Division of Surgery and Interventional Science, University College London, Royal Free London NHS Foundation Trust, London, United Kingdom. Department of Anaesthesia, Royal Free Perioperative Research, Royal Free London NHS Foundation Trust, London, United Kingdom. Royal Free London NHS Foundation Trust, Therapy Services, London, United Kingdom. Royal Free London NHS Foundation Trust, London, United Kingdom. Royal Free Perioperative Research, Royal Free London NHS Foundation Trust, London, United Kingdom.