New method to compute mixed water and electrolyte changes in hyponatremia: a preliminary report.

Background: We developed a new method that diagnoses when either Na or water alone are altered during electrolyte abnormalities. The aim of this study is to describe the preliminary report of an original method which can give exact or useful calculations even in mixed disorders.

Methods: when NaCl is lost, the plasma Cl (PCl) to plasma Na (PNa) ratio falls from 0.75 (the normal value) toward zero, the ratio of plasma anions other than Cl (POAN) to PNa rises toward unity and POAN/PPCl toward infinity. With subscript 0 or normal values and subscript 1 for those during derangement, PCl1/PCl0 falls below unity, while POAN 1 /POAN 0 rises above unity in these hyponatremic disorders. Based on these changes, we developed exact mathematical formulas to compute alterations in solvent and solutes. The boundary conditions of applicability were computer modeled.

Results: regression coefficients between true data fed oo the computer and those calculated with our formulas were 1.00 when the boundary conditions were entirely met (R2=1.00, <0.0001) and 0.93-0.96 (R2>0.94<0.99, <0.001) when the boundary conditions were allowed to extend beyond their limits of exclusive mathematical validity. The method was extended to patients whose data satisfied the boundary conditions. The results show that the computations satisfactorily predict the exact measurements obtained by the change in body weight (R2=0.61, <0.001).

Conclusions: This new method represents useful adjunct in evaluating and treating hyponatremias. Although its validity is limited to rather strict boundary conditions, it represents an original way to evaluate mixed solvent/solute derangements.