Hypertonic Solution: An Explanation for Nursing Students
I’m going to help you understand WHY and HOW a hypertonic solution works so that you can be confident in knowing which situations to use them in for nursing school.
If you haven’t already seen the explanation of results from my Osmosis Experiment, make sure to check that out first. It’s not only interesting, but I’ll be referring to it in this series of articles about IV solutions.
I can’t wait to talk about Hypertonic IV fluids because I think they’re so stinkin’ cool.
And yes, I do realize that makes me a really big nerd 🙂
What is a Hypertonic Solution?
Let me back up a second. Do you understand what “Hypertonic” means? Dictionary.com defines it as:
“A solution of higher osmotic pressure than another solution with which it is compared.”
How’s that for a confusing, vague sounding definition? Let’s see if I can explain it better.
The main thing to remember is that we always have to compare TWO fluids in order to define the “tonicity” (hyper/hypo/iso) of each solution.
So when we say that a solution is hypertonic, what we REALLY mean is that there are more solutes in the hypertonic solution than there are in the solution that we are comparing it to. (Need a refresher on what solutes, solvents, and solutions are?)
And I know exactly what you are thinking right now: “But what about Hypertonic IV fluids? What are THEY being compared to?”
Hypertonic IV Fluids
In nursing, we almost always compare solutions to something in the human body! So when we say that an IV solution is Hypertonic, what we are really saying is that the hypertonic fluid has a higher solute to solvent ratio than blood does.
For the purposes of IV fluid, this means that blood becomes our definition of what “Isotonic” means.
What are some examples of Hypertonic IV Fluids?
Of all the IV solutions that nursing students have to study, Hypertonic IV solutions are probably the least used, and (in my opinion) can be the most confusing to understand. Arguably, they can also pose the highest risk of complications! Here’s a list of the most commonly used Hypertonic solutions:
- 10% Dextrose in Water
- 3% Saline
- 5% Dextrose in 0.45% Saline
- 5% Dextrose in 0.9% Saline
Understanding the Dextrose IV Solutions
Notice that three of the hypertonic solutions listed above contain Dextrose, which is a sugar. The purpose of adding sugar is to provide extra calories to the patient.
The dextrose (sugar) is what makes these 3 solutions hypertonic: there is more solute per liter in the IV solution than there is in the blood. BUT once the solution enters the patient’s blood stream, the dextrose moves into the various cells of the body. With the dextrose out of the blood, all that’s left of the original solution is the Saline or Water (depending on which IV fluid was used, of course).
Which leads us to the potentially confusing part: after the dextrose has been used up by the body, what’s left of the solution is either hypotonic (5% dextrose in 0.45% saline and 10% Dextrose in Water) OR isotonic (5% Dextrose in 0.9% Saline).
So you see that while dextrose solutions are technically hypertonic, they quickly leave the patient with only pure water or saline in the blood vessels, depending on which solution was used. At that point, these solutions behave functionally like either hypotonic or isotonic solutions.
Understanding 3% Saline Solution
3% saline is a “true” hypertonic solution and needs to be administered with extra care. It acts like a hypertonic solution even when it’s inside the body (unlike the Dextrose solutions described above).
3% Saline Solution is typically used to treat severe cases of hyponatremia (low sodium), a situation where you might want to add sodium into the blood stream, but not fluid.
(Just a reminder: saline = NaCl = sodium chloride)
Think about it this way: if your blood is normally isotonic, then decreasing the amount of sodium ions in it will make it a bit hypotonic in comparison to what your blood SHOULD be. This is because there are now fewer solutes (sodium) in the solvent (blood).
In order to get blood back to its normal isotonic state, we would want to add MORE solutes without adding much solvent. In other words, we want to add more sodium without adding water/saline. This strategy can help move blood levels back to their isotonic normal, thus correcting the hyponatremia.
Hypertonic solutions are also useful to patients with fluid overload when they need electrolytes. This includes conditions such as Heart Failure or severe edema. 3% Saline can provide patients with needed electrolytes, all while adding minimal water.
Possible Complications of Hypertonic Solutions
Even though hypertonic solutions can be useful, they require caution. Generally speaking, you’ll want to infuse IV solution of 3% Saline VERY slowly. Watch the patient closely for any evidence of intravascular overload (aka too much fluid in the blood vessels).
Intravascular overload can happen when too much water moves into the blood vessels. This excess water can come from the interstitial space, or even from the blood cells! Osmosis is at work!
Remember that osmosis is the movement of water as it attempts to create equal dilution levels wherever it goes. Since you are purposely injecting a large amount of sodium into the patient’s blood vessels, water may start moving OUT of the interstitial space and INTO the blood vessels, too!
This extra fluid being pulled into the blood vessels can lead to increased blood pressure and pulmonary edema (aka fluid in the lungs). Ironically, it can even lead to hypernatremia from too much sodium being added!
The Role of Hypertonic Fluids in Anatomy & Physiology
Take a look at the “after” pic from my Egg Osmosis Experiment. Now imagine that the third egg is actually your patient’s Red Blood Cells (RBCs)!
When you immerse the egg in a hypertonic solution, the water that was inside the egg is pulled out in an attempt to equalize the hypertonic solution around it. And the exact same thing can potentially happen to RBCs if we inappropriately administer a hypertonic solution such as 3% Saline.
That’s a very serious case of egg-dehydration.
Unlike this egg experiment, the human body has more than one semi-permeable membrane to help regulate how fluid moves throughout the body. One of these many membranes surrounds each RBC. So
if your RBCs are suddenly floating around in a solution that is hypertonic, then they are going to start leaking water via osmosis.
These dehydrated RBCs are at risk of dying unless the water balance is quickly reversed. Obviously, that’s not a good situation for our patient to be in, especially since they already have other health problems going on!
Keep in mind that this state of dehydration is at the cellular level. So even though our patient may appear to be well-hydrated on the outside, perhaps they’re urinating appropriately with good skin turgor, their RBCs can still be dehydrated due to the type of IV solution we injected into their body.
In addition to the RBC membrane, nurses also need to consider the semi-permeable membrane of the blood vessel wall. When you inject hypertonic solution into the blood vessels, you risk pulling water not only the RBCs, but also from the interstitial space! All of this additional water moving by osmosis into the blood vessels can quickly cause high blood pressure (and all the complications that come with it) if not done carefully.