A Case of Severe Hypernatremia

Question 2: What was the cause of hypernatremia?

Hypernatremia is very rarely the result of gigantic loads of exogenous Na⁺ administration. More frequently it is due to water losses:

[Na⁺] = (total body Na⁺ + total body K⁺)/total body water

The only way to increase serum sodium is to lose more water than salt (isotonic fluid losses will lead to hypovolemia, not to hypernatremia), so the cause of hypernatremia is almost always loss of electrolyte-free water (without Na⁺ or K⁺). This may come from renal, intestinal or skin (insensible) loss of water.

Question 4: If not the bowel, where was electrolyte-free water loss coming from?

In this case, all was needed was to look at urine output. Patient was producing urine at a rate of 96 ml/hr. For someone that’s dehydrated, this definitely feels abnormal.

Was this Diabetes Insipidus? Urine Osmolarity was 480 mosm/L. Definitely not DI. However, urine should have been maximally concentrated in the setting of hypernatremia. This was most likely related to still incomplete recovery of tubular function from ATN.

To determine if this urine output is generating loss of electrolyte-free water a sample of urine was sent to the lab:

U Na⁺ = 16 meq/L, U K⁺ = 24 meq/L

Such low values suggest most urine output is actually electrolyte-free water.

The formula for electrolyte-free water excretion is:

Urine Volume x {1 – (U Na⁺ + U K⁺ )/[Serum Na⁺]}

So, using this formula with a urine output of 96 ml/hr (2300 ml/day) we get:

2300 x {1 – (16 + 24)/[156]} = 1710

This means that this patient is losing 1710 ml of electrolyte-free water through the kidneys every day, or 71 ml/hr.

The treatment with oral free water that patient had been receiving the last 3 days was barely enough to replace urine losses!