Everyone in medicine knows about IV fluids, but not everybody knows what they actually are and why certain ones are used. The most common ones are crystalloid and colloid solutions. Obviously if you read the title, we are going to be going over crystalloid IV fluids (yes a little cliff hanger until colloids). So why do we use them? We use them to replenish electrolytes and fluid volume in patients. This is a 101 post on crystalloids so we are not going too far down the rabbit hole of complexity. We will make a more in-depth version in the future but for right now, let’s cover the basics (we also will not be hitting on all of the crystalloids, just the main ones).
What Does it Mean to be a Crystalloid?
A crystalloid is a solution that contains solutes that have a small molecular weight such as chloride, sodium, etc. These solutions have a low tendency to stay in the vascular space because the solutes can easily pass through the semipermeable cell membranes. This should be sounding like a biology lecture you have taken but hopefully there will be less thoughts of you wanting to put your fist through a wall.
Before we talk more about crystalloids, we need to discuss plasma. Plasma is the extracellular liquid portion of blood that carries the RBCs, WBCs, and platelets. Below is a graph that depicts the important constituents of plasma as well as the various crystalloid components. The various crystalloids will vary in their impacts based off of tonicity and osmolality.
Crystalloids are subcategorized into three distinct categories:
- Isotonic- Same number of solutes as plasma.
- Hypertonic– Higher number of solutes than plasma.
- Hypotonic– Less number of solutes than plasma.
For those who need to brush up on their familiarity on these categories, click the link.
Before we get into the three types, here is a nice visual I used in school to help me remember how water would react in various solutions.
That picture is a nice visual, but to really understand how tonicity impacts cells, you need to look at the numbers.
Everything loves being at equilibrium. If a solution is not in equilibrium, it will try to achieve it by shifting fluid. In the example above of the hypotonic solution, you see there is more water outside of the cell than in the cell correct? To correct this, fluid will rush into the cell to equalize it. There is unfortunately too much solution so the cell will swell up with fluid and possibly lyse (burst). In the example next to it in the hypertonic environment, there is more water inside of the cell than in the solution. Just like with the other solution, the fluid will flow to the area with less amount of fluid. In this example, the cells will shrivel up. And finally, the isotonic solution is balanced. It’s a happy solution so it will remain that way with no fluid shifts. So now that we all understand this a little more, lets get back into the crystalloids.
As stated before, isotonic crystalloids have roughly the same number of solutes as plasma which means there won’t be any fluid shift. These types of fluids are used to increase the intravascular volume without impacting the amount of ions. The two main ones we want to hit on here are lactated ringers (LR) and “Normal” saline. Notice the little airquotes? That is because there is nothing normal about normal saline. If I could tell NS to kick rocks, I would but unfortunately inanimate objects do not listen to me. I won’t hit on NS much here because, I made a whole post on why it kills your trauma patients. So here is my shameless plug into another post if you want to read more on that.
Saline is the literal epitome of a dumpster fire so if you have options do not give it to your trauma patients.
So what about Lactated Ringers?
There has been a lot of excitement about this solution over the past several years with good reason. It has almost the exact same amount of ions as our plasma. This solution contains Sodium, Chloride, Potassium, and Calcium in it. One of the buffers, it has is lactate which is useful in patients who are acidotic (such as your trauma patients). LR is a great crystalloid for fluid resuscitation and can even be given in patients who need an increase in some ions.
There are two considerations to remember when administering LR though.
- Lactate is broken down in the liver and converted into bicarbonate so this should be considered when giving this crystalloid to a patient with hepatic dysfunction.
- As stated before, this solution contains calcium which is critical in coagulation. This solution can be given with blood products, but it is best to separate the two to prevent any adverse coagulation.
What about Dextrose 5%?
D5w is a solution containing 5% sugar in it. D5 is initially an isotonic solution but it quickly becomes a hypotonic solution once the body metabolizes the sugars. This solution only has water and sugar in it with no electrolytes. This can be given to combat hypernatremia and will increase fluids in the body. You should not use this for fluid resuscitation though because the patient can become hyperglycemic after a liter of fluid.
Hypertonic crystalloids are solutions where there is more solutes in the solution than there is in the plasma, which will cause a fluid shift. One of the main reasons to administer these kinds of fluids is to combat hyponatremia with neurological dysfunction. For those who don’t know, a severe acute onset of hyponatremia (Serum sodium less than 125 mmol/L) can cause cerebral edema as fluid is pushed into the cells which can cause neurological symptoms. The main hypertonic crystalloids you can administer are:
- 3% Saline
- 5% Saline
Both 3% and 5% Saline are used in patients with hyponatremia and in some patients with increased cranial pressure (ICP). These solutions are used to increase the osmolarity of the patient’s blood which causes a shift of fluid from the extravascular space and into the intravascular space. Patient’s electrolytes should be checked often.
Now for the sugars…
D10 and D50 are similar to D5 which we discussed in the isotonic portion. These solutions are basically water and a ton of sugar. We obviously give these to patients who are hypoglycemic as an emergent treatment to increase their BGL to acceptable ranges. Recheck BGL to ensure patient is not hyperglycemic after treatment.
Hypotonic crystalloids have less solutes than the plasma which causes a shift of fluid into the cells. These should be avoided in patients with suspected/known increased cranial pressure due to fluid shifts into the cells which can cause them to swell. These fluids are not normally used for fluid resuscitation but for maintenance in patients with fluid loss from urine production. The only one we will really talk about is: 0.45% saline (1/2 saline).
0.45% saline is commonly used to treat patients with hypertonic extracellular dehydration. Don’t use this in patients with renal disease, hyponatremia, or hypervolemia.
Always make sure you monitor your patient’s electrolytes and urine output to ensure they are being administered the correct amount of crystalloids. As I said this is just a 101 post on crystalloids so we didn’t want to go too in-depth just yet (that will be for another post).
This site is meant to be used for educational use only. We strive to push evidence based medicine with no bias to help you obtain all the important information. You should always follow your protocols that have been set in place.
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-Scopeducation Team (Matt)
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Mason, A. (2020, July 08). Hypertonic Fluids. Retrieved November 25, 2020, from https://www.ncbi.nlm.nih.gov/books/NBK542194/
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