• Andrea Lassnigg, Michael Hiesmayr, Mohamed Mouhieddine, Christoph Schwarz, and Sandra Stieglmair.
• Department of Nephrology, Krankenhaus der Elisabethinen, Linz, Austria.
• J Crit Care. 2013 Dec 1;28(6):1114.e1-5.

PurposeHyponatremia is frequently observed in intensive care unit (ICU) patients, but there is still lack information on the physiological mechanisms of development.Materials And MethodsIn this retrospective analysis we performed tonicity balances in 54 patients with ICU acquired hyponatremia. We calculated fluid and solute in and outputs during 24 hours in 106 patient days with decreasing serum-sodium levels.ResultsWe could observe a positive fluid balance as a single reason for hyponatremia in 25% of patients and a negative solute balance in 57%. In 18% both factors contributed to the decrease in serum-sodium. Hyponatremic patients had renal water retention, measured by electrolyte free water clearance calculation in 79% and positive input of free water in 67% as reasons for decline of serum-sodium. The theoretical change of serum sodium during 24 hours according to the calculations of measured balances correlated well with the real change of serum sodium (r = 0.78, P < .01).ConclusionsBalance studies showed that renal water retention together with renal sodium loss and high electrolyte free water input are the major contributors to the development of hyponatremia. Control of renal water and sodium handling by urine analysis may contribute to a better fluid management in the ICU population.Copyright © 2013 Elsevier Inc. All rights reserved.

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