Sooner or later, just about all kidney disease dogs will need IV and Sub-Q Fluids. I was reading about how human kidney disease is treated with fluid therapy and found it interesting that the fluids used are "patient specific" in content. What usually happens is the individual patient's fluids are created for them, so they correct any particular failing electrolyte or mineral imbalance, while at the same time offering hydration. Disorders of sodium, chloride, potassium, calcium, and phosphorus are commonly encountered in renal failure and can be life threatening.
Basically the problems that require IV fluids are not only to do with hydration, but also the electrolyte balance and acidosis and the fluids used are based on any predicted problems and lab results. So in polyurea you could predict a low sodium level, oligurea high sodium, there might also be the need to correct acidosis ... but basically it's an individual treatment plan.
I'm not so sure this human kidney treatment plan transfers into veterinary practice. Do dogs get individually tailored fluids - or do they just get an "off the shelf" bag of fluids? Anyone that's currently giving their dog fluids, please ask their vets about this, if you get the chance. I would be very interested in how things are done and whether, in fact, we can encourage a different approach to potentially improve the treatment prognosis.