How to manage a patient with hypernatremia, polyuria, on D5W (Dextrose 5% in Water), with serum osmolality, urine osmolality, and urine sodium levels indicating impaired renal concentration ability?

Management of Hypernatremia with Polyuria and Impaired Renal Concentration

This patient has osmotic diuresis-induced hypernatremia with impaired renal concentrating ability, requiring immediate discontinuation of D5W and replacement with hypotonic fluids at a controlled correction rate of 10 mmol/L per 24 hours.

Diagnostic Interpretation

The laboratory values reveal a critical pattern:

• Serum osmolality 308 mOsm/kg with serum sodium 149 mmol/L confirms true hypernatremia, not pseudohypernatremia 1
• Urine osmolality 264 mOsm/kg is inappropriately low for hypernatremia, indicating impaired renal concentrating ability or ongoing osmotic diuresis 1
• Urine sodium 76 mmol/L with polyuria (5L/day) suggests osmotic diuresis with electrolyte-free water clearance 2
• The combined urinary sodium and potassium losses per liter are likely lower than serum sodium, explaining why hypernatremia persists despite high urine output 2

This pattern indicates osmotic diuresis with inadequate free water replacement - the D5W at 125cc/hr (3L/day) is insufficient to match the 5L urine output 3, 4.

Immediate Management Steps

1. Stop D5W and Switch to Hypotonic Fluids

• Discontinue D5W immediately - it provides inadequate free water replacement for this degree of polyuria 1
• Switch to 0.45% NaCl (half-normal saline) as the primary replacement fluid, which contains 77 mEq/L sodium and provides both free water and some sodium 1
• Calculate free water deficit: (149 - 140) × (0.5 × body weight in kg) = total free water deficit in liters 1

2. Determine Correction Rate

• For chronic hypernatremia (>48 hours), limit correction to 10-15 mmol/L per 24 hours to prevent cerebral edema 1, 5
• Never exceed 10 mmol/L reduction in the first 24 hours - rapid correction causes seizures and permanent neurological injury 1, 6
• If acute hypernatremia (<24 hours) with severe symptoms, correction up to 1 mmol/L/hour is permissible 1

3. Calculate Fluid Requirements

• Replace calculated free water deficit over 48-72 hours 6, 4
• Add ongoing losses: match the 5L urine output with hypotonic fluid replacement 3, 4
• Add insensible losses: approximately 500-800 mL/day for adults 4
• Total fluid requirement = deficit replacement + ongoing losses + insensible losses 4

Specific Fluid Management Protocol

Initial 24 Hours

• Administer 0.45% NaCl at a rate calculated to reduce sodium by 10 mmol/L over 24 hours 1
• Monitor urine output hourly and adjust fluid rate to match ongoing losses 1, 3
• Check serum sodium every 2-4 hours initially, then every 6-12 hours once stable 1

Ongoing Management

• Continue hypotonic fluid replacement until serum sodium normalizes to 135-145 mmol/L 1, 4
• Track daily weights - expect gradual weight gain as free water deficit is corrected 1
• Monitor urine osmolality and specific gravity to assess renal concentrating ability 1, 4

Identify and Treat Underlying Cause

• Osmotic diuresis causes: hyperglycemia, mannitol administration, high-protein tube feeds, urea from tissue breakdown 3, 2
• If diabetes insipidus suspected (urine osmolality <300 mOsm/kg with continued polyuria despite correction): consider desmopressin trial, but only for central DI, not nephrogenic 5, 4
• Measure urine electrolytes: if urine sodium + potassium < serum sodium, confirms electrolyte-free water loss 2

Critical Monitoring Parameters

• Serum sodium every 2-4 hours during active correction, then every 6-12 hours 1
• Daily weights and strict intake/output records 1, 6
• Neurological status: watch for confusion, seizures, or altered mental status indicating overcorrection 1, 5
• Urine output, specific gravity, and osmolality to guide fluid replacement 1, 4

Common Pitfalls to Avoid

• Never use isotonic saline (0.9% NaCl) as initial therapy - it will worsen hypernatremia by providing inadequate free water 1
• Never correct chronic hypernatremia faster than 10-15 mmol/L per 24 hours - this causes cerebral edema and seizures 1, 5, 6
• Never ignore ongoing losses - the 5L urine output must be matched with hypotonic fluid replacement 3, 4
• Never assume D5W alone is adequate - it provides only free water without electrolytes and is insufficient for this degree of polyuria 1

Special Considerations

• If nephrogenic diabetes insipidus is confirmed, ongoing hypotonic fluid administration will be required indefinitely to match excessive free water losses 1
• If patient has heart failure or cirrhosis, fluid restriction may be needed after initial correction, with careful sodium and fluid balance monitoring 1
• In critically ill patients, hypernatremia is an independent risk factor for mortality, making aggressive but controlled correction essential 3