My Shaina a 5lb Chi mix was diagnosed at 12.5 with Congestive Heart Failure. She was on Lasix, Enalapril, Vetmedin. She was a nervous thing, always shaking and timid. Her heart would beat fast too. She lived almost two years after the diagnosis. Keep doing what you are doing.....Good to be aware. I never even thought of her heart beating faster would hurt her.
Also, good that you pup is under the care of a cardiologist. Put a call in to the vet. Shaina's cardiologist was always happy to answer any questions I had.
puppy.jpg Shaina as a puppy.