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Intravenous Therapy 101: The Main Types of IV Fluids

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Intravenous Therapy 101: The Main Types of IV Fluids

Patients are prescribed IV bags for any number of reasons. And to patients who don’t know their medicine, one IV bag can look pretty much the same as the next. 

Here’s the kicker: your bag of saline may have a totally different chemical composition than the person next to you, even if they look exactly the same. It isn’t just a matter of the Myers cocktail versus other nutritional combinations during IV nutrition therapy–the fluid itself may be a totally different solution. 

One solution may be good for dehydration, while another solution might not ever be used for you due to various health complications. 

Want to know what’s in your fluid? Here are the different types of IV fluids, what makes them unique, and what each solution can be used for. 

What’s in an IV Bag?

An IV bag might look like a bag of water, but there’s a lot more going on than good old H2O. 

The exact content of a fluid bag will vary somewhat based on the needs of the recipient, but it always contains a saline solution of some kind as a carrier for fluids and electrolytes. If you’re getting IV therapy, your fluid bag will also contain vitamins and minerals to give you an extra boost. 

Sounds simple, right? Here’s the thing: not all saline solutions are created equal. 

A Few Definitions

But before we delve into why that is, we first need to cover a few basic definitions that will help you to better understand what that little bag of fluid is doing to your cells. 

Osmosis

First is a term you probably remember from high school biology class: osmosis (as in: learning by osmosis, the thing your teacher always told you didn’t actually exist). 

Osmosis is an important mass transport process in biology. It’s the process by which solvent molecules move through a semipermeable membrane from a dilute solution to a concentrated solution. This serves to equalize the concentration of solutes on both sides of the membrane. 

In plain English, that means that molecules move in or out of a membrane. When they do this, they either create a more concentrated solution or a more diluted solution, depending on the circumstance. 

This is best explained by water, the most common culprit in osmosis. Water likes to balance itself out (i.e. rush to places where there isn’t water). So if a cell was low on water, water molecules would rush into the cell, regulated by the cell membrane. If the inside of the cell had more water molecules than the outside of the cell, the water molecules would rush out. 

Osmotic Pressure

Osmosis is regulated by osmotic pressure, which is the pressure necessary to prevent the inward flow of water across a membrane. This is what protects a cell from taking in more water than it can handle, which would cause the cell to burst. 

In other words, it’s the pressure necessary to stop osmosis from occurring. 

Understanding the Basics

Osmosis and osmotic pressure are essential players when it comes to IV fluids and your cells. In fact, IV fluids are specifically designed to create a certain reaction in your cells based on osmosis, depending on what you’re trying to achieve. 

Basically, whether something flows into or out of your cells has a significant impact on how your body responds to it. So when we talk about IV fluids and IV solutions, what we’re really discussing is how these solutions affect osmosis and osmotic pressure in your cells. 

IV fluids are either crystalloids or colloids. A crystalloid or colloid solution can also be isotonic, hypotonic, or hypertonic, and that directly affects what that solution is used for. 

If it all sounds like Greek to you, it’s not as complicated as it sounds. 

Crystalloids

Crystalloid solutions contain small molecules that easily flit across semi-permeable membranes. Think of your cell membrane like a strainer. Crystalloid solutions (or rather, the particles in crystalloid solutions) are small enough that they can get through the holes in the cell membrane. 

This means that crystalloid solutions are good at traveling into your cells and making the contents of the solution available for use. 

Because of this, crystalloids are used when healthcare professionals want to increase fluid volume and intravascular space, as in the case of hypovolemia (loss of plasma) caused by burns, trauma, or post-operative recovery. 

Colloids

Colloid solutions, unlike crystalloid solutions, contain larger molecules. Because of this, they do not cross semi-permeable membranes as readily as crystalloid solutions. 

In healthcare terms, this means that colloid solutions, unlike crystalloid solutions, remain intravascular. In other words, they remain in your bloodstream rather than entering your cells. This also means that they stay in your blood longer than crystalloid solutions. 

