Glucose mediated osmotic diuresis does what to [Na+]?

[CBG23]

I am unsure about what effect the osmotic diuresis due to diabetes mellitus has on the serum sodium concentration. There are two rows in RR3 that make mention of this. For osmotic diuretics (listing glucose as an example), the text says that hypernatremia results. For diabetic ketoacidosis, the text says that the serum sodium concentration falls.

I think I can explain the difference away, but I'm not sure if my reasoning is correct:

Assuming a patient was put on an osmotic diuretic, they would lose some sodium in their urine that they normally wouldn't have and they would lose an equivalent "amount" of water because they water would have followed the sodium if it would have been reabsorbed. Also, they would lose more water - the water that is "trapped" in the tubules by the osmotic diuretic. So they lose relatively more water than sodium. The result is:
- An increase in serum sodium concentration.
- A decrease in ECF volume because of the urine water loss,
- A decrease in ICF volume because water is drawn into the now hypernatremic serum.


If a patient has hyperglycemia due to prolonged diabetes, their liver is pumping glucose into the blood (without water) so the serum becomes hyperosmotic. This causes water to shift out of the cells and dilutes the serum, causing a hyponatremia. Now we filter the blood at the glomerulus. Some glucose can be reabsorbed, but we quickly saturate the glucose transports in the proximal tubule. Now glucose is "stuck" in the tubules and "traps" water with it. This is essentially the same situation as having an osmotic diuretic. So we lose a bit of Na+ in the urine and relatively more water. Based on this, you would expect the serum sodium concentration to rise (hypernatremia), but the liver is still pumping out more glucose into the serum, which dilutes the sodium concentration again and you get hyponatremia. I guess to not end up with hypernatremia in this state, you have to assume that the water shifts due to the hytperglycemia always win out. I haven't been able to find a good explanation anywhere though.

The result is:
- An ?decrease? in serum sodium concentration.
- A decrease in ECF volume because of the urine water loss,
- A decrease in ICF volume because water is drawn into the hyperglycemic serum.

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[turkeyjerky]

so typically in DKA or Hyperosmolar Hyperglycemia, you end up w/ a dilutional (pseudo)hyponatremia, which corrects w/ treatment. You can correct the measured Na for the glucose by adding 1.6 to the [Na] for every 100 mg/dl increase in [glucose] over 100.

 

[theWUbear]

Still trying to figure out the rules governing this double standard...

 

[CBG23]

Still trying to figure out the rules governing this double standard...

Hey Pre-med,

This isn't a homework help question - we don't have homework assignments in my school. There are some questions that come up in the course of learning or reviewing something that aren't explained well by professors, lectures notes, or even several text resources. As a result, I try and ask my colleagues to see if they understand a particular point better or maybe can explain it better. It's for my own understanding more than anything else,

 

[theWUbear]

Hey Pre-med,

This isn't a homework help question - we don't have homework assignments in my school. There are some questions that come up in the course of learning or reviewing something that aren't explained well by professors, lectures notes, or even several text resources. As a result, I try and ask my colleagues to see if they understand a particular point better or maybe can explain it better. It's for my own understanding more than anything else,

Hey Med,

We don't have homework at the med school I'm taking classes in either - I fully understand. I am referring to an auto-response by mods (mostly ksmi117) to about 90% of content-related questions in pre-allo (regardless of whether the poster is trying to better understand a concept, or it is indeed a 'homework question'). I do apologize for hijacking your thread specifically, however.

 

[CBG23]

Hey Med,

We don't have homework at the med school I'm taking classes in either - I fully understand. I am referring to an auto-response by mods (mostly ksmi117) to about 90% of content-related questions in pre-allo (regardless of whether the poster is trying to better understand a concept, or it is indeed a 'homework question'). I do apologize for hijacking your thread specifically, however.

And I apologize, theWUbear, for coming off as condescending by referring to you as pre-med in some negative way as I am only several years removed from the same position.

Thanks Azadre and turkeyjerky for the responses. So it's a dilution effect causing the hyponatremia then.

I came across this tidbit in "CURRENT Medical Dx & Tx":

"While severe hyperglycemia can cause translocational hyponatremia, progressive volume depletion from glucosuria can result in hypernatremia. Osmotic diuresis can occur with the use of mannitol or urea."

Diagnosaurus (don't know how reliable this is) also lists hyperglycemia as a cause for both.

So it looks like hyperglycemia can cause either hypo- or hypernatremia...

I guess hyperglycemia can cause hypernatremia if you don't replenish the renal water losses by increasing water intake? and hyponatremia if your water intake is not impaired?

 

[reedman]

i would guess hypernatremia. even though there is an osmotic diuresis, some Na is going to be reabsorbed. furthermore, once the volume is depleted there will be an increase in aldosterone, which will increase Na absorption in the collecting duct.

 

[MDofIre]

Still trying to figure out the rules governing this double standard...

I remember when I was 13...

 

[Jolie South]

Agree with turkeyjerky about the pseudohyponatremia. After you correct the hyperglycemia, you may see hypernatremia from volume depletion 2/2 osmotic diuresis.

 

[Caesar]

I think it can cause both hypo and hyper. There is a balance between becoming hypovolemic and reuptaking Na, and losing Na in urine as a result of diuresis.

 

[Jolie South]

Hey Pre-med,

This isn't a homework help question - we don't have homework assignments in my school. There are some questions that come up in the course of learning or reviewing something that aren't explained well by professors, lectures notes, or even several text resources. As a result, I try and ask my colleagues to see if they understand a particular point better or maybe can explain it better. It's for my own understanding more than anything else,

This is the difference.

 

[Jolie South]

I think it can cause both hypo and hyper. There is a balance between becoming hypovolemic and reuptaking Na, and losing Na in urine as a result of diuresis.

You don't actually become hyponatremic. It just seems as if you are because the total number of osmoles is increased with the candy factory that's raging in the serum. This leads to an underestimate of the actual serum sodium. Thus, when you're in the midst of raging uncorrected HONK or DKA, you need to correct your sodium value according to the formula indicated by turkeyjerky.