Hyponatremia in Critical Care

by Patrick Neligan (c) 2002

What is it?

Extracellular fluid osmolality is determined, in general, by the serum sodium concentration. A serum sodium of <135mEq represents a hypotonic state. There is a net excess of water in relation to sodium. This may be caused by excessive administration of hypotonic fluids, the inability to excrete free water, or the presence of other osmotically active molecules in the extracellular space.
Hyponatremia is common in ICU an we divide it up into: mild hyponatremia - Na 135-130mEq/L, moderate 129-125mEq/L, severe 124-120mEq/L and life threatening: Na <119mEq/L.

Why is it important in ICU?

Hyponatremia is important for four reasons:

1. If the blood is hypo-osmolar in relation to the brain, water enters the brain and can cause acute cerebral edema, particularly in patients who are euvolemic (such as patients who develop "TURP" syndrome - caused by intravascation of hypotonic fluid during transuretheral resection of the prostate gland). This may lead to a spectrum of neurologic upsets ranging from confusion, to seizures, to coma to brain stem herniation.

2. Patients with chronic hyponatremia tolerate this problem remarkably well, and urgent correction may not be indicated, indeed it may be harmful.

3. Rapid correction of low sodium can lead to osmotic demyelination of the brain / brainstem, due to rapid shrinkage of the brain.

4. Pseudohyponatremia, caused by the presence of alternative osmoles (in large concentrations), can be as harmful as hypo-osmolar hyponatremia.

What is Pseudohyponatremia?

Serum osmolality is governed by contributions from all molecules in the body that cannot easily move between the intracellular and extracellular space. Sodium is the most abundant electrolyte, but glucose, urea, plasma proteins and lipids are also important. A patient with diabetic ketoacidosis may have hyponatremia, but normal osmolality, due to hyperglycemia, hypertriglyceridemia and ketonemia. Patients with acute renal failure may have hyponatremia due to uremia.
If a patient has hyponatremia, with low measured and calculated serum osmolality, we call this hypotonic hypernatremia. If serum osmolality is normal or high, this is isotonic or hypotonic hyponatremia -pseudohyponatremia.

Serum osmolality is calculated from:

2(Na + K) + BUN/2.8   + Glucose/16

or in SI units (mmol/l)
2(Na + K) + Urea + Glucose

Classically, we divide up pseudohyponatremia into conditions in which the measured and calculated serum osmolalities are the same - hyperglycemia or uremia, and those in which there is an osmolar gap: some osmoles are clearly there (measured), but we have not recognised them. The source of unmeasured osmoles may be endogenous (lipids or proteins), or exogenous - alcohols (ethanol, ethylene glycol, methanol or isopropyl alcohol).

What is the Syndrome of Inappropriate ADH secretion?

A number of diseases / drugs cause either ectopic production of ADH (arginine vasopressin) or cause release of this compound from the posterior pituitary gland. The result is a paradoxically concentrated urine with dilute blood (the urinary osmolality is higher (>300mOsm) than the serum osmolality (<300mOsm).
This problem is easily confused with salt wasting syndrome (CSW), which is characterized by hypovolemic hyponatremia: SIADH improves with fluid restriction, CWS does not.

CLICK HERE FOR A LIST OF CAUSES OF SIADH

Algorithm for Assessment of Hyponatremia

flash plug-in required

How do you manage Hyponatremia?

If Na >125, the treatment is water restriction, 500ml-1000ml per day

If Na < 125, or water restriction is not possible, furosemide 40mg-80mg iv repeated as necessary, with replacement of electrolyte losses.

If this strategy is unsuccessful at raising serum sodium, treatment with hypertonic saline may be necessary: NaCl 0.9% contains 1mEq of Na in 6.5ml (i.e. 0.154 mmol/ml), NaCL 1.8%, contains approx 1 mEq Na per 3.25ml, 3% NaCl containe approx 1mEq Na per 2ml.

If the cause is SIADH, and the patient does not respond to fluid restriction, then loop diuretics may be helpful. An alternative treatment is to cause a nephrogenic diabetes insipidis, by administering demeclocycline 300-600 mg BID.

Repleting the Sodium Deficit

4 Steps:

1. Find the patient's normal body weight

2. Calculate the serum sodium deficit, divide by half to find how much you will replace: from this calculate the total body sodium deficit (serum deficit x total body water)

3. Calculate the rate of replacement (replace the serum deficit at 0.5mEq/hour).

4. Calculate how much of your chosen fluid is required.

If you are going to use hypertonic saline, you must calculate the sodium deficit: it is conventional only to correct half of the deficit. The normal serum sodium is 140 mEq/L.

Step 1: find out the patient’s weight is kilograms prior to illness.

Step 2: Calculate the Sodium Deficit

It is usual to correct only half the sodium deficit (NaD): (hence the deficit/2)

NaD = (Desired Sodium - Patients Sodium /2)

If the patient’s weight is 70kg, and the serum sodium is 120, then the desired change is 10 mEq/L

Total body deficit of sodium is the sodium deficit x total body water (TBW)

NaD x (weight in Kg x 0.6) = Total deficit (TD)

Using the formula: 10 x (70 x 0.6) = 420 mEq.

Step 3: calculate the rate of replacement

Most physicians replace the deficit at no more than 0.5 mEq per hour. The patient has a deficit of 10 mEq, so at this rate, it will be replaced over 20 hours (10/0.5). 

Rate of Replacement (RoR) in hours = NaD/ 0.5

Step 4: replace the sodium deficit with the fluid of your choice

The amount of fluid required depends on the sodium content of that fluid:

So,

TD / [Na Fluid/ml] / RoR = per hour fluid replacement

If we are using 3% saline in this 70kg male patient with a serum sodium of 120:

(420/0.513) / 20 = 41ml/hour

That is, after 20 hours, assuming no other fluids are given, the patient's serum sodium will rise to 130mEq/L. If 0.9% saline is given:

(420/0.13) / 20 = 160 ml/hour

Please note: these tutorials are for personal study purposes only.  They are not currently peer reviewed, and no responsibility will be taken for mistakes or inaccuracies. Reproduction of information is forbidden. All material is copyrighted by the GasWorks Group.