Hi Melanie,
Thanks for your question.
There is a simple answer to this. It needs more investigation to determine if there is a concern here or not. There are many reasons for a fast heart in dogs such as an elevated heart rate can be heart disease, pain, toxin ingestion, among others. Most of these things won’t be found on blood work. I would recommend that you perform some imaging as a next step. Radiographs of the heart or even an ultrasound would be a good next step. You could consider getting a second opinion and make sure that the vet doesn’t just focus on the heart but also looks for sources of pain or other conditions that can cause it.
There is not enough information in your questions to be more specific than that.
Good luck and I hope you gain more insight into what is going on here.
Dr. Clayton Greenway