HDCN Abstract:  Am Soc Nephrol Ann Mtg -- Toronto  

Carlotti APCP, Bohn D, Mallie JP, Halperin ML

Comparison of an electrolyte-free water and a tonicity balance: Which provides the best way to determine why the plasma sodium concentration (PNa) changes and its treatment?

Am Soc Nephrol Ann Mtg -- Toronto
J Am Soc Nephrol (Sep) 11:103A 2000

Background: Electrolyte-free water (EFW) terms are commonly used to decide why the PNa changed and to plan goals for its therapy.

Objective: To evaluate whether an EFW balance reveals the basis for a change in PNa and what treatment measures are needed to restore the ECF and ICF volumes.

Methods: A 40 kg boy had a pheochromocytoma removed by neurosurgery. Over the first 9 h, his urine volume was 4 L with a [Na+K] of 50 mM. His PNa rose to 157 mM. The only IV was 3 L of isotonic saline (iso NaCl).

EFW balance: He received only iso NaCl so there was no EFW input. Dividing his urine into iso NaCl and EFW, he excreted 2.7 L of EFW. Based on this, 2.7 L of D5W should be infused to replace his negative balance of EFW.

Tonicity balance: In this calculation Na+K and water are analyzed separately. Input was 450 mmol and output 200 mmol Na+K, a positive balance of 250 mmol Na+K. Water balance was –1 L. Based on this, therapy to lower his PNa would clearly be to create a negative balance of 250 mmol Na+K and a positive balance of 1 L of water.

Hypothetical examples: If the IV was 0 or 4 L of iso NaCl, EFW balance would be identical. Tonicity balance is very different and dictates therapy. With no IV, the 4L water + 200 mmol Na+K deficits must be replaced. With the 4 L iso NaCl input, excrete the excess 400 mmol Na+K.

Conclusions: An EFW balance reveals only that the PNa will change. It does not reveal why PNa changed or how to treat it. A tonicity balance reveals why the plasma [Na] changed and what its treatment goals should be.

Copyright 2000, American Society of Nephrology