Spirometry
FEV1, forced vital capacity (FVC) and forced expiratory flow 25–75% (FEF25-75) were measured using standard spirometry following the ATS/ERS guidelines28 with an ultrasound spirometer (Ecomedics, EXHALIZER-D software 3.3.1). Data were expressed in % predicted using the normative data from the Global Lung Function Initiative software (GLI 2012, Global Lung Function Initiative Task Force).