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Hypernatremia And Free Water Deficit

Sodium (Na) fluctuations portend increased in-hospital death for ICU patients, and when working them up, it’s essential to determine if these disorders are acute or chronic (> 48 hours or unknown duration). For hypernatremia ([Na] > 145 mEq/L), think about situations where there is a free water deficit (FWD) – decreased thirst, limited free water intake, increased fluid loss, etc. Symptoms can span from vague weakness and lethargy to seizures, altered mentation, unconsciousness, and even death.

Remember, water follows sodium. Hypernatremia causes water to shift out of cells leading to cell shrinkage – the mechanism hypertonic saline can help with cerebral or gut edema. Water shifts through semipermeable membranes to equalize solute concentration intra and extracellularly. Our thirst mechanism is regulated by hypothalamic osmoreceptors which are, in turn, are heavily influenced by the sodium concentration. As plasma sodium concentrations increase, vasopressin (antidiuretic hormone) is also released to retain more free water at the level of the kidney. We need to determine the FWD:

FWD = (% water x TBW) x ([Na]/140 – 1)

In this equation, % water is expressed as a fraction (0.6 for adult males and children, 0.5 for adult females and elderly males, 0.45 for elderly females), TBW is total body weight in kilograms, and [Na] is the serum sodium concentration.

Hypernatremia algorithm (citation: Braun MM, Barstow CH, Pyzocha NJ. Diagnosis and management of sodium disorders: hyponatremia and hypernatremia. Am Fam Physician. 2015;91(5):299-307.)

For example, a 45 year old 75 kg adult male with a [Na] of 154 mEq/L has a FWD of 4.5 liters (assuming a goal [Na] of 140 mEq/L) with consideration for ongoing losses adding more to this deficit.

I try to correct FWD using enteral sources of water (encouraging hydration, adding free water to tube feeds, etc.) If this isn’t feasible, I’ll use hypotonic fluids like D5W% or 0.45% sodium chloride. For very acute hypernatremia, the rate of correction can be relatively quick (ie, 0.5 – 1.0 mEq/L/hour); however, in more chronic hypernatremia, the goal is not to exceed 8-10 mEq/L in a 24 hour period. I’ll check serum sodium levels every 4-6 hours initially to gauge the rate of correction, and space lab checks out accordingly.

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