Hypernatremia

This was another common reason for consultation on my Nephrology rotation. Hypernatremia is defined as the serum Na > 145. Before I would see the patient, I would read up on their history and hospital/ED course and ask myself what type of fluid loss or fluid gain most likely occurred? Was it pure free water or did the patient lose sodium and potassium with it? Is the pt receiving TPN? Hypernatremia causes can be divided into 3 main categories and the treatment changes depending on the cause and type of fluid lost. 

1. Water Loss into the Cells

This is the least common cause. Water can momentarily move intracellularily causing a relative extracellular hypernatremia. This can occur after severe exercise of post electroshock induced seizures. 

2. Free Water Loss

This is the most common cause. You can lose free water through the skin, GI tract or urine.

Skin-when you sweat, you lose free water because the sweat glands reabsorb the sodium. 

GI-vomiting and gastric suction removes hypoosmolar fluid. Fluid replacement with gastric suction was a common mistake that I would see on the wards. A pt who had a bowel obstruction would have an NG tube placed. At the same time, the patient was NPO so they would be on IV fluids. The IV fluids would be NS and as a result, the patient would start to trend towards hypernatremia because the fluid that was being removed from the NG tube was hypoosmolar. Diarrhea can cause hypernatremia if it is secondary only to osmotic causes and not secretory causes where you are losing both free water and solutes (VIPOMA and cholera). Osmotic causes of diarrhea include lactulose, charcoal and bacterial. 

Urine-free water loss can occur in the urine if it is secondary to osmotic causes. This can occur in DKA when there is so much glucose being spilled into the urine that osmosis will cause free water loss into the urine to dilute it (This was the most often reason for hypernatremia on my rotation). Other causes of osmotic diuresis would be mannitol administration or if the patient had a prolonged period of azotemia that had resolved and the kidneys were filtering out the excess urea. 

3. Sodium Overload

This could occur several different ways. One cause is iatrogenic causes. Imagine the situations in the free water loss category. Iatrogenic sodium overload could occur when there was free water loss but the patient received NS as fluid replacement. Salt poisoning is another cause and can be manifest in the form of child abuse. FENA will show a Na excretion of > 2% because the kidneys are trying to rid the body of excess Na. A third cause of sodium overload is if the pt is receiving TPN and tube feeds which are hypertonic in solutes. 

Treatment

There are four steps to determining the best treatment. If free water was lost, you will want to give D5W because 2/3 of this fluid will move into the intracellular space which is where you want it to go. If the patient has hypernatremia due to free water loss but has a little bit of vascular depletion as well, then you will want to add a bit of NS to the bag (NS will go to the vascular space and is used for volume repletion). A few of our patients were in this situation so, we gave them D5W1/4 NS. 

First, determine the free water deficit with the following equation

TBW in kg x 0.5(women) or 0.6 (men) x (serum Na/140-1)

Second, determine the appropriate rate of sodium correction. For acute hypernatremia (occurs < 48 hours) the serum Na should be lowered to normal levels within 24 hours. You can use D5W 3-6 mL/kg/hr and monitor the Na level every 1-2 hours until the Na is < 145. For chronic hypernatremia, you want to replace about 10 mEq/24 hours. Give D5W 1.35 mL/hour/kg. 

Third, you design a fluid replacement regimen. 

For acute hypernatremia, you want to replace the entire fluid deficit within 24 hours. 

For chronic hypernatremia, you want to replace the half of the fluid deficit within 24 hours

Fourth, calculate the ongoing fluid loss that occurs every day and add that to the fluid amount. The equation is 

Urine volume output/hour x ((UNa + UK)/serum Na)