The primary reason for giving sub-q's is to maintain hydration.
Surplus amounts of fluid given subcutaneously
aren't introduced immediately into the circulatory system -- first they filter slowly through the interstitial matrix
into the lymphatics where they gradually work their way up to the lymphatic ducts to enter the circulatory system more or less "as needed" (i.e. not under pressure). Thus sub-q's present little risk of the volume overload that can be a concern with IV treatment. However, in some exceptional cases such as existing CHF, we might feel it important to give "just enough and no more".
This discussion deals with "fine-tuning" doses for those cases.
The goal is to balance total daily water intake with total water output. These factors are involved:
Intake includes oral (food and drink) and metabolic (generated in the body by the metabolism of carbohydrates and fat).As a guideline, in a normal cat daily water turnover (intake=output) is about 42ml of water per pound of body weight. This is broken down as follows:
First assume that the sweat and insensible losses are probably insignificant in the order of things, that metabolic input probably approximates loss to new protoplasm, and that moisture in food probably approximates moisture in feces. This narrows it down to two variables: water drunk versus urine. Sub-q's should supply the net difference.
Rather than measuring urine (hard to get all of it) and water (how much lost to evaporation from the bowl?), it's more practical to track changes in body weight. If it's assumed that a cat's body mass is about the same in the morning as in the evening and the same as it was the day before, then it follows that any fluctuations in weight are due either to eating, drinking, urination, or defecation.
What we did with Coco was to establish a nominal base weight and measure the sub-q's to maintain that weight when she was fully hydrated. (Digital scales for infants are ideal for this purpose and readily available at reasonable cost.) The starting base weight was determined relatively arbitrarily... we picked a time to weigh her about halfway between sub-q's when she was at an average hydration state, and neither immediately before nor after eating, urinating, or defecating.
We kept a log of weighings with notes indicating whether each was taken immediately before or after a treatment, before or after urination, meals, etc. We weighed Coco whenever she was due to urinate, and immediately after. (After a while, it became apparent that Coco would normally void about 1 1/2 oz. If she did so four times in a day, that represented 6 oz or about 150 ml.) We weighed her before and after each treatment. This confirmed the amount given, since as we all know the markings on the bags are impossible to read accurately. For simplicity, we used a conversion of 25ml per ounce of weight. This is a couple of grams off, but accurate enough to serve the purpose. Thus if she weighed 5 1/2 or 6 oz more after her treatment than before it confirmed that we had given her 150 ml. We also recorded the amount of each treatment. Having this information on paper helped track her progress. If she weighed a certain amount one morning after urinating and defecating but before her treatment, and the next morning she weighed 2 ounces more, she probably had retained 50ml of fluid. We might take that as an indication to reduce that treatment by 50ml. The reverse was also true.
1. Establish what you consider to be the normal weight AFTER TREATMENT. This may take several days to find a stable average;