessels, so we need to use extreme caution in any patient at risk for volume overload like heart failure or renal failure. So we watch for signs of overload like shortness of breath, decreasing oxygenation, or crackles in the lungs.
Okay let’s recap – remember that a hypertonic solution has more solute than the blood plasma – so it’s going to make the blood more concentrated than it was before. That will cause fluid to shift into the blood plasma and out of the cells and tissues. Examples are hypertonic saline, D5NS or D5LR, and D10W. The main reason we use hypertonic solutions is to correct hyponatremia and treat cerebral edema. Remember that we have to correct sodium SLOWLY to prevent neurologic damage – more about that in the hyponatremia lesson. We also want to use a central line or a central venous catheter when giving hypertonic saline and we use extreme caution to prevent volume overload in patients with heart or kidney failure.
So that’s it for hypertonic solutions – make sure you have also checked out the isotonic and hypotonic solutions lessons, as well as all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!