Acute Hypernatraemia Protocol
The water deficit, and a replacement rate that will not risk cerebral oedema · v1.1- Enter the sodium, the weight and sex, the chronicity, the neurological symptoms and the urine output pattern.
- You get the free water deficit, a safe correction rate, and a protocol with fluid selection and a monitoring plan.
- Add to the deficit it prints for whatever the patient is still losing. The formula assumes a closed system.
- It replaces the water, not the diagnosis. In diabetes insipidus that diagnosis is the treatment.
- Ask at the bedside whether the patient can reach a glass. Thirst is one of the symptom options here; free access to water is not, and it is often the whole problem.
- Neonates, infants and children. The total body water fraction and the correction limits are not the ones used here.
- The cause of the water loss. It calculates the deficit and the rate, and leaves the diagnosis to you.
- Diabetes insipidus beyond naming it and pointing at DDAVP.
- Ongoing losses. The deficit formula assumes a closed system, so the figure it prints is a floor rather than a total.
- The hyperglycaemia, the mannitol or the feed driving an osmotic diuresis.
1. Free Water Deficit: The Core Calculation
Formula: Free Water Deficit (L) = TBW × ((Serum Na / 140) − 1), where TBW = body weight × factor (0.6 for young men, 0.5 for young women and for elderly men, 0.45 for elderly women). That figure is what the patient has already lost. It says nothing about what is still going out through urine, drains and skin, so add 30 to 50% to the calculated deficit and recalculate every 12 to 24 hours. A deficit worked out once, in a polyuric patient, is out of date by the time the bag is up.
2. Why Rapid Correction Is Dangerous (Cerebral Oedema)
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.
The evidence behind the 0.5 mmol/L/hour limit is thinner than its standing suggests. Chauhan and colleagues, 2019, studied 449 critically ill adults and found no excess harm from correction faster than 0.5 mmol/L per hour: no rise in mortality, seizures or altered consciousness, and on chart review not one cerebral oedema attributable to the rate. That is a retrospective cohort, not a trial. This tool still holds to the slower limit in chronic hypernatraemia. The limit rests on physiology and case reports rather than on outcome data. A correction that has run slightly fast in an ICU patient is not, on its own, a reason to alarm the team.
3. Diabetes Insipidus: Central vs Nephrogenic
| 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) |
4. Fluid Selection for Free Water Replacement
| 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. |
Abbreviations
ADH (Antidiuretic Hormone) · CKD (Chronic Kidney Disease) · D5W (5% Dextrose in Water) · DDAVP (Desmopressin) · DI (Diabetes Insipidus) · DKA (Diabetic Ketoacidosis) · FWD (Free Water Deficit) · HCTZ (Hydrochlorothiazide) · HHS (Hyperosmolar Hyperglycaemic State) · I/O (Intake/Output) · IV (Intravenous) · Na (Sodium) · NaCl (Sodium Chloride) · NaHCO3 (Sodium Bicarbonate) · NG (Nasogastric) · NS (Normal Saline) · NSAID (Non-Steroidal Anti-Inflammatory Drug) · ODS (Osmotic Demyelination Syndrome) · Osm (Osmolality) · SC (Subcutaneous) · TBW (Total Body Water) · TDS (Three Times Daily) · TPN (Total Parenteral Nutrition)References
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.
- Sterns RH. Disorders of plasma sodium. N Engl J Med. 2015;372(1):55-65.
- Muhsin SA, Mount DB. Diagnosis and treatment of hypernatremia. Best Pract Res Clin Endocrinol Metab. 2016;30(2):189-203.
- Chauhan K, Pattharanitima P, Gnanasekaran I, et al. Rate of Correction of Hypernatremia and Health Outcomes in Critically Ill Patients. Clin J Am Soc Nephrol. 2019;14(5):656-663.
How to Cite This Tool
DOIhttps://doi.org/10.5281/zenodo.22401528
AMA Style:Umakanth S. Acute Hypernatraemia Protocol. Version 1.1. MEDiscuss Clinical Decision Support System. Published 2026. Accessed . https://mediscuss.org/cdss/acute-hypernatraemia. doi:10.5281/zenodo.22401528
Vancouver Style:Umakanth S. Acute Hypernatraemia Protocol [Internet]. Version 1.1. MEDiscuss.org; 2026 [cited ]. Available from: https://mediscuss.org/cdss/acute-hypernatraemia. doi:10.5281/zenodo.22401528
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