Exacerbation heterogeneity in COPD: subgroup analyses from the FLAME study - Vogelmeier, CF; Chapman, KR; Miravitlles, M; Roche, N; Vestbo, J; Thach, C; Banerji, D; Fogel, R; Patalano, F; Olsson, P; Kostikas, K; Wedzicha, JA (International Journal of Chronic Obstructive Pulmonary Disease, 2018)