B. Relative Intravascular Volume Depletion
This state is also known as “decreased effective arterial volume” or sometimes “hypervolemic hyponatremia”. In simple terms, the patient behaves as if there were intravascular volume depletion, but appears as having excess of overall fluid in the body. The typical examples of this are cirrhosis of the liver, nephrotic syndrome (loss of large amounts of protein through the urine) and decompensated heart failure states. One characteristic that these examples have in common is edema, which represents an increase in total body sodium and total body water. Additionally, these states have a reduced intravascular volume level. The patient with cirrhosis does not generate enough albumin to create the necessary oncotic pressure to retain plasma in the intravascular space. The patient with nephrotic syndrome, by losing protein in the urine, has the same problem. The patient with heart failure has a problem with the heart generating the necessary blood flow for the body sensors to detect a normal blood volume. The end result of all of these processes is similar to what we saw in (A). ADH is secreted leading to a decrease in water excretion and the mechanism of thirst is activated leading to more drinking of water.
Marco A. Ramos MD