Hypernatremia


Normal sodium levels in the human blood are between 135-145mEq/L. Hypernatremia is characterized by sodium values over 145mEq/L.

CAUSES:

Increases in osmolality (that may result from deficit of water or increased sodium input), in normal individuals, stimulates thirst and an increase in urinary sodium (antidiuretic-hormone-ADH, also known as arginine-vasopressin-AVP, which is synthesized by the supraoptic and paraventricular nuclei of the hypothalamus, is released by the posterior pituitary and acts on the V2 receptors of the renal collection ducts). Therefore, hypernatremia is usually a consequence of one (or a combination) of the following mechanisms:

Loss of net water or low fluid intake

Excessive sweating (sweat is hypotonic compared to plasma)
GI losses (vomiting, GI tube drainage, osmotic diarrhea)
Urinary losses (diabetes insipidus, osmotic diuresis)
Incapacity to fell or react to thirst (e.g. old patients with stroke or dementia, babies)

Water shift into cells

Exercise
Seizures

Sodium gain

Salt poisoning
Iatrogenic sodium injection

SIGNS & SYMPTOMS:

Acute hypernatremia can manifest as a range of symptoms from malaise, weakness and irritability to altered mental status and coma (loss of brain water to the vascular compartment may cause demyelinating brain lesions or bleeding).

Chronic hypernatremia, on the other hand, is less likely to cause dramatic symptoms because the body adapts to it. However, it still can present with malaise, weakness, and irritability.

DIAGNOSIS:

The cause of hypernatremia is often easy to diagnose from the history and physical examination (e.g. an old immobilized and dehydrated patient with elevated sodium). If the cause is not evident, extra tests can be obtained.

Urine osmolality should be measured. High urine osmolality (>600mOsmol/kg) is usually due to extrarenal water losses. If it is not high (<600mOsmol/kg) ADH or an analogue (desmopressin) should be given and the urine osmolality and volume should be measured every 30 minutes for 2 hours. Patients with nephrogenic diabetes insipidus will not respond to ADH (the urine osmolality will not increase or will only partially increase – up to 45%). Patients with central diabetes insipidus will respond to ADH (the urine osmolality will increase 100% or more in complete central DI and between 15-50% in partial central DI).

TREATMENT:

Method A:

The first step in treating hypernatremia is estimating the water deficit.

Water deficit = Current Total Body water x { ( Serum [Na] ÷ 140 ) – 1}

Total body water (TBW) = 60% (0.6) of body weight for men, 50% (0.5) of body weight for women, 45% (0.45) of body weight for elderly.

After measuring the water deficit, a rate of correction should be chosen. Chronic (>48h) hypernatremia should be corrected slowly (maximum reduction of 10-12mEq/L/day) to avoid cerebral edema. Acute hypernatremia may be corrected quicker.

After having the water deficit measured and deciding about the rate of correction, a solution should be prepared. 5% dextrose can be used.

An important formula to calculate the change in serum Na+ with 1 liter of solution is:

Change in serum Na+ with 1 liter of the solution = (solution Na+ – serum Na+) ÷ (TBW + 1)

Now that you know how much is the total water deficit and how much 1L of your solution is going to change the sodium, all you have to do is see how much of the solution can you give in a day to not pass over the 10-12mEq/L/day reduction limit. If the total water deficit is low (e.g. 2L), you may be able to correct it all in a day. If it is high, you may need many days to correct it since you are limited to a 10-12mEq/L/day sodium reduction. If you had to put that in equations, it would look like this:

Number of liters to be given in a day  = ( Number of mEqs you want to decrease ) ÷ (Change in serum Na+ with 1 liter of the solution being used)

Infusion rate in mL/h = ( Number of milliliters to be given in a day ) ÷ 24

Expected number of treatment days = Water deficit ÷ Number of liters to be given in a day

These formulas are the most accurate (and I will call them method A).

 

Method B:

Other useful and simple formulas (which I will call method B) can be used IF 5% dextrose or free water are the fluids of choice. These formulas are:

Desired water replacement in the first day in mL  =  3 mL/kg body weight*  x  10**

(*this is the amount of fluid required to change the sodium in 1mEq, derived from the other formula; ** this is the amount of mEqs that have to be changed in the day, 10).

