Friday, August 27, 2010

Board question: Water-1 answer


This was an actual case I saw and thought it generated a multitude of interesting dilemmas. When I initially examined the patient she was in status epilepticus. This, by itself, mandated the immediate correction of her serum sodium. But what was the best way to achieve the correction without adversely affected the patient? This was my thought process…

The patient is 100kg women, so her TBW should be 50 Liters.
(using the Watson Formula)

I decided to separate her water and solute gains/losses to determine what her total body sodium was at that instance.


She had already received 2L of normal saline but lost 3L in urine, so net water loss of 1L. 1L of free water loss would cause that serum sodium to rise 2.5meq/L.

Calculation: 120 (desired Na)-115 (actual Na)/120 (desired Na) times 50L (TBW) is equal to roughly 2L. Therefore, 2L of free water loss would raise serum sodium 5 meq/L.

Therefore, for each 1L of water loss, the serum sodium would rise 2.5 meq/L. (5meq/L divided by 2)

So, just based on water loss, her serum sodium has risen 2.5 meq/L already, so the serum sodium is 117.5 meq/L.
Next, I calculated the impact of solute gain. Total solute for a 100kg woman with a serum sodium of 115 would be 5750 (50L times 115meq/L). I estimated that she gained 133 meq of solute… INPUT: 2L of normal saline 388meq KCL 80meq Total input 468 meq solute input LOSS: Urine sodium of 75 meq/L times 3L urine output is 225 meq Urine potassium of 10 meq/L times 3L urine output is 30 meq Total loss: 255meq

So, total solute input (388)-total solute output (255) equals a gain of 133meq of solute.

How does this impact the serum sodium?
Total body solute 5750 plus gain of 133 equals 5865 5865 meq (New Total Body Solute) divided by TBW (50 L) equals 117.3 Therefore, serum sodium rose 2.3 based on solute gain alone.

If you add the affect of free water loss and solute gain, I anticipated that the serum sodium at that instance was 120 meq/L (2.5 meq/L from water loss and 2.3 from solute gain plus starting sodium of 115). So, I thought that the serum sodium was already in the “safe” zone and that the seizures were related to alcohol withdrawl.

I decided NOT to use 3% saline, and continue with gentle potassium repletion at that time (depite the intensivist’s insistence). What this physician failed to realize was that KCl has the same osmotic potential as sodium!

In fact, there are case reports of central pontine myelinosis induced by rapid correction of hyponatremia via potassium repletion alone.


The patient’s repeat sodium was 121! Her serum sodium stabilized over the coarse of a few days with mere potassium repletion. Her seizures persisted and patient eventually required anti-epileptic therapy.

Given the complexities of this case, there is not a perfect strategy for management. The purpose was to generate thoughts and discussions on the intricacies of hyponatremia management. Would you have managed her differently? Let me know your thoughts…

Michael Lattanzio, DO

10 comments:

Samson said...

That was an excellent explanation.But in real life practice....it will be difficult to attribute ongoing seizures to alcohol alone and withold 3% saline,eventhough her Na has come up to 120(by physiological reasoning). Good work.

Cheeler said...

A question from a budding nephrology fellow . . .

In your thought process about this patient, was volume status taken into account? Does volume status even matter if they're symptomatic from the hyponatremia? It seems that if 3% NS were administered and this patient was simply hypovolemic, overcorrection of her serum sodium concentration could easily happen once she was adequately volume resuscitated. Thanks!

Michael said...

volume status is paramount when considering the etiology and determining the management of hyponatremia. assuming that the patient was volume depleted, normal saline would correct the hyponatremia. Once intravascular volume is repleted, vasopressin secretion will be suppressed. the renal sequela of ADH suppression is free water excretion (aquaresis), with a resultant increase in serum Na. However... this response is not instantaneous! in this case, you have no time to wait for this response, and the sodium must be fixed immediately...hypertonic saline would achieve the desired response. hope that answered the question.

Anonymous said...

This is the 2nd time im reading this.
Two questions....when we calculate the solute gain/loss...it has to be 468-255= 213 (unless we dont take K into the calculation).
Then...her net fluid balance is - 1L ..which means her TBW at this time will be 49L and not 50L.
These two changes will put her sodium level higher than our calculation.Am I confusing this??

Michael said...

you are already accouting for free water loss in the water equation, so you cannot include it in the solute equation, or you will be adjusted twice for water loss. most people dont consider K in the equation. hope this helps.

David said...

You calculate the free water loss to be 1 liter (fluid in -urine output).However this is not solute free or free water.So can you extrapolate the effect on sodium?

Maca said...

Hi, good example. I am puzzled on how you calculated free water clearance; if you calculate electrolyte osmolal clearance taking in account K and Na and then electrolyte free water clearance assuming the three liters of urine were measured in 24 hours (1440 mins) then the free water clearance would just be 0,5 lts.
I also have teeny doubts on other calculations.
Please correct me if I'm wrong

Maca said...

It also seems that for calculating inputs you took in accountance NaCl and KCl (mmol/meqs) for both anion and cations, but for the output calculation you just counted the cations Na and K. Wouldn it have been more accurate to consider also the anions by multiplying by 2 in the outputs?
It seems to have worked on the patient and I see that the principal intention of this was to show that K also matters, but those two things I just can't seem to understand

Anonymous said...

how did you get 388? Each normal saline is 154. 2L. so 308

Anonymous said...

how did you get 388? Each normal saline is 154. 2L. so 308