Wednesday, December 9, 2015

IV Fluid Brain Teaser: Salt versus Saline

Everyone knows that if you give a liter of saline, all of it remains in the extracellular compartment.

But what if you give a patient just the salt from the saline and none of the water? How much does the solute contribute to the increase in the extracellular volume? How does 154 mmols of NaCl affect the size of the extracellular and intracellular compartments?

Assume the patient is a 70 kg lean young male with a serum osmolality of 280 mOsm/kgH2O. Ignore any renal losses during the process.

For full credit fill out the following:

Total body water:
Size of the extracellular compartment:
Size of the intracellular compartment:


Step one calculate the total number of osmoles the patient has:

70 kg lean young male means 60% total body water or 42 liters
42 liters times 280 mOsm/Kg = 11,760 osmoles in the body

Giving 308 mosm of solute will increase that to 12,068. There is no additional water so dividing that by 42 liters gives us a new osmolality of 287 mOsm/Kg water.

Remember that even though the solute is trapped in the extracellular compartment, the osmolality is the same across all body compartments since water can flow from compartment to compartment.

Now we need to find out how much the extracellular compartment expands in osmoles.

Before the addition of solute the extracellular compartment should be one third of total body water, so 14 liters times osmoality of 280 is 3920 mOsmoles. Add 308 and then divide that by the new osmolality to give you the new volume:

That increased volume of course comes from the intracellular compartment, so it goes down by 0.7 liters. You can also get there by taking the original volume of 28 liters multiplying by 280 mOsm to get 7840 miliosmoles and divide that by the new osmolality of 287:

So the addition of 308 miliosmoles from the bag of saline will increase the extracellular compartment by 0.7 liters. Only 0.3 liters less than the increase you would get with a liter of 0.9 NS. 

It's all about the salt.

Total body water: 42 liters
Size of the extracellular compartment: 14.7 liters
Size of the intracellular compartment: 27.3 liters

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