vicinihil

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So I understand the fluid only shifts between the ECF and the ICF if there is a difference in osmolarity.

Does this mean that if I give a patient 3L of pure water, then the ICF will expand by 2L and ECF will expand by 1L?

Also, if I give someone 4L of isotonic saline, the ECF expands by 4L right? Therefore my plasma volume is now increased by 1L (1/4) and interstitial volume is increased by 3L (3/4).

So if I started bleeding out and lose 3L of blood, does this mean to replenish all that (plasma volume) I would have to give myself 4L of isotonic saline to raise my plasma volume by 3?

Just making sure I understand this correctly. Thanks!!!
 

username456789

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So if I started bleeding out and lose 3L of blood, does this mean to replenish all that (plasma volume) I would have to give myself 4L of isotonic saline to raise my plasma volume by 3?


Maybe it's just because it's late, but I'm having trouble following this last part. Can someone spell it out for me so I know whether I'm out in left field or not?
 

hartbot

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Maybe it's just because it's late, but I'm having trouble following this last part. Can someone spell it out for me so I know whether I'm out in left field or not?
Agreed. Based on what was originally said, wouldn't you have to give 12L of isotonic saline to ensure that 1/4 of it (3L) remains in the plasma, allowing for 9L to go into the interstitial volume?
 

illixir

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3 L of isotonic saline will give you an extra 1 Liter of plasma since the other 2 will go into interstitial fluid. With an isotonic solution no exchange occurs with ICF, you basically have the fraction wrong(I believe, I know that the 2/3 - 3/4 thing is hard because different sources say diff things)
 

username456789

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3 L of isotonic saline will give you an extra 1 Liter of plasma since the other 2 will go into interstitial fluid. With an isotonic solution no exchange occurs with ICF, you basically have the fraction wrong(I believe, I know that the 2/3 - 3/4 thing is hard because different sources say diff things)

Right, but if he lost 3L of plasma, replacing 1L of plasma (via 3-4L of isotonic saline, depending on if you subscribe to the 3:1 or 2:1 dogma for interstitial:intravascular) would not be sufficient. Or am I missing something here? From a practical standpoint, it doesn't make sense to me that you'd have to infuse ~12L into the guy (although I understand [i think] having to add 3-4x the target amount of plasma volume you're looking to "add" d/t interstitial redistribution), but mathematically it doesn't make sense to me how it was originally explained either.

And just to clarify, we learned:
ICF: 2/3
ECF: 1/3

and breaking down ECF further:
Interstitial: 3/4
Intravascular: 1/4


But that's beside the point. My confusion isn't with which version of that calculation was used, but rather the math principles in general.
 
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illixir

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Looking at the main question more directly, if 3 L blood are lost, then I would think that 9 L are needed to restore it, since 6 L will go to interstitial fluid. No equilibration with ICF with isotonic saline. But even that seems like too much...
 

username456789

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Looking at the main question more directly, if 3 L blood are lost, then I would think that 9 L are needed to restore it, since 6 L will go to interstitial fluid. No equilibration with ICF with isotonic saline. But even that seems like too much...

Ha yeah that's exactly what I'm saying (except I was doing the 3:1 I learned, in which case 12 would have to go in so that 9 go to interstitial and 3 go to intravascular). Mathematically that makes sense to me, but I can't picture reasonably pushing 12L of fluid into the guy (or even 9).
 

vicinihil

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heh I guess it was really late for me. yeah I meant 12 to get 1/4 of it into the plasma volume (3L). Thanks for clarifying this concept for me guys.
 

lord_jeebus

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In practice, I (and most others) replace blood loss with 3 times the amount in crystalloid.

(Of course, in practice, I would replace 3 liters of bleeding with something better - like blood products.)
 

hartbot

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Giving 12L may seem like too much for 3L of blood loss?

Remember, we only have 5L of blood. So I'm thinking if someone loses 60% of their total blood: a) they're probably dead anyway, b) if they're not dead, you are going to give them blood and not saline, c) if you actually do give them saline, you're gonna have to give them a sh*tload of it to restore the blood volume.
 

Rabbit Hole

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In practice, I (and most others) replace blood loss with 3 times the amount in crystalloid.

(Of course, in practice, I would replace 3 liters of bleeding with something better - like blood products.)
3L blood loss is a class IV hemorrhage which can sometimes be treated with a freakin' miracle.
 

IMGUSMLEStep1

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TBW is ECF (1/3) + ICF (2/3)

ECF is Plasma (1/4) and ISF (3/4)

In a hypovolemic patient, the type of replacement fluid you use determines how much you need to use.

Your goal is to increase plasma volume by the same amount that was lost.

If you are going to use a colloid solution, you need to administer the same amount lost becasue the colloid solution will remian in plasma. Colloid prevents fluid from escaping from plasma into the ISF.

If you are going to give isotonic saline, you need to give 3x what the patient lost. This is because isotonic solutions fill plasma AND interstitial fluid (but does NOT affect ICF). Remember that gain or loss of isotonic fluid from the ECF does NOT affect the ICF. The volume of the ECF does not affect the volume of the ICF. However, the osmolarity of the ECF does affect the ICF.

So if you were to give the patient pure water (which adds to both ECF and ICF), you would have to give 12x the volume the patient lost.
Let's work through this:
add 12 L to TBW --> 2/3 in ICF, 1/3 in ECF (4 L) --> 3/4 ISF, 1/4 plasma (1 L)
This is theoretical of course because I'm sure the effects of administering 12 L for every L of blood lost would be catastrophic.

I hope this helps.