How do I assess and treat hypernatremia emergently, including calculation of free‑water deficit, fluid selection, correction rate, and management of underlying etiologies?

Emergency Assessment and Treatment of Hypernatremia

For hypernatremia, immediately calculate the free-water deficit, initiate hypotonic fluid replacement at a controlled rate not exceeding 10 mmol/L per 24 hours (0.4 mmol/L/hour), and address the underlying cause—typically excessive free water loss or impaired thirst mechanism.

Immediate Assessment

Calculate Free-Water Deficit

Use the following formula to determine the volume of free water needed 1, 2:

Free-water deficit (L) = 0.6 × body weight (kg) × [(current Na / 140) - 1]

• For women, use 0.5 instead of 0.6 as the multiplier 2
• This calculation provides the baseline deficit; ongoing losses must be added separately 2

Determine Acuity

• Acute hypernatremia (<24-48 hours): Symptoms develop rapidly; more aggressive correction may be tolerated 3, 4
• Chronic hypernatremia (>48 hours): Brain cells have adapted by increasing intracellular osmolytes; rapid correction risks cerebral edema 3, 4

Assess Volume Status and Etiology

Hypovolemic hypernatremia (most common):

• Look for signs of dehydration: orthostatic hypotension, dry mucous membranes, decreased skin turgor 1, 2
• Causes: inadequate water intake, excessive GI losses (diarrhea, vomiting), osmotic diuresis 1, 2
• Urine sodium <20 mEq/L suggests extrarenal losses; >20 mEq/L suggests renal losses 2

Euvolemic hypernatremia:

• Diabetes insipidus (central or nephrogenic) 1, 2
• Urine osmolality <300 mOsm/kg with polyuria indicates diabetes insipidus 2
• Measure copeptin or ADH levels if available to distinguish central from nephrogenic 2

Hypervolemic hypernatremia (rare):

• Excessive sodium administration (hypertonic saline, sodium bicarbonate) 2, 4
• Urine sodium typically >100 mEq/L 2

Fluid Selection and Correction Rate

Choose Hypotonic Fluids

For moderate to severe hypernatremia:

• 5% dextrose in water (D5W) is preferred as it delivers no osmotic load and allows controlled sodium reduction 1, 2
• 0.45% NaCl (half-normal saline) provides both free water and some sodium (77 mEq/L); appropriate for moderate hypernatremia with volume depletion 1, 2
• 0.18% NaCl (quarter-normal saline) contains 31 mEq/L sodium; more aggressive free water replacement 2

Avoid isotonic saline (0.9% NaCl) in hypernatremia as it delivers excessive osmotic load—3 liters of urine are required to excrete the osmotic load from just 1 liter of isotonic fluid, potentially worsening hypernatremia 2

Critical Correction Rate Limits

For chronic hypernatremia (>48 hours):

• Maximum correction: 10 mmol/L per 24 hours (or 0.4 mmol/L/hour) 2, 3
• Safer target: 8 mmol/L per 24 hours to minimize cerebral edema risk 3
• Exceeding this rate risks osmotic demyelination syndrome and cerebral edema 3, 4

For acute hypernatremia (<24 hours):

• More rapid correction may be tolerated, but still monitor closely 3
• Hemodialysis is an option for severe acute hypernatremia requiring rapid normalization 3

Calculate Infusion Rate

Initial fluid administration rate 2:

• Adults: 25-30 mL/kg/24 hours as baseline maintenance
• Children: 100 mL/kg/24h for first 10 kg, 50 mL/kg/24h for 10-20 kg, 20 mL/kg/24h for remaining weight

Add ongoing losses:

• Insensible losses: ~500-800 mL/day (increases with fever, tachypnea) 2
• Measured urine output and other losses 2

Adjust rate to achieve target correction:

• Recheck sodium every 2-4 hours initially 2, 4
• Adjust infusion rate to stay within 0.4 mmol/L/hour reduction 2

Management of Underlying Etiologies

Diabetes Insipidus

Central diabetes insipidus:

• Desmopressin (DDAVP): 1-2 mcg IV/SC or 10-20 mcg intranasal every 12-24 hours 1, 2
• Continue hypotonic fluid replacement until desmopressin takes effect 2

Nephrogenic diabetes insipidus:

• Requires ongoing hypotonic fluid administration to match excessive free water losses 2
• Thiazide diuretics and amiloride may reduce urine output 2
• Avoid isotonic fluids as they worsen hypernatremia 2

Impaired Thirst or Access to Water

• Most common in elderly, critically ill, or patients with altered mental status 1, 4
• Ensure adequate free water access once alert 1
• Consider enteral or IV hydration if unable to drink 1, 4

Excessive Sodium Administration

• Discontinue hypertonic saline or sodium bicarbonate infusions 2, 4
• Consider loop diuretics to promote renal sodium excretion 4
• Replace with hypotonic fluids 2

Osmotic Diuresis

• Hyperglycemia: Correct with insulin; each 100 mg/dL glucose >100 mg/dL adds ~1.6 mEq/L to measured sodium 1
• Mannitol or urea: Discontinue if possible 2

Monitoring Protocol

Laboratory monitoring:

• Serum sodium every 2-4 hours during active correction 2, 4
• Daily electrolytes, glucose, BUN, creatinine 2
• Urine osmolality and sodium to guide therapy 2

Clinical monitoring:

• Neurological status: confusion, seizures, altered consciousness 1, 4
• Volume status: vital signs, urine output, weight 2, 4
• Signs of overcorrection: headache, nausea, seizures (cerebral edema) 3, 4

Common Pitfalls

• Correcting chronic hypernatremia too rapidly (>10 mmol/L/24h) causes cerebral edema and can be fatal 3, 4
• Using isotonic saline in hypernatremia worsens the condition by delivering excessive osmotic load 2
• Failing to account for ongoing losses (insensible, urine output) leads to inadequate correction 2
• Not adjusting correction rate when starting renal replacement therapy in chronic hypernatremia can cause rapid sodium drops 3
• Ignoring underlying diabetes insipidus results in persistent hypernatremia despite fluid replacement 1, 2