Sometimes panting or quick breathing can
be indicative of pain in a dog, sbainbr. Have you asked Beau's vet about the arthritis--about the possibility Beau is feeling some discomfort from it? If so, pain meds might help
If the pain meds don't seem to have an effect and you're still worried about the possibility of heart disease, there is such a thing as a canine cardiologist. Tests might be pricey, but it might lay your fears to rest once and for all.
Meanwhile, welcome to the board. We'd love to see pics of Beau if you have any to share with us!