Why Free Water Flushes May Not Be Improving Hypernatremia
Free water flushes are likely failing to correct hypernatremia because the patient has ongoing free water losses that exceed the replacement being provided, or there is an underlying condition causing persistent water loss (such as nephrogenic diabetes insipidus, renal concentrating defects, or inadequate prescription of free water relative to losses). 1, 2
Primary Causes of Treatment Failure
Inadequate Free Water Prescription
Hospital-acquired hypernatremia is frequently iatrogenic due to insufficient water prescription relative to ongoing losses. 3, 4 The free water flushes being administered may simply be inadequate in volume to match the patient's physiological water deficit plus ongoing losses.
Calculate the actual free water deficit using: Water deficit = 0.6 × body weight (kg) × [(current Na/140) - 1], then add ongoing losses (insensible losses, urine output, gastrointestinal losses). 2 Many clinicians fail to account for both components, leading to persistent hypernatremia despite treatment attempts.
Ongoing Excessive Free Water Losses
Patients with renal concentrating defects, such as nephrogenic diabetes insipidus, will develop or maintain hypernatremia even when receiving isotonic fluids. 1 If the patient has unrecognized diabetes insipidus (central or nephrogenic), free water flushes alone will be insufficient without addressing the underlying polyuria.
Evaluate urine osmolality and volume: if urine osmolality is inappropriately low (<300 mOsm/kg) in the setting of hypernatremia with high urine output (>3 L/day), diabetes insipidus should be suspected. 2, 5
Extrarenal free water losses from voluminous diarrhea, severe burns, or high fever may exceed replacement rates. 1 These ongoing losses must be quantified and replaced in addition to correcting the existing deficit.
Treatment Approach Issues
Incorrect Fluid Choice
If isotonic saline (0.9% NaCl) is being used instead of hypotonic fluids, this will worsen hypernatremia by adding sodium load. 2 The initial fluid of choice for hypernatremia should be hypotonic solutions such as 5% dextrose in water (D5W) or 0.45% saline, not normal saline. 2, 6
Patients with significant renal concentrating defects require hypotonic fluid replacement specifically. 1 Using isotonic maintenance fluids in these patients perpetuates the hypernatremia.
Inadequate Correction Rate
The correction rate may be too slow for the severity of hypernatremia, though overly rapid correction (>8-10 mmol/L/day for chronic hypernatremia >48 hours) risks osmotic demyelination. 5, 4
For chronic hypernatremia, aim for sodium reduction of 8-10 mmol/L per day maximum. 5 Monitor serum sodium every 2-4 hours initially to ensure adequate but not excessive correction.
Critical Diagnostic Steps
Assess Volume Status
- Determine if the patient is hypovolemic, euvolemic, or hypervolemic, as this guides both the underlying cause and treatment approach. 2 Hypovolemic hypernatremia requires more aggressive volume repletion with hypotonic fluids.
Evaluate Urine Studies
- Measure urine osmolality and urine sodium to differentiate between renal and extrarenal water losses. 5, 6
- High urine osmolality (>600-800 mOsm/kg) suggests extrarenal losses or inadequate water intake
- Low urine osmolality (<300 mOsm/kg) with polyuria suggests diabetes insipidus
Identify Underlying Causes
Review medications that may impair renal concentrating ability (lithium, demeclocycline, amphotericin B). 4
Assess for diabetes insipidus: if suspected, a trial of desmopressin (DDAVP) should produce concentrated urine and reduced urine output within hours. 5 If nephrogenic diabetes insipidus is present, desmopressin will be ineffective and the patient requires hypotonic fluid replacement plus treatment of the underlying cause.
Common Pitfalls
Failing to account for ongoing losses when calculating free water replacement leads to persistent hypernatremia despite treatment. 3, 4 Serial measurements of serum and urine electrolytes are essential, as formulae treat the patient as a closed system and don't account for variable ongoing losses.
Using isotonic fluids in patients with renal concentrating defects will maintain or worsen hypernatremia. 1, 2
Inadequate monitoring frequency: serum sodium should be checked every 2-4 hours during active correction to adjust the replacement rate appropriately. 4
In patients with cirrhosis and hypernatremia, fluid restriction should be avoided and hypotonic fluids are still appropriate. 2