[1] Univ London Imperial Coll Sci Technol & Med, Chelsea & Westminster Hosp, Dept Surg & Canc, Sect Anaesthet Pain Med & Intens Care, London SW10 9NH, England
[2] Univ London Imperial Coll Sci Technol & Med, Dept Surg & Canc, St Marys Hosp, London SW10 9NH, England
[3] Royal Brompton & Harefield NHS Fdn Trust, Dept Anaesthet, Harefield Hosp, Harefield, Middx, England