RT Journal Article SR Electronic T1 Difficult asthma JF Thorax JO Thorax FD BMJ Publishing Group Ltd and British Thoracic Society SP 555 OP 556 DO 10.1136/thorax.58.7.555 VO 58 IS 7 A1 B D W Harrison YR 2003 UL http://thorax.bmj.com/content/58/7/555.1.abstract AB