Serum sodium concentration and body fluid distribution during interdialysis: importance of sodium to fluid intake ratio in hemodialysis patients.

Changes in the serum Na+ concentration and transcellular body fluid distribution during the interdialytic period were simulated as functions of body weight gain assuming that the effective extracellular osmolality consists only of sodium. Our model shows that these changes are mainly determined by the relative ratio between sodium intake and weight gain in this period. If the sodium intake-to-net fluid intake ratio is equal to the postdialytic serum [Na+], neither changes in serum [Na+] nor transcellular fluid shifts occur. When sodium intake is relatively greater than the net water intake, serum [Na+] is increased and transcellular fluid shifts will occur out of the cells. On the other hand, when the net water intake is relatively greater than the sodium intake, serum [Na+] is decreased and fluid is distributed to both the intracellular and extracellular compartments. The combined application of our models for both the intradialytic interval described previously and the interdialytic interval described here is very useful in quantitatively analyzing the overall sodium and water metabolism in dialyzed patients.