My dog accidentally took extra Carprofen. Is it safe for her heart?

Accidents happen! Fortunately, the carprofen won't impact her heart disease or interact with the pimobendan. However, there is a chance that she could experience some stomach upset with vomiting and/or diarrhea, particularly since this is a pretty big dose for a dog her size. With this kind of dose, the worst case I'd expect to see could be ulceration of the GI tract. Normally, carprofen is prescribed at a dose of 4.4 mg/kg over 24 hours. Today, your dog consumed 7.86 mg/kg. In most cases, we don't see toxicity impacting the kidneys until a dose of 40 mg/kg is ingested. With all of that in mind, your dog should be absolutely fine! In order to be extra cautious, I'd recommend feeding her a bland diet of boiled white meat chicken and white rice for a few days, then gradually mix it in with her normal food over 5-7 days. You can also call your vet to see if they can give a dose for an antacid, like Pepcid. (It's illegal for me to do so without having examined your dog.) If you see any vomiting, diarrhea, lethargy, or blood in the stool, you should call your vet right away. I hope that everything goes well!