[1] Lister Hosp, Dept Resp Med, E&N Herts NHS Trust, Stevenage SG1 4AB, Herts, England
[2] Lister Hosp, Dept Resp Med, Primary Care Trust, Stevenage SG1 4AB, Herts, England
[3] Queen Elizabeth II Hosp, Welwyn Garden City AL7 4HQ, Herts, England
[4] Univ Hertfordshire, Physiotherapy Dept, Sch Hlth & Emergency Profess, Hatfield AL10 9AB, Herts, England