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Forum » Acid-Base-Electrolytes » Sodium, Potassium, Water, Divalent Cations, and Strong Ion Theory » Question about a 10-minute rounds video on Hyponatremia (Calculating the amount of water lost)

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Author	Topic: Question about a 10-minute rounds video on Hyponatremia (Calculating the amount of water lost)

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Question about a 10-minute rounds video on Hyponatremia (Calculating the amount of water lost) on: August 28, 2011, 16:08
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This is a question asked on the YouTube discussion channel about the following video: http://www.youtube.com/watch?v=7Jhd4f9sPoQ

Why are you using the total body water and not the extracellular fluid volume?

The sodium concentrations you are using (110 vs. 140) are for ECV measurements. This is a disconnect for me.

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Re: Question about a 10-minute rounds video on Hyponatremia (Calculating the amount of water lost) on: August 28, 2011, 16:16
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This is a great question. I'll give you a simple answer to help explain this a little better.

Simple answer:
The sodium concentrations used in the 10-minute rounds video (110 and 140 meq/L) are measured in the extracellular compartment. Thus, one would believe that the total sodium (in meq) would be the product of the extracellular fluid volume (ECF vol) * the sodium concentration. However, the [Na+] is not a measure of ECF volume. It is a surrogate measure of the intracellular (or total body) water content.

Multiplying ECF volume with [Na+] would be a misuse of the [Na+] because the latter reflects body water. Thus, total sodium (in meq; not meq/L) would be calculated by multiplying body water * [Na+].

I recommend going through the following teaching resource to understand the difference between [Na+] (and what it measures) versus total sodium content (in meq) and what it measures.

http://blog.ecu.edu/sites/nephrologyondemand/?p=6205

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