1. Population, Policy and Practice Research and Teaching Department, Great Ormond Street Institute of Child Health (R.L.K., D.R.), University College London, UK.
2. Clinical Operational Research Unit (Q.H., F.E.-P.), University College London, UK.
3. Paediatric Cardiology, Royal Brompton and Harefield National Health Service Foundation Trust, London, UK (R.C.F.).
4. Cardiology, Birmingham Women’s and Children’s National Health Service Foundation Trust, UK (A.N.S.).
5. Paediatric Cardiology, Evelina London Children’s Hospital, UK (H.B.-R.).
6. Cardiac, Critical Care and Respiratory Division, Great Ormond Street Hospital for Children National Health Service Foundation Trust, London, UK (K.L.B.).