Hello and thanks for trusting me to help you and Misty today. I'm a veterinarian with over 25 years experience and would be happy to work with you.
Based on her age and the description that you give, there is a very good chance that she has some degree of kidney
failure, but the question remains it that all that is going on? With a cat of her age, it would be expected that her kidney function would be compromised somewhat, but when they become dehydrated by not eating or drinking and vomit on top of that, it puts further strain on the kidneys. When this happens, the naturally made "toxins" that are made as protein is broken down will increase in the blood (called the blood urea nitrogen (BUN) and creatinine). When this happens, it makes her feel like eating even less and leads to further vomiting.
As far as what can be done for her, fluid therapy can help a lot. When we treat with fluids, the goal is to diurese or flush the toxins out of the body by making whatever residual kidney function left work harder to filter out the urea. It would be important to know her values today or before starting the fluids so as to know if we are improving things. If after 24 hours of fluids, the BUN/creatinine decreases, then we are making progress. I will continue fluids until it drops as low as it will go (which is usually still not into the normal range) but many older cats are walking around with elevated BUN and feeling well. That is the first hurdle, can we decrease it and if so does she feel better? Remember that we are not only treating the numbers on the lab sheet, but her as well. If her BUN goes down, but she is still weak and wobbly, that is not good and maybe there is something else going on.
The second hurdle is after we bring it as low as it will go and send her home, how long can she compensate and keep it down. I have to be honest in that if we send them home without the benefit of continued SQ fluids (given under the skin), it will be less time before it starts to rise again. Sometimes the IV fluids get them back "in balance" and they can maintain for awhile (months or more), but the SQ's really do increase that interval of feeling good and not returning to a state of uremia.
I definitely think her situation sounds critical enough that you take her to the ER today. There is not much that can be done for her at home at this stage until we see if we can get her feeling better. As far as putting her down, it is always your choice as to where and when, and if not treated, these cats often hang on for days, but are definitely not enjoying any quality of life. Even if she is to the point where humane euthanasia is a consideration, you may be able to employ the services of a house call veterinarian. I would start with a physical exam and see if there is any hope we can improve her situation at all first.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.