Hyponatremia is the Electrolyte Abnormality Caused by Volume Excess
Sodium 129 mEq/L is the laboratory value most directly attributable to this patient's volume overload, representing dilutional hyponatremia from excessive fluid accumulation between dialysis sessions. 1, 2
Pathophysiology of Dilutional Hyponatremia in Volume Overload
The clinical picture clearly demonstrates severe volume excess:
- Interdialytic weight gain of 4-4.7 kg indicates massive fluid accumulation 1, 2
- Grade 2 leg edema with stasis dermatitis, facial edema, and bibasal crackles confirm extracellular fluid expansion 1, 2
- Hypertension (150-170/90-100 mmHg) is directly driven by this volume overload 1, 2
When excessive fluid accumulates in the extracellular space without proportional sodium retention, the serum sodium concentration becomes diluted, resulting in hyponatremia. 1 This is the hallmark electrolyte disturbance of volume excess in hemodialysis patients. 2
Why the Other Values Are NOT Caused by Volume Overload
Hyperphosphatemia (10.6 mg/dL)
- Elevated phosphorus results from inadequate dialytic removal, dietary phosphorus intake, and secondary hyperparathyroidism—not from volume status 3, 4
- Phosphorus kinetics are independent of extracellular fluid volume expansion 5
- This value reflects the patient's underlying CKD stage 5 and dialysis adequacy, not fluid overload 3
Hyperkalemia (5.9 mEq/L)
- Potassium elevation in dialysis patients results from dietary intake, cellular shifts, and inadequate dialytic removal 1
- Volume overload does not cause hyperkalemia; in fact, volume expansion would theoretically dilute potassium concentration 1
- This represents the patient's interdialytic potassium accumulation from dietary sources and reduced renal excretion 1
Hypocalcemia (6.96 mg/dL)
- Low calcium is a consequence of CKD-mineral bone disorder, hyperphosphatemia, and secondary hyperparathyroidism 3
- Volume status does not directly affect serum calcium concentration 1
- This reflects the patient's underlying renal osteodystrophy, not fluid overload 3
Clinical Management Implications
The presence of hyponatremia in this volume-overloaded patient mandates aggressive ultrafiltration with lowered dialysate sodium concentration (135-138 mmol/L) to facilitate sodium and water removal. 2
Key management priorities include:
- Extending dialysis treatment time or adding sessions to achieve safe ultrafiltration rates below 10 ml/kg/hr 2
- Implementing strict dietary sodium restriction to <2g/day 2
- Reassessing dry weight downward to achieve euvolemia 1, 2
Critical Pitfall to Avoid
Do not interpret the hyponatremia as requiring sodium supplementation or fluid restriction alone—this patient needs aggressive volume removal through intensified dialysis to correct both the fluid overload and the dilutional hyponatremia. 2 The hyponatremia will correct as volume status normalizes through adequate ultrafiltration. 1