1 Department of Anaesthesia and Intensive Care, Papworth Hospital, Papworth Everard, Cambridge, United Kingdom.
2 Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London, United Kingdom.
3 Department of Physiotherapy, Papworth Hospital, Papworth Everard, Cambridge, United Kingdom.
4 Department of Cardiothoracic Surgery, Papworth Hospital, Papworth Everard, Cambridge, United Kingdom.
5 Department of Research and Development, Papworth Hospital, Papworth Everard, Cambridge, United Kingdom.
6 Department of Anaesthesia and Intensive Care, Papworth Hospital, Papworth Everard, Cambridge, United Kingdom. Electronic address: andrew.klein@nhs.net.