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A computer-based model has been developed to predict the changes in serum sodium, urea concentration, and osmolality as well as transcellular fluid distribution which occur during hemodialysis. Sodium and urea transfers across the dialyzer membrane and transcellular fluid shifts in response to the sodium transfer were modeled assuming that only sodium and its accompanying anions are important as effective osmotic substances in extracellular fluid. Model predictions were consistent with values measured in five patients who were studied on hemodialysis at three different dialysate sodium concentrations equal to 7% below and 7% above the predialysis serum concentration. The measurements and model predictions indicate that serum sodium concentration decreases and intracellular fluid volume increases in dialyses with dialysate Na+ concentration used in conventional hemodialysis, whereas serum sodium concentration increases and intracellular volume decreases in high sodium dialyses. An analysis of model predictions indicates that a reasonable estimation of total body water and the intracellular to extracellular volume ratio enables us to accurately predict the magnitude of transcellular fluid shifts induced by hemodialysis as well as the postdialysis serum sodium concentration and osmolality.

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