For this reason, they’re used as plasma expanders as a form of fluid resuscitation in cases of severe hypovolemic shock. 

Isotonic, Hypotonic, Hypertonic

We mentioned earlier that crystalloid and colloid solutions can be isotonic, hypotonic, or hypertonic. 

Isotonic solutions have solute concentrations that are different than those of your cells. This means that there is no concentration gradient across the cell membrane, which means that your cells neither expand nor shrink in the presence of an isotonic solution. 

Hypotonic solutions have solute concentrations lower than those of your cells. This means that, in an effort to balance the solute concentration, water will rush into the cell, causing it to expand. 

Hypertonic solutions have higher solute concentrations than those of your cells. In order to balance the solution, water will flow out of the cell, causing it to shrink. 

A professional will make a choice between isotonic, hypotonic, and hypertonic solutions based on which way they want osmosis to work. For example, since isotonic solutions maintain osmotic pressure on the inside and outside of the cell, they’re often used to treat vomiting, diarrhea, shock, and metabolic acidosis, among other conditions. 

On the other hand, because hypotonic solutions create cell swelling, patients who receive these solutions have to be monitored for hypovolemia and hypertension. These solutions should not be administered to patients with increased intracranial pressure as it may exacerbate the effects of cerebral edema (a.k.a. brain swelling). 

Hypertonic solutions draw fluids out of the cells, which means that patients must be monitored carefully. Any IV solution comes with its own associated risk, but hypertonic solutions are especially risky because they can result in intravascular fluid overload and pulmonary edema (excess fluid in the lungs). 

For this reason, hypertonic solutions cannot be used for extended periods. 

Saline Solutions

We said earlier that not all saline solutions are created equal. As you can see, one bag of saltwater is not the same as another bag of saltwater–at least, not if one is isotonic and one is hypertonic. 

The gulf between crystalloid and colloid saline solutions is significant, even before you factor in whether the solution is isotonic, hypotonic, or hypertonic. 

Nonetheless, the basic content of a saline solution remains the same: sodium (a.k.a. salt) dissolved in sterile water. 

Why Saline? 

This simple solution has been a staple of modern medicine for more than 150 years–doctors go through enough saline solution to fill an Olympic pool every five days. 

And while there’s some debate about the salt content of your saline, the fact remains that saline solution is the fluid of choice for most doctors in the United States. 

Surprisingly, the transcendence of saline (specifically normal saline) is based on the experiments of one man, a Dutch scientist named Hamburger. In 1883, Hamburger argued that human blood was 0.9% salt and that a solution of equal concentration would be a “normal” composition for intravenous fluids, which is where the solution gets its name. 

The fact that Hamburger was wrong in his argument about salt has had apparently little effect on its widespread adoption by the medical community at large. Normal saline actually has a much higher concentration of sodium and chloride than the average human body. It’s actually closer to 20 snack-size bags of potato chips. 

If that sounds surprising to you, keep in mind that saline solution hasn’t actually been studied that closely in clinical trials. 

Nonetheless, doctors use various saline solutions all the time. They’re kept on hand to flush wounds, replenish fluids, sustain patients through surgery, or deliver nutrients (as in the case of IV nutrient therapy). 

Chemically speaking, saline solution does have its uses as a vehicle of delivery, particularly if you’re concerned with controlling osmosis. This is perhaps why the use of saline solutions remains so widespread (at least in modern times). 

Types of IV Fluids

With that in mind, let’s take a closer look at some of the most commonly used IV fluids currently in use in medicine. 

All of these solutions can be classified as crystalloid or colloid and as isotonic, hypotonic, or hypertonic, which has a direct impact on how the fluids can be used. Crystalloid solutions remain by far the most common, largely due to the overwhelming presence of normal saline in most hospital and healthcare settings. 

