National Heart and Lung Institute, Imperial College London, London, UK k.philip@imperial.ac.uk.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.Brunel University London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.National Heart and Lung Institute, Imperial College London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.National Heart and Lung Institute, Imperial College London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.Respiratory Medicine, Imperial College Healthcare NHS Trust, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.National Heart and Lung Institute, Imperial College London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.Department of Behavioural Science and Health, University College London, London, UK.National Heart and Lung Institute, Imperial College London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.National Heart and Lung Institute, Imperial College London, London, UK.Respiratory Medicine, Royal Brompton and Harefield NHS Foundation Trust, London, UK.