An Equation for Calculating Postdialysis Plasma Sodium

Well defined dry weight is a must for adequate UF control during haemodialysis (HD). However, interdialytic weight gain (ΔBW) must not be excessive. ΔBW is closely related to interdialytic thirst which in turn is strongly influenced by postdialysis plasma sodium (C_P^Napost), but little is known about the desired Cp^Napost. 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 ΔBW 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 predialysis osmotic steady state, it can be calculated that the desired C_P^Napost should be higher than the predialysis value by half of the intradialytic plasma potassium drop, i.e., approx. C_P^Napost = C_P^Napre + 1 to 2 mmol/l.