It is very likely Callie has nothing wrong with her heart.
My Reese was diagnosed via an x ray by an ER vet with an enlarged heart. This was not the reason for our visit.... the x ray was to view his lungs. however, I still made an appointment with a cardiologist. She explained unlike specialists...veterinarians compare heart sizes to an picture image of hearts. As a breed...yorkies have larger hearts (don't we know this?
) and are
mis-diagnosed all the time. I had an ultrasound performed for my peace of mind....and there is nothing wrong with his heart.
HTH.