Normal Saline

The best-known name is normal saline, sometimes called 9% normal saline, NS, or 0.9NaCL. 

Normal saline is a sterile, nonpyrogenic solution. It’s a crystalloid fluid (easily passes through the cell membrane) and is generally isotonic. 

It’s the most widely used fluid because it’s the most effective fluid for the widest variety of conditions. It’s the fluid of choice for fluid resuscitation and works well for most hydration needs due to hemorrhage, vomiting, diarrhea, or even shock. 

It’s most often used to increase the volume of circulating plasma (assuming that the patient has sufficient red blood cells). It can be used for things like: 

  • Blood transfusion
  • Fluid replacement for patients suffering from diabetic ketoacidosis
  • Metabolic alkalosis
  • Hypercalcemia
  • Hyponatremia

Normal saline is also the only fluid that can be used in conjunction with blood administration. 

That said, it has to be used with caution in patients who have cardiac or renal complications, as the high sodium content can cause excess fluid retention, which in turn puts additional stress on the already-weakened heart and kidneys. 

Half Normal Saline

Half normal saline is also a widespread fluid. It’s sometimes called 45% normal saline or 0.45NaCl. 

It’s a hypotonic, crystalloid solution of sodium chloride dissolved in sterile water (as opposed to normal saline, which is an isotonic solution). The difference is that half normal saline contains half the chloride concentration of normal saline. 

It’s designed to treat patients suffering from cellular dehydration and can be used for things like: 

  • Raising your overall fluid volume
  • Water replacement
  • Sodium chloride depletion
  • Gastric fluid loss
  • DKA after normal saline and before dextrose infusions

It’s especially helpful for patients who cannot handle additional glucose (i.e. diabetics). 

It is always avoided in patients who have burns, liver disease, or trauma, as the solution depletes intravascular fluids in a way that can be dangerous for patients whose intravascular fluid levels are already low. Like normal saline, the solution can pose a risk for those with cardiovascular disease or increased intracranial pressure. 

Keep in mind that half normal saline is less useful in replenishing sodium chloride deficit than normal saline, as it has half the concentration of sodium chloride. That said, it does still have uses in maintaining daily fluid levels, much like normal saline. 

Lactated Ringers

Lactated Ringer’s is another highly common IV fluid used in fluid resuscitation. In fact, it’s been offered up in many circles as an alternative to normal saline. Either way, if you’ve been injured and received surgery, there’s a decent chance you’ve received an injection of lactated ringers. 

It’s a funny name that comes from the physician who invented it. Sydney Ringer, a physician in the late 1800s, came up with a solution containing sodium, chloride, calcium, and potassium. The “lactated” part of Lactated Ringers comes from one Alexis Hartmann, who figured out that adding lactate to the solution made it more suitable for use in pediatric patients. 

For those who are rusty on their biology, lactate is a chemical that’s most commonly encountered in milk, though our muscles also produce it when we exercise. 

It’s basically normal saline with the addition of electrolytes and a buffer (lactate), which helps explain why the solution is also isotonic. 

It’s the solution that’s most similar to the body’s natural plasma and serum concentration (but unlike serum, it doesn’t contain magnesium). It’s used to treat: 

  • Dehydration
  • Burn victims
  • Hypovolemia resulting from third-space fluid shifts
  • Fluid loss in the lower gastrointestinal tract
  • Acute blood loss
  • Replacement of fluid and pH buffers

Because Lactated Ringer’s contains potassium, it cannot be used in patients with renal failure or renal complications as it can result in hyperkalemia. It also should not be used in patients with liver disease, as they cannot successfully metabolize the lactate. It also should not be administered if patients have a pH level greater than 75. 

Dextrose

Finally, there are many variations on dextrose. 

Dextrose itself is a type of simple sugar made from corn. It’s chemically identical to glucose, which you should recognize as your old pal sugar. It’s often used in processed foods and added to baking products as a sweetener, but it has a number of uses in a medical setting. 

