what can I do to lessen the discomfort of my small dog who has a hart problem?

I too had a puppy with a severe heart defect. Just give him a lot of love and affection. Enjoy the time you have with him. Keep things cheerful, but low key. And remember that the most important thing about a dog's life is to feel loved, well-cared for and pain free. It's hard, but a responsible pet owner needs to make tough decisions sometimes to make sure their dog does not suffer. I eventually had to put my dog to sleep because he had fainting spells, breathing issues and stopped eating. There were other things (like surgery) they could try-but it sounded like it was a long shot and he would have had a long and painful recovery. I couldn't put him through that.

You put him out of his misery & suffering.

If the vet has said that surgery might not work, it's honestly the best choice for the dog.

You do whatever the vet says. Or euthanize the dog.

adjust diet to higher quality, may be raw. Don't over feed, a major issues with chi's. exercise, with permission from the vet.