Anemia and Fluid Therapy

The idea has been circulated that rehydrating cats with low hematocrits will worsen anemia. This is not supported by any textbooks or scientific studies but is more likely based on misplaced intuitive logic. The amount of circulating hemoglobin remains the same after fluid therapy even though the percentage that we measure may change.

The conclusion drawn from the studies cited below is that fluid therapy should not be withheld in cases of anemia and that oxygen is in fact delivered less effectively when running dry.

Websites by veterinarians emphasize the need to maintain hydration in anemic pets:

"The most important thing that you can do for your pet with renal failure is to maintain adequate hydration."
http://www.theinternetpetvet.com/natural-solutions-treating-anemia-dogs-cats

"Even with a low PCV, it is important to expand the circulating volume with crystalloid fluids so that vital organs receive an increased amount of the existing hemoglobin."
https://studyres.com/doc/7845354/effectively-treating-cats-with-fhm

Clinical studies cited by the NCBI explain the reasons:

Directly addressing the question is a study which compared measured hematocrits to a "normalized" hematocrit (adjusted to the patient's ideal blood volume) and found that

A study on the physiology of oxygen transport found that patients are better able to compensate for anemia in a fully hydrated state:

"When intravascular volume is preserved (e.g. normovolemic dilutional anaemia), reductions in O2 content are tolerated to a much higher degree than in hypovolemic anaemia...."
https://www.ncbi.nlm.nih.gov/pubmed/10175156

An in-depth investigation into oxygen delivery explains how the system adjusts to compensate, noting that

"Since [oxygen delivery to the tissues] exceeds oxygen demand under physiological conditions by a factor of three to four, the organism's oxygen demand ... can be met over a large range despite ... decreasing [oxygen delivery]."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2782802/

It should also be noted as mentioned in the second study that while most oxygen is carried by hemoglobin a certain amount is absorbed as free oxygen in solution in the blood. To this extent the greater the volume of circulating liquid, the greater its capacity to carry free oxygen. In addition, dehydration forces the heart to work harder increasing the need for oxygen at a time when it's in reduced supply!