Choosing a saline in SIADH follows a different rule from the one used in every other condition. To see why, start with the usual rule.
Why is saline usually judged against plasma?
When the osmolality of the extracellular fluid is abnormal, a counter-osmolar saline is usually given, and it is chosen with respect to plasma. The reason is that the sodium of the infused saline keeps the fluid in the extracellular compartment. This plasma-referenced rule holds in almost every condition.
What changes in SIADH?
Plasma is the usual reference, but in SIADH the kidney does not behave the same way. SIADH is the most common cause of euvolemic hyponatremia, and it is the one condition in which the choice of saline follows a different rule.
The kidney’s fault in SIADH is with water, not with sodium. Sodium excretion is intact, so the kidney still excretes the infused sodium. The sodium leaves in the urine while the water is kept. That is why the plasma rule no longer decides the outcome, and in SIADH the saline must be judged against the urine, not the plasma.
Where does the water go in SIADH?
Because the kidney keeps the water, the urine is more concentrated than the plasma. When ADH is high the kidney keeps water, so the urine is concentrated while the plasma is dilute. Excessive ADH secretion therefore produces both the hyponatremia and the concentrated urine. In SIADH the urine osmolality is almost always higher than the plasma osmolality.
A saline is hyperosmolar to a fluid when its osmolality is higher than that fluid’s, and hypo-osmolar to it when its osmolality is lower. The comparison that matters now is with the urine.
What does a saline more concentrated than plasma do?
So the water balance is set by the urine, and the plasma comparison is misleading here. Because the infused sodium leaves in the urine, what decides whether the saline adds water or removes it is the urine’s concentration. A saline less concentrated than the urine adds water. A saline more concentrated than the urine removes it.
An example shows this. A patient with SIADH has a plasma osmolality of 120 and a urine osmolality of 300. One liter of saline with an osmolality of 200 is infused. Sodium excretion is intact, so all the sodium in the saline is excreted into the urine. The sodium leaves at the urine osmolality, not at the saline osmolality. The volume that carries the sodium is therefore set by the urine, not by the saline. At a urine osmolality of 300, about 670 ml of the saline is excreted with all its sodium.
What happens to the rest? The remaining 333 ml of water is not excreted. It stays in the plasma, where it dilutes the sodium that is there, and so it worsens the hyponatremia. If isotonic saline is administered, the water will be retained and sodium will be excreted in the urine, leading to possible worsening of hyponatremia.
This saline is hyperosmolar to plasma, since 200 is above 120. Under the plasma rule it would be expected to help. Yet it worsens the condition, because it is hypo-osmolar to the urine: the urine is more concentrated still, so the sodium leaves and the water stays.
What if the saline is more concentrated than the urine?
The same arithmetic run in the other direction shows the intended effect of the saline. Now 1000 ml of saline with an osmolality of 600 is infused. This saline is hyperosmolar to the urine as well as to the plasma. Sodium excretion is still intact, so all the sodium is excreted.
The sodium leaves in a fluid whose osmolality has to be brought down to the urine osmolality of 300. To bring 600 down to 300, the excreted volume must double. The 1000 ml of infused saline is excreted, and a further 1000 ml of body water is drawn in with it to dilute the excreted fluid from 600 to 300.
The body has therefore given up water that had been diluting its sodium, and the extracellular fluid is reduced. This is the intended effect. Plasma sodium concentration depends on how much exchangeable sodium and potassium there is and how much total body water holds them, as the sodium regulation note showed.
That is the conclusion of both examples: in patients with SIADH, the saline to be chosen should be hyperosmolar with respect to the urine, not the plasma. So what must be known about the urine before the bag is hung?