What it is
Hypernatremic dehydration (serum Na > 145 mEq/L) is intracellular water loss; brain cells generate idiogenic osmoles to defend volume. Rehydrate too fast and water rushes back into those still-hyperosmolar cells, producing cerebral edema and seizures. This tool estimates the free-water deficit and frames replacement over ~48 hours, with the correction-rate guardrail front and centre.
Formula
$$ TBW = 0.6 \times weight,(kg) $$ $$ Free\text{-}water\ deficit,(L) = TBW \times \left(\frac{Na_{measured}}{140} - 1\right) $$
A bedside cross-check, the 4 mL/kg-per-mEq rule, is also shown:
$$ FWD,(mL) \approx 4 \times weight \times (Na - 145) $$
Maintenance (Holliday-Segar) is added, and the ~48h total uses the larger of the free-water deficit or the clinical %-dehydration deficit.
When to use
A dehydrated child with confirmed hypernatremia, to plan staged rehydration. It estimates volume; it does not replace serial sodium monitoring.
Worked example
An 8 kg infant, serum Na 158 mEq/L, 8% dehydration:
- TBW: 0.6 × 8 = 4.8 L
- Free-water deficit: 4.8 × (158/140 − 1) = 4.8 × 0.129 ≈ 617 mL
- 4 mL/kg-per-mEq cross-check: 4 × 8 × (158−145) = 416 mL
- Maintenance: 800 mL/day
- Approx total over 48 h: (800×2) + max(617, 640) ≈ 2240 mL
Na 158 falls in the moderate band (>150).
Safety: the correction-rate ceiling
- Lower serum Na by no more than ~0.5 mEq/L per hour (≈ 10 mEq/L per 24 h).
- Replace the deficit gradually over ~48 h — never bolus free water to normalize quickly.
- Recheck Na every 2–4 h; if Na falls faster than the cap, slow or hold free water.
- Restore circulating volume first with isotonic fluid if the child is shocked — that is resuscitation, separate from the deficit replacement.
Pitfalls
- Totals exclude ongoing losses (diarrhoea, polyuria) — titrate to serial Na, not to the printed number.
- The safe rate depends on acute vs chronic onset; chronic hypernatremia tolerates even slower correction.
- A rapidly falling Na is the warning sign of impending cerebral edema — act on the trend, not just the target.
- Watch for rebound hyperglycemia/osmotic diuresis confounding the sodium trajectory.
Run it: Hypernatremic Dehydration
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.