I (and my poor kitty) can appreciate your concern for this! He has just recovered from surgery to fix a perforated rectum. The cause was either from constipation or from an enema to relieve it. Do be careful.
I've had the same experience. I guess it depends on how significant the murmur is. My vet wouldn't do the dental until I had an EKG and the cat was cleared. So, you could always have your kitty's heart checked out by a cardiologist if you're super-concerned.
I realize this thread is years old, but I'm just headed into this same situation. My cat also has CRF, and started having seizures. His first was actually while he was in the hospital receiving the antibiotic Baytril through an IV. The second and third were several weeks later. Blood work...