Short Physical Performance Battery and long term prognosis following severe acute exacerbation of COPD: a prospective cohort study - Barker, R; Kon, SSC; Jones, SE; Maddocks, M; Gao, W; Nolan, CM; Patel, S; Kwan, HY; Clarke, SF; Polkey, MI; Cullinan, P; Man, WD-C (EUROPEAN RESPIRATORY SOC JOURNALS LTD, 2018)