Management of Hypovolemic Hypernatremia
For hypovolemic hypernatremia, restore intravascular volume with hypotonic fluids (0.45% NaCl or D5W) while correcting sodium at a maximum rate of 10 mmol/L per 24 hours (0.4 mmol/L/hour) to prevent cerebral edema. 1
Initial Assessment and Pathophysiology
Hypovolemic hypernatremia occurs when sodium concentration exceeds 145 mmol/L in the setting of volume depletion 1. This develops from either:
- Renal losses: Osmotic diuresis, post-obstructive diuresis, or intrinsic renal disease 1
- Extrarenal losses: Gastrointestinal losses (diarrhea, vomiting), excessive sweating, or burns 1, 2
- Inadequate water intake: Impaired thirst mechanism or lack of access to water 3, 1
The key distinguishing feature is evidence of volume depletion: orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, and flat neck veins 4.
Fluid Selection Strategy
Primary fluid choice is 0.45% NaCl (half-normal saline) containing 77 mEq/L sodium with osmolarity of 154 mOsm/L 4. This provides both free water replacement and some sodium, making it appropriate for moderate hypernatremia correction 4.
For severe hypernatremia or patients requiring more aggressive free water replacement, use 0.18% NaCl (quarter-normal saline) containing 31 mEq/L sodium 4. Alternatively, D5W (5% dextrose in water) delivers no renal osmotic load and allows controlled decrease in plasma osmolality 4.
Critical pitfall: Never use isotonic saline (0.9% NaCl) in hypernatremic dehydration 4. This 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 4.
Correction Rate Guidelines
The maximum safe correction rate is 0.4 mmol/L/hour or 10 mmol/L per 24 hours 4, 1. This prevents cerebral edema from overly rapid correction 4.
- Acute hypernatremia (developed over hours): Rapid correction improves prognosis by preventing cellular dehydration effects 1
- Chronic hypernatremia (developed over days): Slow correction at no more than 0.4 mmol/L/hour is mandatory 1
Initial Fluid Administration Rates
For adults: Start at 25-30 mL/kg/24 hours 4
For children: Calculate based on physiological maintenance requirements 4:
- 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
High-risk populations (infants, malnourished patients): Consider smaller-volume frequent boluses of 10 mL/kg due to reduced cardiac output capacity 4.
Special Clinical Scenarios
Nephrogenic diabetes insipidus: Requires ongoing hypotonic fluid administration to match excessive free water losses 4, 1. Isotonic fluids must be avoided as they worsen hypernatremia 4.
Voluminous diarrhea or severe burns: Match fluid composition to ongoing losses while providing adequate free water 4. These patients require continuous hypotonic fluid replacement 4.
Renal concentrating defects: Patients cannot excrete free water appropriately and will develop worsening hypernatremia if given isotonic fluids 4. Hypotonic replacement is essential 4.
Monitoring Protocol
- Check serum sodium every 2-4 hours during active correction to ensure rate does not exceed 0.4 mmol/L/hour 1
- Monitor volume status: Track urine output, vital signs, and clinical signs of rehydration 3
- Adjust fluid rate based on sodium response and clinical improvement 3
Common Pitfalls to Avoid
- Using isotonic saline in hypernatremic patients: This worsens hypernatremia by delivering excessive osmotic load 4
- Correcting too rapidly: Exceeding 10 mmol/L per 24 hours risks cerebral edema 4, 1
- Failing to address underlying cause: Identify and treat the source of water loss or inadequate intake 3, 2
- Inadequate monitoring: Failure to check sodium levels frequently during correction can lead to overcorrection complications 1