1 Royal Brompton Campus, Faculty of Medicine, National Heart and Lung Institute, Imperial College, London, United Kingdom.
2 Centre for Health Services Research, Postgraduate Medical School, University of Hertfordshire, Hertfordshire, United Kingdom.
3 Adult Critical Care, Royal Brompton Hospital, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom.
4 Division of Anaesthetics, Pain Medicine and Intensive Care, Department of Surgery and Cancer, Faculty of Medicine, Imperial College London, London, United Kingdom.