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Electrolytes & Fluids

Hypernatremia water deficit

Free-water deficit and correction plan for hypernatremia in dogs and cats. Takes serum Na and body weight, classifies the mechanism (pure water loss, hypotonic loss, or rare sodium gain), and returns a correction rate that keeps Na decline under 10–12 mEq/L per 24 hr. Slow correction matters: faster decline risks cerebral edema.

Why mechanism matters

Hypernatremia presents three different fluid problems. Pure water loss needs free-water replacement. Hypotonic loss needs ECF volume restoration first, then free water. Sodium gain is a solute problem, not a water problem. The correct fluid depends on which mechanism caused the elevated Na.

See "Mechanism detail" below for examples and clinical clues.
Default 145 mEq/L from a worked example in the published literature. Override with the patient's documented previous normal sodium when known.
Standard published default is 48 hours. Extend if predicted Na drop exceeds 12 mEq/L per 24 hr.
Awaiting input

Enter a patient weight to see the result.

Reference

Mechanism detail

Pure water loss

Free water lost without proportional sodium loss. ECF volume preserved or only mildly contracted.

Examples
  • Water deprivation (locked in garage, no access to water)
  • Central diabetes insipidus
  • Nephrogenic diabetes insipidus
  • Hypodipsia / adipsia syndromes
Strategy

Replace the calculated water deficit with 5% dextrose in water. Glucose is metabolized leaving essentially free water.

Fluids
  • 5% dextrose in water (D5W)

Hypotonic loss

Water and sodium both lost, but water lost in greater proportion. ECF volume contracted; signs of hypovolemia may be present.

Examples
  • Osmotic diuresis (DKA, mannitol therapy)
  • GI losses with ongoing dehydration
  • Third-spacing (peritonitis, severe pancreatitis)
  • Heat stroke with significant ongoing losses
Strategy

Two-stage approach. Restore ECF volume with isotonic crystalloid first, then transition to hypotonic fluids. Up to 4× the suspected intravascular deficit may be needed.

Fluids
  • Initial volume restoration: 0.9% NaCl, LRS
  • Subsequent free-water replacement: 0.45% NaCl, half-strength LRS

Gain of impermeant solute

Sodium gained without proportional water loss. ECF volume may be expanded; signs of volume overload (pulmonary edema) may be present.

Examples
  • Hypertonic saline administration (iatrogenic)
  • Sodium bicarbonate during cardiac resuscitation
  • Salt toxicosis (paintball ingestion, dough ingestion, seawater)
  • Sodium phosphate enemas
  • Hypertonic feeding tube formulas in compromised patients
Strategy

Administer 5% dextrose IV. Risk of further ECF expansion in patients already volume-overloaded, loop diuretic may be needed.

Fluids
  • 5% dextrose in water (D5W)

Formula

$$\text{water deficit (L)} = \text{Wt}_{\text{present}} \times \left( \frac{P_{Na}(\text{present})}{P_{Na}(\text{previous})} - 1 \right)$$

Weight in kilograms multiplied by the ratio of present-to-previous serum sodium minus one. The result is in liters of free-water deficit.

Worked example with current inputs

Enter a patient weight and sodium values to see the worked example.

Correction rate ceiling

Correction at less than 10–12 mEq/L per 24 hours (≈ 0.42–0.5 mEq/L/hr) minimizes risk of cerebral edema from idiogenic osmoles. Faster correction is particularly risky in chronic hypernatremia.

Sources

  • DiBartola SP, ed. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 4th ed. St. Louis, MO: Elsevier Saunders; 2012. Chapter 3 (Disorders of Sodium and Water: Hypernatremia and Hyponatremia), pp. 60–61.