Question 1

How do you calculate free water deficit in hypernatremia?

Accepted Answer

`Deficit (L) = TBW × ((current Na / target Na) − 1)`

Total body water (TBW) is estimated as roughly **0.6 × weight (kg)** for young men, **0.5 × weight** for young women or elderly men, and **0.45 × weight** for elderly women.

Question 2

How fast can you safely correct hypernatremia?

Accepted Answer

The general limits are **≤0.5 mEq/L/hr** and **≤10 mEq/L per 24 hours**. Faster correction in chronic hypernatremia (>48 hours) risks cerebral edema as brain cells release accumulated idiogenic osmoles. Acute hypernatremia can be corrected somewhat faster, but onset is rarely known precisely.

Question 3

Why is D5W used to correct hypernatremia?

Accepted Answer

D5W effectively delivers free water once the dextrose is metabolized — it's the standard IV vehicle for free water replacement. Enteral water via NG/OG tube is an equally valid (and often more practical) alternative, especially in patients tolerating tube feeds.

Question 4

Does the free water deficit formula account for ongoing losses?

Accepted Answer

No. The deficit calculation is a **closed-system estimate** — it doesn't include insensible, GI, or ongoing renal losses. In practice, patients usually need more than the calculated deficit. Sodium should be rechecked every 4–6 hours during active correction and the rate adjusted accordingly.

Free Water Deficit

References

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