I am sorry to read of the problems your Granddaughter is having. I cannot imagine how scared you must feel as this is just a child and to be burdened with health issues so young is tough.
I have a granddaughter who has Aspergers and has been on meds since she was 4 years old.
For most people, PVCs occur infrequently and are benign. If your Granddaughter was on one of the stimulant medications that may have caused her PVCs thus stopping the medication should give the Drs. a clue. Tough case and wish I had more to offer.