What is the first sign of a heart issue in dogs?

What is the first sign of a heart issue in dogs? - Brunswick Veterinary Hospital

The first sign of a heart issue in a dog varies. In some cases, a dog that used to be able to run three miles a day may now barely make it down the block. Another sign could be a persistent cough that lasts for several months, which doesn't seem to be going away, even though the dog doesn't appear ill in any other way.