A method to estimate urinary electrolyte excretion in patients at risk fordeveloping cerebral salt wasting

Object. Two major criteria are necessary to diagnose cerebral salt wasting (CSW): a cerebral lesion and a large urinary excretion of Na+ and Cl- at a time when the extracellular fluid (ECF) volume is contracted. Nevertheless, it is difficult for the physician to confirm from bedside observation that a patient has a contracted ECF volume. Hyponatremia, although frequently p resent, should not be a criterion for a diagnosis of salt wasting. A contra cted ECF volume is unlikely if there are positive balances of Na+ and Cl-. The goal of this study was to assess the accuracy of calculating balances f or Na+ plus K+ and of Cl- over 1 to 10 days in an intensive care unit (ICU) setting. Methods. A prospective comparison of measured and estimated quantities of N a+ plus K+ and of Cl- excreted over 1 to 10 days in 10 children and 12 adul ts who had recently received a traumatic brain injury or undergone recent n eurosurgery. Plasma concentrations of electrolytes were recorded at the beg inning and end of the study period. The total volumes infused and excreted and the concentrations of Na+, K+, and Cl- in the infusate were obtained fr om each patient's ICU chart. The electrolytes in the patients' urine were m easured and calculated. Correlations between measured and calculated values for excretions of Cl- and of Na+ plus K+ were excellent. Conclusions. Mass balances for Na+ plus K+ and for Cl- can be accurately es timated. These data provide information to support or refute a clinical dia gnosis of CSW. The danger of relying on balances for these electrolytes mea sured within a single day to diagnose CSW is illustrated.