Can I give normal saline to a patient with hyponatremia who might have SIADH?

On the inpatient medical wards, not a month goes by without a case of a patient  with hyponatremia who gets 2 liters of normal saline, and when their sodium is rechecked, there’s no change or it’s even lower than it was before. Was giving normal saline the right thing to do?

If the patient has a good story for hypovolemia (has been taking twice the prescribed amount of diuretic, has not eaten in four days, etc.) then it makes sense that normal saline, with an osmolarity of 154 mEq/L, should help. It’s also a no-brainer that if a patient is grossly hypervolemic (has massive ascites, an acute heart failure exacerbation, etc.) then you should not give extra fluids, and diuresis will improve the sodium. The tricky one is “euvolemic hyponatremia,” and the most common cause of this is SIADH.

SIADH (syndrome of inappropriate ADH secretion) is a water problem. The body has too much water but continues to secrete ADH to retain free water (hence, inappropriate). One dictum you will hear is, “Don’t give normal saline to a patient with SIADH.” The argument is that in SIADH, salt handling is intact, so if you give a patient normal saline, all the sodium will be excreted but only half of the free water will be, which will make hyponatremia worse.

Is this true? Like so many things in medicine, it’s not an absolute rule. More important than whether someone has SIADH is how “saline responsive” they are likely to be. That is, will their sodium increase with normal saline or will it not?

How saline responsive might a patient be? The urine lytes are the key. 

  • urine osmolality: Uosm <100 is consistent with hypovolemia, whereas Uosm >200 is almost definitely not hypovolemia
  • urine Na: UNa <20 is considered consistent with hypovolemia (the body is trying to hold on on sodium to retain water) and >20 is considered fair game for a diagnosis of SIADH, although >50 seems to be the point of non-saline responsiveness. (paper here) UNa is unreliable in patients taking diuretics.
  • fractionated excretion of urea: FEurea >45% is considered to be non-saline responsive. (paper here)
UOsm <100, UNa<20
Malnutrition, beer potomania, surreptitious diuretic use, reset osmostat, hypovolemia already treated with normal saline
Uosm >300, UNa >40
SIADH, hypothyroidism, adrenal insufficiency

One review says that giving normal saline can unmask patients who are thought to be hypovolemic but actually have SIADH: “A rapid increase in FENa (>0.5% after 2 L of isotonic saline over 24 h), without correction of PNa, correlates with inappropriate ADH secretion.” This Hospitalist article recommends, “In salt depletion, plasma Na usually increases ≥5 mmol/L after 2 L saline infusion, which is not the case with SIADH.” So, a trial of normal saline can be diagnostic and therapeutic.  

The only patients you should NOT give normal saline to as a trial are those with an Na <120, as further driving down Na could put them at risk for seizures and coma. Patients with malnutrition or alcoholism are also at higher risk for overly rapid correction and osmotic demyelination, so use caution when repleting with fluids.

 

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