VARIATIONS IN PLASMA SODIUM CONCENTRATION IN POSTOPERATIVE-PATIENTS DEPEND ON AN ELECTROLYTE-FREE WATER-BALANCE, PART OF A TONICITY BALANCE

Background. There is an inverse relationship between changes in the co ncentration of sodium in plasma (P-Na) and intracellular fluid (ICF) v olume. 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 P-Na: when EFW is added to body fluid s, P-Na decreases. Moreover, the concept of EFW permits a good underst anding of the renal contribution to the defence of body tonicity. Purp ose. To illustrate that the measurement of a tonicity balance provides the best estimate of changes in P-Na in an ICU setting. Methods. Twen ty-two patients were admitted to the Post-Operative Intensive Care Uni t. We investigated how well changes in EFW balance correlated with P-N a variations and what is the best formula to calculate EFW in this set ting. Results. P-Na changes depend on EFW balance; there is no signifi cant relationship with other classical factors such as urinary osmolal ity or Na-free water. Conclusion. The utility of a tonicity balance is demonstrated. A formula is derived facilitating at the bedside the pr ediction of changes in P-NA following fluid therapy: P-Na2 = [(P-Na1 . TBW) + balance (Na + K)]/[TBW + balance H2O]. P-Na changes can be und erstood and/or modified exclusively by a careful measurement of intake s and losses of Na, K and water.