Hypernatremia in Hypervolemic Patients
Primary Cause
In hypervolemic patients, hypernatremia results from excess sodium administration that exceeds the body's ability to excrete it, leading to both sodium and water overload—but with proportionally more sodium than water. 1
Pathophysiology
The mechanism differs fundamentally from other forms of hypernatremia:
Iatrogenic sodium loading is the predominant cause, typically from administration of hypertonic saline solutions (3% NaCl), sodium bicarbonate infusions, or excessive isotonic saline in patients with impaired renal sodium excretion 2, 3
Hypervolemic hypernatremia is the most common type in ICU settings, accounting for the majority of cases in critically ill patients recovering from acute kidney injury 1
The average weight gain in these patients exceeds 9 kg despite ongoing fluid losses, with edema present as clear evidence of volume overload 1
Clinical Context in ICU Patients
Post-AKI diuresis plays a critical role—patients recovering from acute kidney injury cannot maximally concentrate urine, leading to ongoing free water losses while retaining the previously administered sodium load 1
Even though patients may be in negative fluid balance at the time hypernatremia is detected, earlier aggressive saline administration has caused massive volume overload that persists 1
Urine studies reveal high osmolality (average 436 mmol/kg) with sodium and potassium contributing only 172 mmol/L, while urea accounts for most of the remaining osmoles (204 mmol/L) due to elevated serum urea from renal dysfunction 1
Acute vs. Chronic Hypervolemic Hypernatremia
Acute hypervolemic hypernatremia (<24-48 hours) typically results from rapid sodium loading with hypertonic solutions 3
Chronic hypervolemic hypernatremia may indicate underlying conditions like primary hyperaldosteronism, though this is less common in ICU settings 3
Key Distinguishing Features
The hypervolemia is evident through:
- Peripheral edema
- Weight gain (often >9 kg from baseline)
- Jugular venous distention
- Pulmonary congestion 1
This contrasts sharply with hypovolemic hypernatremia (from water losses) or euvolemic hypernatremia (from diabetes insipidus), where volume depletion or normal volume status would be present. 3, 4