Question about normal saline

[JustinPA]

Does anyone know why the majority of infused normal saline ends up in the interstitial space, as opposed to staying in the intravascular space? It's my understanding that infused whole blood stays in the intravascular space. What keeps it in the vascular space but allows normal saline to leave? My best guess is the lack of colloidal proteins in the saline lower the oncotic pressure in the plasma allowing for an increased amount of filtration from the capillary to the interstitial space. Does anyone have an answer for this? I asked my pathophys teacher but she didn't know. I searched online and can't find a specific answer either. Everything says that roughly 80 percent of normal saline ends up in the interstitial spaces, but doesn't explain why.

[andersenpa]

There is no restriction to the components of NS- water, sodium, and chloride- from equilibrating freely among the fluid compartments.

The ratio of interstitial to intravascular space is apporx 3:1. If you administer 1 L NS, 750 cc will go to the interstitium and 250 cc will remain intravascular. So roughly 75% extravasates.

 

Hydrostatic and oncotic changes contribute to these shifts. Aggressively resuscitated pts will have low oncotic tone and will be more prone to lose fluid that way as well.

[JustinPA]

Thanks a lot. This leads me to one other question. If someone is hemorrhaging are they losing only intravascular fluid, or a combination of intravascular and interstitial fluid? Is there anything stopping some of the interstitial fluid from crossing back into the circulation when there's a low volume of blood? I guess what I'm asking is if there is a deficit of fluid during hemorrhage across the entire extracellular fluid compartment, or is it simply a deficit of intravascular volume?

[ohiovolffemtp]

If someone is hemorrhaging they are losing blood from the vascular space. While some interstitial fluid may migrate back from the interstitial space, it won't be significant. It also won't contain any red blood cells, so it will provide no help in tissue perfusion - no O2 transport. That's why hypertonic IV fluids really aren't that helpful in fixing blood loss. You need to: 1) provide blood to carry O2; 2) surgically fix the holes. There's a concept called "permissive hypotension" in trauma care. Basically, you try to maintain a systolic of 80-90 to maintain organ perfusion, but don't do any fluid resuscitation beyond that until the holes are fixed. Think of it as not putting any pressure into the system that will cause the leaks to flow more.

 

On the other hand, during sepsis, the patient is losing plasma into the tissue - third spacing, but the RBC's remain in the vasculature. These folks benefit from fluid resuscitation. In fact, there are studies which indicate that for every hour delay in regaining a decent systolic, the mortality % increases by 50%.

 

Net: don't just think fluid - think about what's being lost: blood with RBC's or just plasma.

[andersenpa]

Thanks a lot. This leads me to one other question. If someone is hemorrhaging are they losing only intravascular fluid, or a combination of intravascular and interstitial fluid? Is there anything stopping some of the interstitial fluid from crossing back into the circulation when there's a low volume of blood? I guess what I'm asking is if there is a deficit of fluid during hemorrhage across the entire extracellular fluid compartment, or is it simply a deficit of intravascular volume?

 

You will likely lose blood (colloid) only. In acute blood loss there is a neurohumoral axis activated which includes a huge surge in ADH release, causing water restriction. Blood loss plus free water retention- and then add in crystalloid fluid resuscitation- results in the anemia you see w/ hemorrhage. Depending on the severity of blood loss you may even see migration of plasma water into the interstitial space with shock states. The intravascular low oncotic tone and free water administration gives you the typical resuscitation patient- in some degree of shock with generalized tissue edema.

 

The inflammatory state from trauma/shock/major bleeding/blood product transfusions can also cause capillary leak, leading to more interstital fluid accumulation.

[JustinPA]

That helped a lot. Thanks again!

[andersenpa]

Realize I misspoke- ADH causes free water retention not restriction

[katana]

If you go on I Tunes there is a series of free lectures called "ICU Rounds" from Dr. Jeff Guy at Vanderbilt. I'd highly recommend his lectures on IV fluids and Traumatic Shock and Fluid Resuscitation. All of his lectures were very helpful throughout PA school, but these ones in particular are really good.