Free Water Deficit in Pediatric Hypernatremia: Replace Slowly or Cause Cerebral Edema

What it is

In hypernatremia the body’s water is depleted relative to sodium. The free water deficit estimates how much electrolyte-free water must be added to return serum sodium to a normal target (here 140 mEq/L). It sizes the replacement; it does not set the infusion rate — that is governed by a hard safety ceiling.

Formula

$$ TBW = fraction \times weight $$ $$ Deficit\ (L) = TBW \times \left(\frac{Na_{measured}}{140} - 1\right) $$

The total body water fraction defaults to 0.6 (children / older infants); use 0.7 for infants under 1 year, who run a higher TBW. The deficit excludes ongoing losses and any isotonic resuscitation given for shock.

When to use

Hypernatremic dehydration (Na >145 mEq/L) once any haemodynamic resuscitation with isotonic fluid is complete, to plan the maintenance-plus-deficit water replacement over 48–72 hours.

Worked example

A 10 kg infant under 1 year, sodium 160 mEq/L, fraction 0.7.

$$ TBW = 0.7 \times 10 = 7\ \text{L} $$ $$ Deficit = 7 \times \left(\frac{160}{140} - 1\right) = 7 \times 0.143 = 1.0\ \text{L} $$

So ~1.0 L (1000 mL) of free-water deficit, replaced on top of maintenance and ongoing losses, spread over 48–72 hours.

Safety: correction rate is the whole game

Rapid correction of chronic hypernatremia drives water into brain cells that have accumulated idiogenic osmoles, causing cerebral edema and seizures. The limits this tool enforces in its note:

• Do not lower serum Na faster than ~0.5 mEq/L per hour.
• Stay within ≤10–12 mEq/L per 24 hours.
• Correct the deficit over 48–72 hours, not acutely.
• Recheck serum Na every 2–4 hours and adjust the rate; paediatric data show a higher cerebral-edema risk at 1.0 vs 0.5 mEq/L/hr.

Pitfalls

• The deficit is not the prescription. Add maintenance fluid and replace ongoing losses separately; the formula gives free water only.
• Resuscitate shock first with isotonic fluid — that volume is excluded from this number.
• Use the right TBW fraction. Picking 0.6 for a young infant underestimates the deficit.
• New-onset seizures during correction signal cerebral edema from lowering Na too fast — slow or pause, do not push more hypotonic fluid.

Run it: Free Water Deficit (Hypernatraemia)

Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.