Hi Dr. Topf,
I hope that you are doing well. My name is XXXX and I am an M2 at OUWB. I was just reviewing your Disorders of Sodium and Water Metabolism lecture again, and I have a quick question.
In a patient with decompensated heart failure presenting with hypervolemic hyponatremia, are loop diuretics appropriate here? While they address volume overload, is there a risk of worsening the hyponatremia? I know in the lecture you mentioned treating the underlying issue and volume restricting.
Great question.
The key to fixing the hyponatremia in these patients is fixing the heart failure.
Remember why the patient with decompensated heart failure is hyponatremic in the first place. The problem is not that they don’t have enough volume. They have too much volume. But because the heart is failing, the arterial circulation is underfilled. The kidneys interpret this low effective circulating volume as a threat and respond by retaining sodium and water. Most importantly for the hyponatremia, ADH is released despite the low serum osmolality.
So the patient retains water and becomes hyponatremic.
This creates what seems like a paradox: the patient is overloaded with fluid, but the kidney is acting like the patient is volume depleted.
The way out of this is to successfully treat the heart failure and restore effective circulation. And in a congested patient, that means getting rid of volume with loop diuretics.
But XXXX has identified an important wrinkle: loop diuretics change the composition of the urine.
Without a loop diuretic, a patient with heart failure and lots of ADH may make a small volume of very concentrated urine. A loop diuretic increases urine volume, but it also increases the amount of sodium and potassium in that urine. That makes each liter of urine less effective at getting rid of electrolyte-free water.
This is the concept of electrolyte-free water clearance.
Very roughly, what matters for the serum sodium is not simply:
How much urine did you make?
but:
How much water did you excrete relative to sodium + potassium?
So loop diuretics push things in two different directions. They increase urine volume, which tends to increase water loss and raise the serum sodium. But they also increase urinary electrolyte excretion, which means each liter of urine carries away less electrolyte-free water.
In most patients with decompensated heart failure, the net effect of successful diuresis is still favorable: they lose more water than electrolytes and the serum sodium rises.
But not always.
You absolutely can have a patient whose serum sodium falls during aggressive diuresis. This is particularly likely if the heart failure remains decompensated, ADH remains high, the patient continues drinking water, and the diuretic-induced urine is relatively rich in sodium and potassium.
But that possibility doesn’t mean you should avoid loop diuretics.
The main thrust of treatment is still to get the patient from decompensated heart failure to compensated heart failure. If they are congested, that requires volume removal.
And when you succeed, effective circulation improves. The non-osmotic stimulus for ADH goes away. ADH falls. The collecting duct becomes impermeable to water and the kidney starts dumping dilute urine.
Aquaresis.
At that point the serum sodium will correct.
So yes: use the loop diuretic.
Don’t let the hyponatremia scare you away from treating the disease that is causing the hyponatremia. Follow the serum sodium while you diurese, restrict water as appropriate, and get the heart recompensated.
Fix the heart failure → remove the stimulus for ADH → excrete the excess water → fix the hyponatremia.
