Formula: Free Water Deficit (L) = TBW × ((Serum Na / 140) − 1), where TBW = body weight × factor (0.6 for young males, 0.5 for young females, 0.45 for elderly). This tells you how much free water the patient has lost. However, it does not account for ongoing losses (insensible losses, urine, drains). You must add 30 to 50% to the calculated deficit to account for ongoing losses, and recalculate every 12 to 24 hours.
In chronic hypernatraemia, brain cells adapt by generating intracellular osmolytes (idiogenic osmoles) to prevent cellular shrinkage. If the extracellular Na is lowered too rapidly, water rushes into the adapted neurons, causing cerebral oedema (swelling, herniation, and death). This is the mirror image of ODS in hyponatraemia. The safe correction rate for chronic hypernatraemia is 10 to 12 mEq/L per 24 hours. For acute hypernatraemia (< 48 hours), correction can be faster (up to 1 mEq/L/hour) because idiogenic osmoles have not yet accumulated.
| Feature | Central DI | Nephrogenic DI |
|---|---|---|
| Mechanism | Deficient ADH production | Renal resistance to ADH |
| Common causes | Post-neurosurgery, head trauma, pituitary tumour | Lithium, hypercalcaemia, CKD, sickle cell |
| Urine osmolality | < 300 mOsm/kg (dilute) | < 300 mOsm/kg (dilute) |
| Response to DDAVP | Urine concentrates (> 50% increase in urine osm) | No response |
| Treatment | DDAVP (Desmopressin) 1-2 mcg IV/SC q12h or nasal spray | Treat cause, thiazide diuretics (paradoxically reduce polyuria), low-salt diet, NSAIDs (indomethacin) |
| Fluid | Free Water Content | When to Use |
|---|---|---|
| D5W (5% Dextrose) | 100% free water | Pure free water deficit. IV route when oral not feasible. |
| 0.45% NaCl (Half-NS) | 50% free water | Combined volume + free water deficit. Safer than D5W for large volumes (avoids hyperglycaemia). |
| 0.225% NaCl (Quarter-NS) | 75% free water | Balanced approach. Common choice for paediatric use. |
| Plain water (oral/NG) | 100% free water | Preferred route if patient can drink or has NG tube. Safest and most physiological. |
AMA:
Umakanth S. Acute Hypernatraemia Protocol. MEDiscuss. Published 2026. Accessed .
Vancouver:
Umakanth S. Acute Hypernatraemia Protocol [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:
