1 From the Directorate of Anaesthesia and Critical Care, London North West University Healthcare NHS Trust, Northwick Park Hospital, Watford Road, Harrow, HA1 3UJ, UK (RSC, DV, SVS); King's College Hospital Critical Care Department, Denmark Hill, Brixton, London, SE5 9RS, UK (MB); Anaesthetics Department, Level 4, Paul O'Gorman Building, Great Ormond Street Hospital, Great Ormond Street, London, WC1N 3JH, UK (RS); Division of Anaesthetics, Pain Medicine and Intensive Care, Imperial College London, Chelsea and, Westminster Campus, 369 Fulham Road, London, SW10 9NH, UK (SS).