Table 1.

Interpreting hyponatremia in hemodialysis (HD) patients from a mechanical perspective

No.	HD characteristics	Pathophysiologic explanations
1	Hemodialysis patient represents a closed system with 2 main compartments (intracellular volume/extracellular volume)	Law of mass conservation applies to fluid and sodium imbalance (in/out) in the patient-HD system	Input: diet intake, endogenous production Output: hemodialysis system (Role of residual kidney function is quite limited and most likely absent)
2	Lack of kidney function (anuric)	Exclude all causes referring to decrease of free water excretion (vasopressin, cortisol, tubular defect)	Orients toward free water excess intake or compartmental translocation
3	Free water excess is quite unlikely in HD patient	Except in case of excessive thirst (i.e., hyperglycemia, angiotensin II) or potomania (mental disorders)	Combined fluid disorders (extracellular fluid [ECF] and intracellular fluid [ICF] excess) are most likely to be present
4	Fixed dialysate sodium concentration is used in general practice	Dialysate Na prescription ranges between 135 and 142 mM/l	Hyponatremia is unlikely reflecting low dialysate sodium concentration except with technical failure or human error
5	Interdialytic weight gain is a marker of fluid and sodium accumulation	Hyponatremia may be used to quantify free water excess (hypotonic fluid imbalance) and reflect combined ECF and ICF excess	Normonatremia suggests isotonic accumulation of fluid with predominant ECF expansion