What is the appropriate treatment approach for hypernatremia, including volume resuscitation and controlled free‑water replacement?

Treatment of Hypernatremia

For hypernatremia, administer hypotonic fluids (0.45% NaCl or D5W) to replace free water deficit, targeting a maximum correction rate of 10-15 mmol/L per 24 hours for chronic cases (>48 hours), while avoiding isotonic saline which will worsen the condition. 1

Initial Assessment and Volume Status Determination

Before initiating treatment, assess the patient's volume status and chronicity of hypernatremia, as these factors fundamentally guide your therapeutic approach 1, 2:

• Hypovolemic hypernatremia: Clinical signs include orthostatic hypotension, dry mucous membranes, decreased skin turgor, and tachycardia 1
• Euvolemic hypernatremia: Normal volume status with intact thirst mechanism failure or water access issues 2
• Hypervolemic hypernatremia: Presence of edema, ascites, or jugular venous distention 1

Check urine osmolality and sodium to differentiate causes 2. A urine osmolality <300 mOsm/kg with hypernatremia suggests diabetes insipidus, while >600 mOsm/kg indicates extrarenal water losses 2.

Fluid Selection and Administration Strategy

The cornerstone of treatment is hypotonic fluid replacement 1, 3:

• 0.45% NaCl (half-normal saline): Contains 77 mEq/L sodium with osmolarity ~154 mOsm/L, appropriate for moderate hypernatremia 1
• 0.18% NaCl (quarter-normal saline): Contains ~31 mEq/L sodium, providing more aggressive free water replacement for severe cases 1
• D5W (5% dextrose in water): Delivers no renal osmotic load and allows controlled decrease in plasma osmolality; preferred as primary rehydration fluid 1

Never use isotonic saline (0.9% NaCl) as initial therapy, as it delivers excessive osmotic load requiring 3 liters of urine to excrete the osmotic load from just 1 liter of fluid, which risks worsening hypernatremia 1. This is particularly critical in patients with nephrogenic diabetes insipidus or renal concentrating defects 1.

Correction Rate Guidelines

The single most important safety principle is avoiding overly rapid correction 1, 4:

• Chronic hypernatremia (>48 hours): Maximum correction of 10-15 mmol/L per 24 hours to prevent cerebral edema, seizures, and permanent neurological injury 1, 4
• Acute hypernatremia (<24-48 hours): Can be corrected more rapidly, up to 1 mmol/L/hour if severely symptomatic 1

The rationale for slower correction in chronic cases is that brain cells synthesize intracellular osmolytes over 48 hours to adapt to hyperosmolar conditions 1. Rapid correction causes water to shift into brain cells faster than osmolytes can be eliminated, resulting in cerebral edema 1.

Calculate Free Water Deficit

Use this formula to determine fluid requirements 1:

Free water deficit = 0.5 × ideal body weight (kg) × [(current Na ÷ desired Na) - 1]

For adults, initial fluid administration rate is 25-30 mL/kg/24 hours 1. For children, use 100 mL/kg/24 hours for first 10 kg, 50 mL/kg/24 hours for 10-20 kg, and 20 mL/kg/24 hours for remaining weight 1.

Treatment Based on Volume Status

Hypovolemic Hypernatremia

Administer hypotonic fluids to replace free water deficit at an initial rate of 4-14 mL/kg/h 1. Replace ongoing losses from diarrhea, vomiting, or burns with appropriate hypotonic solutions 1.

Euvolemic Hypernatremia

Provide hypotonic fluid replacement while ensuring access to free water 1. Consider desmopressin (Minirin) if diabetes insipidus is confirmed 4. For nephrogenic diabetes insipidus, ongoing hypotonic fluid administration is required to match excessive free water losses 1.

Hypervolemic Hypernatremia

In cirrhosis patients: Focus on attaining negative water balance rather than aggressive fluid administration 1. Discontinue intravenous fluid therapy and implement free water restriction 1.

In heart failure patients: Combine sodium restriction (<2 g daily) with fluid limitation (1.5-2 L/day) 1. Diuretics remain essential for volume management but must be carefully balanced with hypernatremia correction 1. Consider combining IV hypotonic fluids with free water via nasogastric tube if needed, targeting 10-15 mmol/L correction per 24 hours 1.

Monitoring Protocol

Intensive monitoring is mandatory during active correction 1:

• Check serum sodium every 2-4 hours initially during active correction, then every 6-12 hours 1
• Monitor daily weight, supine and standing vital signs 1
• Track fluid input and output with careful attention to urine output, specific gravity/osmolarity, and urine electrolyte concentrations 1
• Assess renal function (BUN, creatinine) to evaluate for worsening azotemia 1

Special Populations and Considerations

Older adults are at higher risk for both hypernatremia and complications from correction due to reduced renal function and cognitive impairment that may prevent recognition of thirst 1. Use more conservative fluid rates and monitor closely 1.

Patients with chronic kidney disease require more conservative fluid rates with close monitoring for worsening azotemia during correction 1.

Traumatic brain injury patients: Prolonged induced hypernatremia to control intracranial pressure is NOT recommended, as it requires an intact blood-brain barrier to be effective and may worsen cerebral contusions 1. There is a risk of "rebound" ICP elevation during correction as brain cells synthesize intracellular osmolytes 1.

Common Pitfalls to Avoid

• Using isotonic saline in patients with renal concentrating defects will exacerbate hypernatremia 1
• Correcting chronic hypernatremia too rapidly (>10-15 mmol/L per 24 hours) leads to cerebral edema, seizures, and neurological injury 1
• Inadequate monitoring during correction results in overcorrection or undercorrection 1
• Failing to identify and treat the underlying cause, which is often iatrogenic, especially in vulnerable populations 1
• Starting renal replacement therapy without adjusting for chronic hypernatremia can cause rapid sodium drops 4