Especially in hospitalized or institutionalized patients (who may not have the ability to drink when they are thirsty), we often encounter people who are so dehydrated that the salts in their blood have become extremely concentrated. The key salt to consider in such a situation is sodium.
When sodium in the bloodstream has become too concentrated (hypernatremia), that in itself is a bad thing and can lead to quite profound depression of consciousness.
That said, if someone is identified as having a very high sodium concentration, it is usually not a good idea to immediately provide sufficient water to dilute their sodium back down to normal levels. If you do, there is a real risk of causing brain swelling (cerebral edema). I imagine the risk would exist whether you give the water by mouth or by IV.
The reason that suddenly restoring someone’s blood sodium concentration back to normal by providing them with the water they lack can cause brain swelling has to do with the role that sodium concentration plays in pulling water in and out of body compartments.
For example, when blood sodium levels rise, the result (by osmosis) is to pull water out of the body’s cells (and into the blood). In the case of the brain, the same thing occurs, with high sodium concentration in blood tending to pull water out of the brain cells. That would lead to brain (and brain cell) shrinkage which is something your body guards against. It does so by generating intracellular osmoles in the brain which serve to balance the osmotic pull of the high sodium concentration in the blood, i.e. those extra brain cell osmoles allow the brain cells to retain their water in the face of high sodium, and thus high osmolality, in the blood.
If, at a time when the brain cells have generated such extra osmoles you intervene to lower the blood sodium concentration by providing water to the patient, you will, of course, lower the osmolality in the blood. But, because of those intracellular osmoles in the brain cells, a gradient is thereby created where water will flow from the blood and into the brain cells. Brain swelling can occur with fatal consequences.
What you must do is only slowly give back the ‘missing’ water and thus only gradually lower the blood sodium concentration and osmolality. That gives time for the brain cells to dissipate any osmoles they generated in the face of the high sodium in the blood (which would have pulled water from the brain cells if they hadn’t been produced).
Again, the key is to do it slowly. Empirically, restoring an elevated blood sodium concentration to normal by the administration of water over 48 to 72 hours provides sufficient time for the brain cells to get rid of their extra osmoles, and prevents brain swelling.
I see no reason why the above would not apply pretty much equally for water given IV or by mouth and have to admit that I, too, have wondered whether people who had been dying of thirst (i.e. whose blood sodium was too concentrated) would be at risk if able to suddenly take in all their ‘water deficit’ by mouth. I know they are (as above) if they get the water by IV.
(NB: the above ‘explanation’ takes some liberties by equating blood with the extracellular space which is not strictly the case, but the general idea is not affected by doing so)
One of the funniest things I’ve read on the boards.
My neighbor the nurse was my favorite person that day, when she showed up with an enema bag and a huge bottle of saline and told me where to shove it!