A 6-month-old infant is found to have serum sodium of 158 mEq/L with clinical signs of 5% dehydration. What type of fluid is most appropriate as initial replacement to safely lower the sodium?
- A 5% Dextrose in water
- B 0.45% Normal saline with 5% dextrose (hypotonic)
- C 0.9% Normal saline (isotonic) ✓
- D 3% Saline
Correct answer: C. 0.9% Normal saline (isotonic)
Explanation
In hypernatraemic dehydration, the goal is slow sodium correction (no more than 0.5–1 mEq/L/hour or 10–15 mEq/L per day) to prevent cerebral oedema due to rapid osmotic shifts. Normal saline (0.9% NaCl, 154 mEq/L) is used initially as it expands intravascular volume while avoiding too rapid a drop in serum sodium. Hypotonic fluids (0.45% saline or 5% dextrose) may cause excessively rapid sodium fall and cerebral oedema. 3% saline would worsen hypernatraemia.
Reference: Ghai Essential Pediatrics, 10th ed.
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