Hyponatremia Part 1: Salt versus free water

Hyponatremia is difficult to understand. So I’ve decided to devote two whole posts to unravelling this enigma. The first part deals with normal salt physiology and the often forgotten difference between salty fluids and free water, while part 2 will deal more with the management.

It is important to distinguish between salty water and free water. Salty water refers to your resuscitation fluids like normal saline, Hartmann’s and plasmalyte, which contain 140-150mmol/L of Na. Because they are isotonic with body fluids, these fluids will be confined to the EXTRACELLULAR space, which you might recall is about 15L in a 70kg male. Turns out having fluid that stays in the extracellular space is a good thing if your goal is to expand the blood volume.

In contrast free water is solute free fluid which is hypotonic. Therefore it will distribute itself among TOTAL BODY WATER, which again you might recall is about 40L in a 70kg male. Now, we don’t often give pure water intravenously, but we do give fluids like 5% or 10% dextrose. This essentially functions as free water because the glucose will be taken up by cells leaving the free water component. From the point of view of the blood compartment, this free water is a muddy pig- it slips its way out of your grasp and disappears into the vast expanse of total body water, which is why it is wholly useless for resuscitating someone. The commonly used fluid dex-saline (which is 4% dextrose plus 0.17% saline) contains some salt but one can kind of think of it as free water also.

Herein the questions arises of what kind of fluid do we lose everyday? Most people will remember that the daily requirement for fluid is about 2-3L per day. But what is the composition of this fluid? Does it matter?

confusion

Of course it does. A human’s daily requirement for Na is about 70mmol. Read that again. That’s not 70mmol/L. That’s 70mmol TOTAL. So you are losing far more water than you are sodium (relative to normal body concentrations). This is from sweat, respiratory losses, the GI tract and the kidneys. Even though the kidneys can produce highly concentrated urine, they still lose more water than they do sodium. Another factoid you might recall is that the normal urine sodium concentration is 20mmol/L. Even if you are being hammered with a diuretic and losing lots of sodium in the urine, the urine sodium might get up to 80-90mmol/L, but this still falls far short of normal plasma sodium concentrations.

Given the above information it reasons that the function of the 2L of maintenance fluid we chart patients who are nil by mouth should be to maintain plasma osmolarity (given that salt is the main determinant of this). Therefore one should prescribe a maintenance fluid that achieves this, which is the reason that dextrose saline is favoured. Going without maintenance fluid will steadily raise your plasma osmolarity/sodium.

The purpose of 2L of maintenance fluid is not really to maintain blood volume (assuming your patient is euvolemic to begin with). Of course it fulfils this purpose to a degree but the more important determinant of blood volume is the solute load. A sodium load activates vasopressin which allows us to reabsorb water in the kidneys. Water follows sodium in the kidneys and this “sodium attached water” stays in the extracellular space, allowing maintenance of blood volume. As mentioned however the daily sodium load required to achieve this is only 70mmol/day (pretty much what you get with two bags of dex-saline).

The last two paragraphs may be confusing, so let me recapitulate. To maintain blood volume your kidneys must reclaim a certain amount of “water attached to sodium” or “salty water”, and this reclamation is dependent on an adequate sodium load and adequate water intake. However most of the fluid lost by your kidneys is actually “sodium free water” which must be replaced orally or intravenously to maintain serum osmolarity. Of the 2-3L a day of “fluid” one loses (not just from the kidneys), most of it is in the form of “sodium free water”, while “salty water” is a smaller component.

So what happens if I give a euvolemic patient normal saline as maintenance? In order to cope with the solute load (one bag already has twice the daily sodium requirement) and maintain a normal serum sodium concentration the kidneys will activate vasopressin and hold onto more water. The amount of “salty water” will increase, while the amount of “sodium free water” the kidneys can excrete is reduced. The serum osmolarity will be maintained, but at the expense of an expanded extracellular fluid volume. This may not be so much of an issue in healthy patients but you can bet it will significantly increase tissue oedema in those with heart failure, or with inflamed leaky capillaries. People with good kidney function will be able to increase the sodium loss in their kidneys, but remember even when they are losing heaps of sodium in the urine they are still losing more water than they are sodium. The effect will be worse in those with impaired kidney function who are less able to increase urinary sodium losses.

Of course such a maintenance regime may be appropriate in those with ongoing sodium losses. Those with ongoing sodium losses will have trouble maintaining their extracellular fluid volume (and therefore their blood volume) so ongoing sodium replacement will be appropriate. These patients may have vomiting, diarrheoa, losses from stomas or fistulae or renal losses from diuretics or acute tubular necrosis. Which brings us to managing hyponatremia in Part 2…

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