How Likely are Feline Heart Murmurs to be Dangerous?
Heart murmurs are funny things. Until you know how to listen for them, you can't hear them. That means that many vets (myself included) graduated from veterinary school having heard only a few heart murmurs. Now I hear a few every work day.
When I advise an owner of a pet's heart murmur, the owner wants to know what the murmur means (blood is flowing through the heart in a turbulent fashion) and whether it is dangerous (some are, some aren't).
Today I learned (by way of a lecture at the American Veterinary Medical Association Conference) a statistic that is more precise than some are and some aren't, at least for cats. It turns out that 30% of persistent heart murmurs in cats are completely harmless (I suspect that a much higher proportion of the murmurs I hear are harmless, incited by the stress of being sick and at the hospital).
If your cat has a persistent murmur, diagnostic imaging (radiographs and ultrasound of the heart) will tell you whether he's among the 1/3 with harmless cardiac changes or in the 2/3 with disease that may need treatment.