I don't know of one who is specialized in heart problems but there are a couple of really good vets in the Riverside area that I can highly recommend.
One is Dr. Michael Butchko (909) 686-2242 at the West Riverside Veterinary Hospital. But call and see if he is there the day that you go, and make sure to request him. His son is also there but IMO he is not nearly as good as his father.
Also, at Sunneymead Animal Hospital there is Dr. Martha Eaton (909) 242-3118. She is great, she used to work with Butchko but had the chance to open her own practice out in Moreno Valley.