An Equation for Calculating Postdialysis Plasma Sodium

Abstract

Well defined dry weight is a must for adequate UF control during haemodialysis (HD). However, interdialytic weight gain (delta BW) must not be excessive. delta BW is closely related to interdialytic thirst which in turn is strongly influenced by post-dialysis plasma sodium (CPNa post), but little is known about the desired CPNa post. The points below serve as a basis for establishing this value. a) Thirst is mediated by osmoreceptors. b) A strong correlation has been found between delta BW and intradialytic increase in plasma sodium but no such correlation exists with the interdialytic increase in plasma urea. This indicates that fluid intake between dialyses depends solely on electrolytes. c) Pre-dialysis plasma sodium in an individual is stable, indicating that the patient is at his "set value" of electrolyte osmolality. d) Half of the potassium removed during HD comes from the extra- and half from the intracellular space. Assuming that it is desirable not to disturb a patient's pre-dialysis osmotic steady state, it can be calculated that the desired CPNa post should be higher than the pre-dialysis value by half of the intradialytic plasma potassium drop, i.e., approx. CPNa post = CPNa pre + 1 to 2 mmol/l.

Clinical consequences of an individualized dialysate sodium prescription in hemodialysis patients

“…This is in agreement with previous studies showing that the predialysis sodium concentration is constant independently of the sodium gradient established between blood and dialysate in the previous session [6]. This "set point" dictates the interdialytic fluid intake to bring one's osmolality back to its set point; if the post-HD Na + is higher, greater IDWG will inevitably occur.…”

“…However, predialysis plasma sodium concentration is constant in HD patients, and these patients seem to have an individual osmolar set point [6]; consequently, the use of a dialysate Na + concentration higher than plasma could contribute to an increase in the interdialytic fluid ingestion in the attempt to achieve this set point [7].…”

Individualized Sodium Prescription in Hemodialysis: An Ally for Better Dialysis Outcomes?

“…1,14 This value is highly conserved. Addition of extra sodium to the body will increase the thirst, thus increasing fluid intake so as to maintain the sodium and osmolar setpoints.…”

Are Dialysate Sodium Levels Too High?

“…Another concern that arises is whether or not dialysate [Na+] might also influence serum [Na+]. The traditional wisdom is that humans have an individual natremic set point, and studies such as the DOPPS data referred to above have generally not shown any cross‐sectional correlation between dialysate and serum [Na+] (41,61,71–76). However, it is likely that natremic adaptation does in fact occur, at least to some degree.…”