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
Why is it important in ICU?
Hyponatremia is important for four reasons:
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.
Serum osmolality is calculated from:
2(Na + K) + BUN/2.8 + Glucose/16
or in SI units (mmol/l)
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).
Algorithm for Assessment of Hyponatremia
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
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 patientís 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 patientís 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:
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
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