Hourly infusion rate (mL/hour)  =  Desired water replacement in the first day in mL  ÷  24 hours

 

Method C:

In addition to these formulas, an even quicker simplification (which I will call method C) may be used. This is derived from all the formulas mentioned and it may be used for calculation of the amount of fluids to be given in a given day:

 Give 5% dextrose at 1.35 mL/hour x patient’s weight in kg

Practical example of the methods:

A 50 year old male with 70kg and a sodium of 170 needs to receive treatment.

Let`s try method C:

Quick calculation of how much dextrose 5% should be provided: 1.35 x 70 = 94.5mL/h or 2.2L/day.

Confirmation by the other formulas:

Let`s try method B:

Desired amount of water replacement in the day in mL = 3mL x 70 x 10 = 2100mL (2.2L/day)

Notice that the values of method B and C are very similar (2.2L vs 2.1L). Remember: These volumes should be able to reduce the sodium to about ~10mEq a day. Let`s see if that`s true with method A.

Let`s try method A, the slower (but most accurate) way:

Verifying total water deficit:

TBW = 70×0.6 = 42L.

Water deficit = 42 x { ( 170  ÷  140) – 1} = 8.82L

Calculation of how much 1 liter of solution (in this case dextrose 5%) is going to reduce the sodium = (0 – 170) ÷ 42 + 1 =  3.95mEq/L.

So, 2.2L of the solution is going to reduce the sodium by 8.69mEq. This value is lower than the 10mEq used in the other formulas because the other formulas are a simplification that takes into consideration a TBW calculated based only on 50% of lean body weight (and for our male patient we used 60%). Still, it’s not a very different result and these methods can be used in clinical practice. JUST REMEMBER: METHOD B AND C WERE CALCULATED CONSIDERING A SOLUTION WITH ZERO SODIUM (D5% or free water). If you are using any other solution, use method A since it takes into consideration the Na+ concentration of the solution (see appendix).

Regardless of the method selected, serum electrolytes should be measured every 4-6 hours. If the rate of correction is proper (the sodium concentration is not falling too quick) the next measurements can be obtained every 12-24h. If the rate is improper, the infusion should be adjusted accordingly and new sodium should be obtained in 4-6h.

In addition to the sodium replacement, if there is an obvious cause (iatrogenic, diuretics, medications, diabetes insipidus) it should be addressed properly.

Appendix:

Content of sodium in different solutions:

  • 5% dextrose in water (D 5W): 0 mmol/L
  • 0.2% sodium chloride in 5% dextrose in water (D 52NS): 34 mmol/L
  • 0.45% sodium chloride in water (0.45NS): 77 mmol/L
  • Ringer’s lactate solution: 130 mmol/L
  • 0.9% sodium chloride in water (0.9NS): 154 mmol/L
  • 3% saline (hypertonic saline): 513 mmol/L

 

SOURCES & FURTHER READING:

  1. Adrogue HJ, Madias ANE. HYPERNATREMIA. N Engl J Med 2000; 342:1493-1499.
  2. Hoorn EJ. Preventing a Drop in Effective Plasma Osmolality to Minimize the Likelihood of Cerebral Edema During Treatment of Children with Diabetic Ketoacidosis. The Journal of Pediatrics. Volume 150, Issue 5, May 2007, Pages 467-473.
  3. Al-Absi A et al. A Clinical Approach to the Treatment of Chronic Hypernatremia. American Journal of Kidney Diseases. Volume 60, Issue 6, December 2012, Pages 1032-1038.
  4. Lukitsch I. Hypernatremia Treatment & Management. Medscape. Updated: Aug 24, 2016.
  5. Braun MM et al. Diagnosis and Management of Sodium Disorders: Hyponatremia and Hypernatremia. Am Fam Physician. 2015 Mar 1;91(5):299-307.
  6. Lindner G, Funk GC. Hypernatremia in critically ill patients. Journal of Critical Care (2013) 28, 216.e11–216.e20.