SIADH euvolemic

[ulikedaggers]

I understand that SIADH is euvolemic because the transient increase in intravascular volume will trigger renal sodium excretion, thus bringing you back to a euvolemic state.

However, I've read sources that use SIADH as an example of hyposmotic volume expansion. Are those sources just ignoring the fact that the transient volume expansion will eventually be dealt with?

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[BlueArc]

I think you are talking about BRS physiology. Have you come across a question concerning this?

 

[Brain Bucket]

Think of it as the SIADH causing the euvolemic hyponatremic state rather than trying to classify SIADH as such. The important bit is to understand the physiology. If you understand all this as an MS 1, colour me impressed.

As water retention goes up, a hypervol. state is reached and the excess stretch on the right atrium of the heart leads to release of ANP. ANP leads to increased GFR through it's action on mesangial cells and decreased Na+ reabs. in PCT causing Na+ loss in urine.

The RA system is suppressed during the initial phase of water retention but activated later on due to the loss of Na+. As long as ANP mediated natriuresis is counteracted by the RA system, low-normal serum Na+ lvls are maintained reaching a static level, but anything not playing nice, well the possibilities are endless.

Note that euvolemic doesn't mean TBW is normal, it is increased, but not to a level that causes obvious edema, so it is a hyposmotic volume expansion, just not edematous. They're both right.

 

[Transposony]

I understand that SIADH is euvolemic because the transient increase in intravascular volume will trigger renal sodium excretion, thus bringing you back to a euvolemic state.

However, I've read sources that use SIADH as an example of hyposmotic volume expansion. Are those sources just ignoring the fact that the transient volume expansion will eventually be dealt with?

Hyponatremia is classified into three categories based on serum osmolality (2 X Na+10):

1. Isotonic
2. Hypertonic
3. Hypotonic

Hypotonic is further subdivided into three based on volume status:

1. Hypovolemic
2. Hypervolemic
3. Normovolemic

SIADH is Hypotonic (Low Na) Normovolemic Hyponatremia since ADH will reabsorb free water but Na (with Cl, Urea, K etc) will be excreted along with mandatory water making the urine concentrated.
Also, SIADH can be Chronic or Acute depending on the cause and, therefore, the compensation will give you different scenarios.

 

[ulikedaggers]

Thanks guys. That's what I understood prior. I might need to put BRS down for a bit.