Giving fluids may or may not help. One of my past dogs developed kidney issues later in life. I was able to manage it via diet for 2yrs then she developed Pneumonia so was on lots of drugs to get her through that which worked but that pushed her into renal failure. For a week we tried fluid therapy where she would spend the day at the vets getting IV fluids then come home at night with me and get Sub Q fluids. Sadly at the weeks end her kidney values had not changed so I had her put down. At the same time someone I work with had her dog at the same vets getting fluids for kidney failure I knew this as one night when I was picking my dog up she was there picking hers up so we talked and found out we both were dealing with the same issue
. Her dog did show better kidney values a few days later and lived about another year. So it is worth trying but results can go either way. Depending on what your dogs heart issue is they may have to be careful with fluids to avoid putting the dog into congestive heart failure. Good luck with your pup.