How should I lower serum sodium in a patient with hypernatremia?

How to Lower Serum Sodium in Hypernatremia

For hypernatremia, lower serum sodium by administering hypotonic fluids (0.45% NaCl or D5W) at a controlled rate not exceeding 10 mmol/L per 24 hours, while simultaneously addressing the underlying cause of water deficit or excessive sodium intake. 1

Initial Assessment and Fluid Selection

Determine the underlying mechanism by evaluating volume status, urine osmolality, and access to free water 2:

• Hypovolemic hypernatremia (dehydration from GI losses, burns, excessive sweating) requires initial volume resuscitation followed by hypotonic fluid replacement 2
• Euvolemic hypernatremia (diabetes insipidus, impaired thirst mechanism) necessitates free water replacement and treatment of the underlying cause 2
• Hypervolemic hypernatremia (rare, from excessive sodium administration) requires diuretics plus hypotonic fluid replacement 3

Hypotonic Fluid Options

Choose the appropriate hypotonic solution based on severity and clinical context 4:

• 0.45% NaCl (half-normal saline) provides 77 mEq/L sodium and is appropriate for moderate hypernatremia 1
• D5W (5% dextrose in water) delivers no osmotic load and allows the most controlled decrease in plasma osmolality 1
• 0.18% NaCl (quarter-normal saline) contains approximately 31 mEq/L sodium for more aggressive free water replacement 1

Avoid isotonic saline (0.9% NaCl) in hypernatremia as it delivers excessive osmotic load—requiring 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid—which risks worsening hypernatremia 1

Critical Correction Rate Guidelines

The maximum safe correction rate is 10 mmol/L per 24 hours to prevent cerebral edema from rapid osmotic shifts 2, 3:

• For chronic hypernatremia (>48 hours duration), do not exceed 8-10 mmol/L per day 3
• A rate of 0.5 mmol/L per hour is the traditional upper limit, though recent evidence in critically ill adults suggests faster correction may be safe in acute settings 5
• Neonates and preterm infants require slower correction over 48-72 hours due to immature renal function and higher risk of pontine myelinolysis 1

Calculation of Fluid Requirements

Calculate the free water deficit using the formula 2:

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

Administer replacement fluids at a rate that achieves the target correction 2:

• For adults: 25-30 mL/kg/24 hours as baseline maintenance 1
• For children: 100 mL/kg/24 hours for first 10 kg, 50 mL/kg/24 hours for 10-20 kg, 20 mL/kg/24 hours for remaining weight 1

Special Clinical Scenarios

Diabetes insipidus (central or nephrogenic) requires specific management 2:

• Central diabetes insipidus: Administer desmopressin (Minirin) plus hypotonic fluid replacement 3
• Nephrogenic diabetes insipidus: Ongoing hypotonic fluid administration is required to match excessive free water losses; isotonic fluids will worsen hypernatremia 1

Patients with renal concentrating defects need continuous hypotonic fluid administration to prevent hypernatremia recurrence 1

Monitoring Protocol

Check serum sodium frequently during correction 3:

• Every 2-4 hours initially during active correction
• Adjust fluid rate based on response to avoid overcorrection
• Monitor for signs of cerebral edema (confusion, seizures, altered consciousness) 4

Watch for CNS complications which represent the highest morbidity and mortality risk in hypernatremic dehydration 4:

• Confusion, lethargy, or coma indicate severe CNS dysfunction 3
• Rapid correction can paradoxically cause cerebral edema as brain cells reaccumulate osmoles 4

Common Pitfalls to Avoid

Never use isotonic saline as primary therapy for hypernatremia—it worsens the sodium concentration by providing inadequate free water 1

Do not correct chronic hypernatremia rapidly (>10 mmol/L per day) as this causes cerebral edema from osmotic shifts 3

Avoid starting renal replacement therapy without adjusting dialysate sodium in patients with chronic hypernatremia, as standard dialysis can cause dangerously rapid sodium drops 3

For acute hypernatremia (<24 hours), hemodialysis with adjusted sodium concentration in the dialysate is an effective option to normalize levels more rapidly 3