It’s useful specifically because it’s a simple sugar–that is, your body can quickly make use of it for energy. 

There are three main versions of dextrose solutions:

  1. Dextrose in water
  2. Dextrose in saline
  3. Dextrose in Lactated Ringer’s

Regardless of the type of dextrose solution, the basic principle is the same. The IV fluid acts as a carrier for dextrose, which acts as sugar readily available for cells to gobble up and use as energy. 

Dextrose in Water

One of the more common iterations of dextrose is a solution of dextrose in water. 

Dextrose in water is a crystalloid solution. In the bag, it’s isotonic, but the solution itself is physiologically hypotonic. That’s a fancy way of saying that it enters your body as an isotonic solution (that is, when there’s still sugar in the solution), but when the sugar is absorbed by your cells, you’re left with a hypotonic solution. 

Unlike other fluids we’ve listed thus far, dextrose in water is not used in fluid resuscitation, as it can cause hyperglycemia. Instead, it is used to: 

  • Raise your total fluid volume
  • Rehydration
  • Hypernatremia (an electrolyte problem caused by a decrease in total body water relative to electrolyte content)

Dextrose in water is often used to treat diabetic patients who are not eating anything by mouth for various reasons. That said, although the solution contains about 170 calories per liter, it is not sufficient to replace normal daily calories and should not be used for long-term food replacement. 

Interestingly, it’s sometimes used as a diluent for preparing injectable medications for an IV bag (a lot of fluid in which to dilute a small dose of medicine). This is likely because the dextrose is absorbed so readily. 

It should always be avoided in patients with cardiac problems, renal failure, and increased intracranial pressure (much like the other fluids on this list) as it can cause fluid overload. 

Dextrose in Saline

Another common alternative is dextrose in saline, which is pretty much exactly what it sounds like. 

It’s a sterile, nonpyrogenic solution. As the name implies, it’s a solution of 5% dextrose in normal saline. Like normal saline, it’s isotonic at first, but it becomes hypertonic when the dextrose is absorbed (remember earlier when we said that hypertonic solutions are particularly risky to work with?)

Because dextrose in saline is such a specialized fluid, it’s used for extremely specific cases, including: 

  • Temporary treatment of circulatory insufficiency, but only if other plasma expanders are unavailable
  • Hypotonic dehydration
  • Addisonian crisis (a potentially life-threatening condition resulting from acute insufficiency of adrenal hormones)
  • Syndrome of inappropriate antidiuretic hormone/SIADH (when the brain makes too much antidiuretic hormone)

Like many other fluids on this list, dextrose in saline should not be used in patients with renal or cardiac complications, as it can cause heart failure or pulmonary edema. 

Dextrose in Lactated Ringers

Finally, dextrose in Lactated Ringer’s is…well, exactly what the name implies. 

It’s a solution of 5% dextrose in Lactated Ringer’s, a sterile, nonpyrogenic solution used for fluid and electrolyte replenishment. Like other dextrose solutions, it’s isotonic until the dextrose is absorbed. This particular solution becomes hypotonic after the dextrose is metabolized. 

It serves much the same purpose as Lactated Ringer’s, with the addition of 180 calories per liter, though it can be used as an alkalinizing agent. 

Because it’s basically Lactated Ringer’s with dextrose, it has many of the same contraindications as Lactated Ringer’s. It isn’t advisable in patients with renal issues due to hyperkalemia concerns, nor should it be used in patients with liver failure (again, they cannot metabolize the lactate). 

The addition of dextrose means that it should be used with care if patients have issues with glucose (i.e. diabetics). It also should not be used in infants less than 28 days old, even if separate infusion lines are used. 

Putting Your Fluids Into Action

Now that you know your way around the various types of IV fluids, it’s time to put your fluids into action. 

We know our way around IV nutrition and we know how to work with you to make sure that you get the perfect solution to meet your unique health needs. Click here to check out our available packages or get in touch today to set up your appointment. 

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