Oh, you’re in Colorado? Didn’t notice that before. I have a MARVELOUS vet in Fort Collins who (at the very least) sure has a boatload of experience with my Ridgebacks. He’s fabulous and brilliant and I trust him implicitly with my critters. Send me a PM if you’d like his info.
You could also head up to CSU to meet with their cardiac team, although I’ve ALWAYS been unhappy when I go there. Ugh.
VCA Loveland also has an excellent team of people who might be useful for a second opinion.