The question: I am going through the real well made sodium and water guide you made and there is a concept I am having trouble with regarding ADH stimulation by decreased BP. In the text, one of the reasons that ADH release is triggered is:
Decrease in blood pressure: ADH is a potent vasoconstrictor. 10% drops in blood pressure are required to release ADH. This drop in pressure is usually due to a drop in the blood volume from volume loss but can also be due to heart failure or cirrhosis. ADH is relatively insensitive to changes in pressure, this is why it takes a full 10% drop in pressure to start to stimulate ADH.
I thought blood pressure goes up with heart failure as hypertension is a risk factor for heart failure. How do we get decreased blood pressure from heart failure?
The Answer: Hypertension can cause heart failure. Hypertension is a risk factor for hypertension. Heart failure can present with hypotension or hypertension. Regardless of the blood pressure, heart failure can cause poor perfusion, and stimulate ADH release, with or without a low blood pressure. Sometimes the heart pumps so poorly that blood backs up in the venous circulation, this venous congestion slows perfusion since the back pressure blocks flow, even though blood pressure it good.
So the important point is that heart failure, cirrhosis and volume depletion all stimulate ADH release through decreased perfusion/blood pressure predisposing to hyponatremia.