MATERIALS AND METHODS
We conducted a cross-sectional, multicenter study at 11 hospitals from 5 Spanish regions. The study was sponsored by the Working Group of Perinatal Respiratory Diseases of the Spanish Society of Pediatric Pulmonology and was approved by the ethics committees at each participating institution. Written consent was obtained from the adolescents and their parents/caregivers after providing a full explanation of the study protocol. The inclusion period was from May 2017 to June 2019.