1. Department of Cardiology, The Northwick Park Hospital, Watford Rd, Harrow, HA1 3UJ, UK
2. The Royal Brompton Hospital, Sydney Street, Chelsea, SW3 6NP, UK
3. National Heart and Lung Institute, Imperial College London, Exhibition Rd, South Kensington, London, SW7 2BU, UK
4. The Ealing Hospital, Uxbridge Road, Southall, UB1 3HW, UK