Effects of 5500 ft altitude on child with Hypoplastic Left Heart Syndrome?

I have a child with hypoplastic left heart syndrome. We live in PA in a mountainous area with an elevation of 2500 feet. She sees a specialist in Pittsburgh PA. We were comtemplating a move to an area of colorado with an elevation of aound 5500 ft. She is otherwise in excellent health given her circumstance. We are planning our move on several factors, one most importantly to be near a good children's hospital. Aurora Colrado has a ranking slightly behind Pittsburgh in cardiology. We are very happy with her care at Pittsburgh Children's. She has had three heart surgeries and is one of the healthiest patients with a single ventrical condition that her doctor currently sees. I of course will be seeking opinions from her doctor but I am hoping for other opinions, perhaps from specialists in mountainous regions. Thank you.