[1] Ninewells Hosp, Dept Resp Med, Dundee DD1 9SY, Scotland
[2] Univ Dundee, Ninewells Med Sch, Dundee, Scotland
[3] Univ London Imperial Coll Sci Technol & Med, Natl Heart & Lung Inst, Resp Sci, London, England
[4] Royal Edinburgh Infirm, Dept Resp Med, Edinburgh, Midlothian, Scotland