[1] Univ Oxford, Nuffield Dept Clin Neurosci, Oxford, England
[2] Oxford Univ Hosp NHS Fdn Trust, Nuffield Dept Anaesthesia, Oxford, England
[3] Univ Oxford, Radcliffe Dept Med, Oxford, England
[4] Manchester Univ NHS Fdn Trust, Dept Anaesthesia, Manchester, Lancs, England