When you are taking care of patients, especially those who are critically ill, you may hear the term “third spacing” thrown around in report or in rounds. What does this term mean and, more importantly, what are you going to DO about it?

The fluid compartments

The first thing to understand is the concept of fluid compartments in the body. Though it varies by gender and muscle mass, about 60% of your body weight is due to water…we call this the “total body water” or TBW.

All that water is distributed throughout the body in what we call “fluid compartments.” Some are contiguous compartments (like the intravascular space) and others are virtual compartments like the intracellular space. Let’s take a look at how these fluid compartments are dispersed throughout the body, shall we?

third spacing

ICF = intracellular fluid; ECF = extracellular fluid

As you can see from the graphic, we have total body water making up about 60% of body weight. Total body water itself is made up of two main types…intracellular fluid and extracellular fluid. The intracellular fluid is within the body’s cells, so that’s pretty easy to understand. The extracellular fluid, however, is dispersed throughout the body in various smaller compartments. The ones that we care about the most are:

  • The interstitial fluid: surrounds the cells and bathes them in nutrients and other chemicals necessary for the body to function normally; acts as a transport medium for things going in and out of the cells
  • The intravascular fluid: plasma and lymphatic fluid
  • The transcellular fluid: fluid in epithelial-lined spaces…this includes fluids of the gut, synovial fluid, cerebrospinal fluid, the aqueous humor and even fluid hanging out in the bladder

If you’re a perfectionist like me, then yes…there are two more fluid compartments, but this fluid is not easily mobilized so we don’t really consider it playing a role in physiology as we do the others. These two compartments are the bone and dense connective tissue. See? I knew you’d be underwhelmed.

Third spacing simplified

Third spacing occurs when fluid moves from the intravascular compartment (where it DOES contribute to cardiac output) into a body compartment where it cannot contribute to cardiac output. In simpler terms, the fluid leaves the intravascular space and leaks into another compartment where it can’t really do anything useful for your patient. Typically this is the interstitial space, but can also include the transcellular space as well (leading to ascites).

What causes third-spacing?

Basically, third-spacing occurs due to decreased oncotic pressure in the intravascular space. This drop in oncotic pressure means fluid will “leak out” of the intravascular space into the interstitial space (and yes, this can include the interstitial spaces in the brain leading to cerebral edema!).

In most cases, you’ll care greatly about your third-spacing patient because the fluid that would normally contribute to their blood pressure is now hanging out in place it shouldn’t be. So even though your patient’s weight may be unchanged (or even increased in many cases) he can actually be severely hypovolemic and hypotensive.

Along with decreased oncotic pressure due to low albumin/protein levels, third-spacing can also occur with trauma (burns are a big one and so is abdominal surgery). So, if you’ve got a post-op belly surgery or a big burn…watch closely for hypotension and massive fluid shifts!

Another instance where you could have third-spacing is in sepsis. The leakiness of the capillaries causes fluid to “leak out” into the interstitial space and is just one of the reasons your septic patient is hypotensive.

LATTE for third-spacing

Let’s take a quick look at your third-spacing patient, using our LATTE methodology (not sure what LATTE is? Check out this post here or this podcast!). 

L (How will the patient LOOK?): Your patient who is “third-spacing” will have edema which can, at times, be pretty extensive. They may have marked weight gain and look “swollen” overall.

A (How will you ASSESS this patient?): Assess for edema, noting its severity. Monitor cardiac output. Monitor urine output. Check daily weights. Assess lung sounds and for difficulty breathing. Monitor O2 sats, HR and BP. Monitor neurological status if cerebral edema is suspected. Measure abdominal pressures and girth in cases of surgery or ascites.

T (What TESTS will be conducted?): Because there are multiple causes for and consequences of third-spacing, the chosen diagnostics and lab tests vary. These can include albumin and pre-albumin levels, liver function tests, cardiac ECHO, chest x-ray, and abdominal X-ray or ultrasound (to assess ascites).

T (How will you TREAT this patient?): For starters you’ll want to treat the underlying cause of the third-spacing. Many times we will give colloids which are large-molecule fluids to help increase the oncotic pressure within the intravascular space. The most common one we give is albumin, but there’s also Dextran and Hetastarch…all of these are large, heavy fluids that increase oncotic pressure in the intravascular space and help “pull” that interstitial fluid back into the vasculature where it can: A) contribute to blood pressure; and B) be released as urine. When the fluid shifts back into the intravascular space we say that the fluid has been “mobilized.”

E (How will you EDUCATE the patient/family?): As always, you want to provide education on all medications/fluids you administer. Family will often find the edema very troubling (especially when it is extensive), so calm explanations about how fluid shifts occur can be helpful in assuaging their anxiety.

Hope that helps you understand the basics of third-spacing, something you will definitely see a LOT in the clinical setting! What other topics would you like to see covered? Let us know in the comments below!

Get this on audio in episode 113 of the Straight A Nursing Podcast.