Causes of Hyponatremia in Hemodialysis vs Peritoneal Dialysis
Hemodialysis Patients
In hemodialysis patients, hyponatremia is primarily caused by excessive free water intake relative to sodium intake, typically driven by high dialysate sodium concentrations (140+ mmol/L) that stimulate thirst and promote fluid consumption between dialysis sessions. 1
Primary Mechanisms in HD
Dilutional hyponatremia results from positive water balance exceeding sodium balance, creating a fundamental mismatch between water and sodium accumulation 1
High dialysate sodium (increased from 135 mmol/L in the 1960s to 140+ mmol/L by the 1990s) aggravates thirst through osmotic mechanisms, leading to excessive interdialytic fluid gain and subsequent hyponatremia 1
Sodium-driven thirst cycle: When patients consume excess sodium, the resulting osmotic gradient triggers thirst and water consumption, creating isotonic fluid gain that becomes hyponatremic when water intake exceeds proportionate sodium 1
Contributing Factors in HD
Volume expansion from sodium and water retention is especially marked in HD patients with poor residual kidney function 1
Hyperglycemia stimulates thirst independent of sodium intake, and elevated blood angiotensin levels can promote drinking 1
Iatrogenic causes include normal saline administration during dialysis to treat hypotension, which expands volume without proportionate sodium correction 1
Medications that increase water consumption contribute to the water-sodium imbalance 1
Dialysate composition errors can inadvertently cause hyponatremia (or hypernatremia), particularly from faulty conductivity monitors 2
Peritoneal Dialysis Patients
In peritoneal dialysis patients, hyponatremia has distinct etiologies that depend on body weight changes and intracellular/extracellular fluid volume ratios, requiring assessment of both fluid compartments to identify the underlying cause. 3
Hyponatremia with Weight Loss in PD
Negative sodium balance with increased ICV/ECV ratio: Primarily driven by excessive sodium removal relative to water, resulting in weight loss but preserved or increased intracellular volume 3
Malnutrition with reduced ICV/ECV ratio: Associated with poor nutritional status, low serum albumin, and reduced hand grip strength, reflecting both intracellular and extracellular volume depletion 3
Hyponatremia with Weight Gain in PD
Rapid loss of residual renal function (RRF) with reduced ICV/ECV ratio: Combined with higher peritoneal membrane permeability, leading to inadequate sodium and water removal despite weight gain 3
Excessive water intake with normal to increased ICV/ECV ratio: Similar to HD patients, driven by high water consumption exceeding sodium intake 3
PD-Specific Factors
Overnight ultrafiltration amount shows negative correlation with serum sodium concentrations, suggesting that aggressive peritoneal ultrafiltration can contribute to hyponatremia 3
Peritoneal membrane characteristics influence sodium and water removal efficiency, with high transporters potentially at greater risk 3
Lower dialytic efficiency compared to HD means PD achieves only 10-20% of normal kidney clearance, limiting sodium and water regulation 4
Continuous nature of PD creates different sodium and water balance dynamics compared to intermittent HD 4
Key Differences Between HD and PD
HD-Specific Patterns
Hyponatremia in HD is predominantly thirst-driven and dialysate-related, with high dialysate sodium being the central culprit 1
Intermittent treatment creates cyclical patterns of sodium and water accumulation between sessions 1
Dialysate composition errors are a unique HD risk factor 2
PD-Specific Patterns
Hyponatremia in PD has multiple distinct etiologies requiring assessment of ICV/ECV ratios and weight trends to differentiate 3
RRF loss plays a more prominent role in PD hyponatremia than in HD 3
Continuous treatment means sodium and water balance is ongoing rather than intermittent 4
Peritoneal membrane transport characteristics directly influence sodium handling in ways not applicable to HD 3
Critical Management Pitfall
Advising water restriction without concurrent sodium restriction is a critical mistake in both HD and PD patients, as excessive sodium intake will continue to stimulate water consumption through osmotic mechanisms. 1 Simultaneous sodium restriction (≤5g sodium chloride or 85 mmol daily) and avoidance of high dialysate sodium concentrations (using 135-138 mmol/L instead of 140+ mmol/L) are essential to prevent and treat hyponatremia 1, 5