[1] UCL, Div Surg & Intervent Sci, London, England
[2] Royal Free London NHS Fdn Trust, Dept Anaesthesia, London, England
[3] Royal Free London NHS Fdn Trust, Royal Free Perioperat Res Grp, London, England
[4] Royal Free London NHS Fdn Trust, Dept Surg, London, England
[5] East & North Hertfordshire NHS Trust, Lister Hosp, Dept Radiol, Stevenage, Herts, England