Hypernatremia occurs due to net water loss or excess sodium intake and it is more common in infants or elderly population with neurological or physical impairment. The Free Water Deficit is calculated in hypernatremia and determines the volume (L) of water required to correct dehydration or, to reach the desired level of sodium in the blood serum, during the initial stages of fluid-replacement therapy. The goal of therapy is to correct both the serum sodium and the intravascular volume. In patients with severe dehydration or shock, the initial step is fluid resuscitation with isotonic fluids before free water correction. It is important to remember that hypernatremia should be corrected over 48 hours with a decrease in serum sodium not exceeding 0.5 mEq per hour. Patients should be carefully monitored for the rate of correction, urine output, and ongoing losses. Rapid correction can lead to cerebral edema and seizures. It is crucial to identify acute versus chronic onset hypernatremia before correcting the free water deficit.

The Free Water Deficit in Hypernatremia is calculated by estimated total body water fraction, patient current sodium values and patient age sex range by using below equation.

— Free Water Deficit = %Total Body Water, fraction x [(plasma sodium/140) -1]

— %Total Body Water = Age / Sex coordinate x body weight(kg)


Where Age / Sex coordinate is for

— Child: 0.6

— Adult Male: 0.6

— Adult Female: 0.5

— Elderly Male: 0.5

— Elderly Female: 0.45

References

Sonani B, Naganathan S, Al-Dhahir MA, et al. Hypernatremia. 2023 May 20. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. PMID: 28722989.

Barma MA, Soiza RL, Donnan PT, et al. Serum sodium level variability as a prognosticator in older adults. Scand J Clin Lab Invest. 2018 Nov-Dec;78(7-8):632-638. doi: 10.1080/00365513.2018.1543893. PMID: 30755097.

Rubin AN, Espiridion ED, Kattan M, Desmarais EC et al. Serotonin Syndrome with Atypical Hypernatremia. Cureus. 2018 Nov 19;10(11):e3616. doi: 10.7759/cureus.3616. PMID: 30680272; PMCID: PMC6340410.

Contributed by

J Villanueva MD

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