[1] Oxford Univ Hosp NHS Fdn Trust, Nuffield Dept Anaesthet, Oxford OX3 9DU, England
[2] North Bristol NHS Trust, Southmead Hosp, Dept Anaesthet, Bristol, Avon, England
[3] London North West Univ Healthcare NHS Trust, Dept Anaesthet, Northwick Pk Hosp, Harrow, Middx, England
[4] Univ Hosp Bristol NHS Fdn Trust, St Michaels Hosp, Dept Anaesthet, Bristol, Avon, England
[5] Imperial Coll Healthcare NHS Trust, Queen Charlottes & Chelsea Hosp, Dept Anaesthet, London, England