My own 16-year-old Jake was diagnosed last November with end-stage kidney failure (renal failure), and he's doing great! He gets subcutaneous fluids (not really an IV, it goes under the skin not into a vein) every day, plus 1/4 of a tiny pill (Norvasc) for the high blood pressure that comes with kidney failure. It was high blood pressure that was giving him a headache so he would yowl, and I thought he was confused and looking for us….
So it's been nine months and Jake is doing so well. I also got slippery elm bark (in capsules, I open them up, or in bulk) to add to his food. I sprinkle it over his wet food and add a little water to help it dissolve, so he basically gets soup). I also give him half a Pepcid tablet when I give him his Norvasc. This is because he was so miserable with excess stomach acid, it made him gag and try to throw up many times a day. Now he doesn't do that at all, and he is a happy cat, no howling, no gagging, he purrs and eats and sleeps very well.
This end-stage renail failure can be managed, and sometimes you can keep them pretty well for years. Good luck with your cat!