Variations in plasma sodium concentration in post-operative patients depend on an electrolyte-free water balance, part of a tonicity balance.

BACKGROUND

There is an inverse relationship between changes in the concentration of sodium in plasma (PNa) and intracellular fluid (ICF) volume. Intakes and losses of sodium (Na), potassium (K) and water can be divided into two volumes: isotonic and electrolyte-free water (EFW). Calculations of these volumes assess a tonicity balance, a tonicity imbalance results in a change of PNa: when EFW is added to body fluids, PNa decreases. Moreover, the concept of EFW permits a good understanding of the renal contribution to the defence of body tonicity.

PURPOSE

To illustrate that the measurement of a tonicity balance provides the best estimate of changes in PNa in an ICU setting.

METHODS

Twenty-two patients were admitted to the Post-Operative Intensive Care Unit. We investigated how well changes in EFW balance correlated with PNa variations and what is the best formula to calculate EFW in this setting.

RESULTS

PNa changes depend on EFW balance; there is no significant relationship with other classical factors such as urinary osmolality or Na-free water.

CONCLUSION

The utility of a tonicity balance is demonstrated. A formula is derived facilitating at the bedside the prediction of changes in PNa following fluid therapy: PNa2 = [(PNa1.TBW) + balance (Na + K)]/[TBW + balance H2O]. PNa changes can be understood and/or modified exclusively by a careful measurement of intakes and losses of Na, K and water.