We went to the cardiologist today. The good news is that other than a teeny heart murmur, Isabelle's heart is fine. It's in pretty good shape for a pug her age and in her condition. No enlarged heart, no congestive heart failure. So that's great news.
She still has a cough that we have to address though. We aren't sure whether her breathing problems are in her upper or lower airways. Her regular vet has to read the cardiologist's report, and then we'll figure out the next steps. She got a new cough medicine that will hopefully help better than the syrup.
In the meantime, Isabelle is enjoying her new heated bed in her pugjamas.