A 60 kg man with serum Na 115 mEq/L (severe hyponatraemia from psychogenic polydipsia) requires correction. Calculating Na deficit: Target Na = 125 mEq/L. Using the Na deficit formula:

• A Na deficit = 360 mEq; correct with 3% saline over 24 h
• B Na deficit = 240 mEq; correct slowly at 0.5 mEq/L/h ✓
• C Na deficit = 180 mEq; give as bolus immediately
• D Na deficit = 600 mEq; not safe to correct

Explanation

Na deficit = TBW × (desired Na − current Na). TBW in a 60 kg man = 0.6 × 60 = 36 L. Na deficit = 36 × (125 − 115) = 36 × 10 = 360 mEq. However, to avoid osmotic demyelination syndrome (ODS), Na should be raised no faster than 8–10 mEq/L in 24 hours (0.5 mEq/L/h) in chronic hyponatraemia (>48 hours). The first step is to target correction to 125 mEq/L initially, not full normal. Option B captures the correct strategy; the deficit to target 125 from 115 is 360 mEq but slow correction rate is mandated.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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