Synopsis
Restore circulation and free water safely, identify impaired access or renal water loss, and prevent dangerous over-rapid tonicity change.
- Hypernatraemia usually means water deficit relative to body sodium, caused by inadequate access or thirst, extrarenal water loss, osmotic diuresis or arginine vasopressin deficiency or resistance.
- Older, cognitively impaired, intubated, postoperative and care-dependent adults are vulnerable because they cannot obtain water even when renal physiology and thirst would otherwise protect them.
- Assess circulation before calculating free water: shock requires isotonic resuscitation first. In decompensated AVP deficiency with persistent excessive dilute urine, Society guidance advises desmopressin 1–2 micrograms IV or IM and serum sodium every 4 hours during resuscitation.
Key red flags
Lethargy, irritability, confusion, hyperreflexia, weakness, seizure or coma suggests clinically important hypertonicity and requires urgent neurological and biochemical assessment.
Investigation priorities
Confirm hypertonicity and quantify the actual response to fluid and desmopressin treatment.
Management branches
The hypernatraemic adult has shock, hypotension or clinically important hypovolaemia.
- Use ABCDE, obtain urgent serum and urine studies, secure accurate input and output measurement and identify ongoing gastrointestinal, renal, skin or iatrogenic losses.
- Give isotonic crystalloid resuscitation according to NICE CG174 and repeated haemodynamic assessment despite the raised sodium, because organ perfusion takes immediate priority.
Haemodynamics are stable but hypertonicity and a water deficit remain.
Key medicines
Desmopressin for central AVP deficiencyContinue the person's verified stable formulation and dose when safe. In decompensated AVP deficiency, restore fluid first; if excessive dilute urine persists, give desmopressin 1–2 micrograms IV or IM, with serum sodium every 4 hours during resuscitation and response review before any repeat dose.
Enteral water or intravenous glucose 5%Prescribe the calculated but frequently revised free-water amount over the next monitored interval, preferring oral or nasogastric delivery when safe and using glucose 5% intravenously only within local fluid governance.