Why should people with congestive heart failure drink less fluid?

[wywong]

People with congestive heart failure (CHF) are advised by doctors to restrict fluid intake. For example, while 8 cups of fluid per day is recommended for healthy individuals, people with CHF are recommended not to exceed 4 cups a day. The explanation, according to a medical student friend, is so that the blood volume is reduced, and therefore the heart doesn't have to work that hard. I am baffled by such an explanation. According to physics, the mechanical power output of the heart is given by the volume of blood passing through the heart per second times the pressure difference between the inlets and outlets of the heart. If I understand correctly, a reduced blood volume affects neither the hemoglobin concentration nor the blood pressure due to homeostasis, so a reduced blood volume shouldn't alter the heart's workload. Am I mistaken or is my friend's explanation wrong?

 

[B0b-A]

Reduced fluid intake in CHF is an attempt to reduce edema , particularly pulmonary edema ...

Congestive Heart Failure ... the total amount of fluid consumed must be regulated. Although many people with congestive heart failure take diuretics to aid in the elimination of excess fluid, the action of these medications can be overwhelmed by an excess intake of water and other fluids. The maxim that "drinking eight glasses of water a day is healthy" certainly does not apply to patients with congestive heart failure. In fact patients with more advanced cases of congestive heart failure are often advised to limit their total daily fluid intake from all sources to 2 quarts ...

http://providence.net/medical-services/internal.php?page=heart-failure

 

[wywong]

Thanks very much. But my friend's textbook and other internet sources do list workload of the heart as one of the reasons. In the end we agree that to excrete the excess water, the kidney needs to derive more power from the blood, which causes the heart to work harder.