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Before we start, 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 isotonic solution - that means it has an osmolarity that is similar to the blood plasma, typically between 250 and 375 mOsm/L. Remember blood is between 275-295 mOsm/L.
Some examples are normal saline - which is 0.9% sodium chloride. It has an osmolarity of 308 mOsm/L. The other common fluid we see is Lactated Ringers or LR. This fluid is a mixture of sodium chloride, sodium lactate, potassium chloride, and calcium chloride in water. So it has more than just sodium in it, really important to know that. Also - another fact that is good to know - sodium lactate will actually convert bicarb in the body - so we’ll see this used specifically to treat metabolic acidosis because the bicarb can help buffer the acids. We also classify D5W, or 5% dextrose in water, to be isotonic in the bag because it has an osmolarity of 252 mOsm/L (so it’s over 250). The big distinction here, though, is that once it enters the bloodstream, this dextrose - which is just sugar - is actually used up as energy by the body. So it leaves us with something a lot closer to just water. So in the BAG it’s isotonic - but in the body it acts as a hypotonic solution. And finally any colloids are considered isotonic - those are blood products, so that’s red blood cells, fresh frozen plasma, albumin, etc., as well as Hetastarch.
As we already mentioned, the osmolarity of an isotonic solution is between 250 and 375 mOsm/L. So the solution we’re introducing to the bloodstream has the same (or similar) concentration to the blood plasma itself. So what we see is that there is actually NO net shift in fluids. That means it really doesn’t affect the cells because the concentrations will balance out easily. The benefit of this is that it means we can effectively increase the extracellular fluid volume - specifically we can increase the volume inside the blood vessels - or the intravascular volume.
And we’re going to see that that is one of our biggest benefits - increasing intravascular volume. So we’d give these fluids if a patient has experienced blood loss - maybe because of a trauma or a surgery - or is experiencing dehydration - or really any other extracellular fluid loss like excessive vomiting or diarrhea. We can also use these during fluid resuscitation - again remember they help increase our blood volume so they’re perfect for that. We can even put these in a pressure bag and give them rapidly if necessary. We can also just use these for general hydration or maintenance fluids, especially if someone is NPO and can’t take oral fluids. A couple cautions to know - if you’re administering blood products, you can ONLY use normal saline. Even though the other solutions are considered isotonic, they can still cause issues in the IV tubing with red blood cells - so we ONLY use 0.9% sodium chloride - or normal saline. Another thing to consider is patients with heart failure or kidney failure because they are at high risk for volume overload if we give them too much too fast. And again, we know that LR can convert to bicarb in the blood, so we wouldn’t want to give it to a patient who has metabolic alkalosis, because they already have too much bicarb. Check out the lessons on metabolic acidosis and metabolic alkalosis to understand that a little better.
Okay let’s recap - remember that a isotonic solution has about the same solute amount as blood plasma - so it’s actually not going to cause any fluid shifts in or out of the vessels or the cells. Examples are 0.9% sodium chloride, or normal saline, Lactated Ringers or LR, D5W in the bag, and colloids. Remember also that 5% dextrose will actually be hypotonic in the body because the dextrose gets used up and what’s left is just sterile water. The main reason we use isotonic solutions is to increase intravascular volume when someone has lost a bunch of blood or fluids or even when they just need good maintenance hydration. We are cautious in heart failure because of volume overload - so we don’t give too much too fast, and we know that NS is the only solution that should EVER be given with blood products.
So that’s it for isotonic solutions - make sure you have also checked out the hypotonic and hypertonic 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!!
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