[1] Univ Hosp Birmingham NHS Fdn Trust, Dept Anaesthesia, Birmingham, W Midlands, England
[2] Western Gen & St Johns Hosp, Dept Anaesthesia, Edinburgh, Midlothian, Scotland
[3] St Georges Univ Hosp NHS Fdn Trust, Dept Anaesthesia, London, England
[4] Guys & St Thomas NHS Fdn Trust, Dept Anaesthesia & Perioperat Med, London, England
[5] Kings Coll London, London, England