Effect of IV fluids-treatment-of-hyponatremia

complex as depends not only on [Na]infusate but also UOP and Uosm

If minimal UOP and none of infused Na excreted

• 分子: (current [Na] serum ×TBW) + [Na]infusate
• 分母：TBW + 1

If euvolemic (eg, in SIADH) & all infused Na excreted

• 分子: (current [Na] serum x TBW) + [Na]infusate - [Na]infusate
• 分母: TBW+1 - (Infusate osm/U osm)

In SIADH “fixed” high Uosm.

∴ in SIADH w/ Uosm 616, 1 L NS (154 mEq Na or 308 mOsm solute in 1 L H2O) will be excreted in 0.5 L H2O → net gain 0.5 L H2O. ∴ NS worsens [Na]serum if Uosm >infusate(osm) . In contrast, 1 L 3% NaCl (1026 mOsm) would be excreted in ~1.7 L urine → net loss 0.7 L H2O. ∴ 3% saline ↑ [Na]serum.