1. Department of Emergency Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA
2. Clinica Pediatrica, Department of Medicine and Surgery, University of Parma, 43126 Parma, Italy
3. National Heart and Lung Institute, Imperial College, London SW7 2AZ, UK
4. Centre for Paediatrics and Child Health, Imperial College, London SW7 2AZ, UK
5. Royal Brompton Hospital, London SW3 6NP, UK