Inclusion Criteria: common to the 2 groups studied are as follows:  child aged 6 to 17 inclusive; child diagnosed with cystic fibrosis (CF) and follow-up at the pediatric reference center of University Children's Hospitals of Nancy and Trousseau Hospital in Paris child for whom data from a complete routinely pulmonary function testing are available child who did not object to his/her participation child whose parental authority have received full information on the current research and have not objected to the participation of their child  The criterion for inclusion in the group "with gas trapping" is gold standard (FRCpleth-FRCHe) / FRCpleth> 10% The criterion for inclusion in the group "without gas trapping" is gold standard (FRCpleth-FRCHe) / FRCpleth ≤10%  The number of subjects potentially recruited "with gas trapping" and "without gas trapping" is unbalanced (imbalance estimated at ½ and taken into account in the calculation of the number of subjects required). In order not to go beyond this imbalance, recruitment in one of the two groups (instruction applied by center) will be stopped as soon as this group has reached the planned number of subjects and will continue only with the inclusion of subjects in the other group.  Exclusion Criteria:  other chronic respiratory pathology (bronchopulmonary dysplasia, virosis sequelae, inhalation pathology, thoraco-pulmonary malformation, tracheomalacia); anatomical or functional abnormalities of the pharyngolaryngeal pathway (tonsils hypertrophy grade 3 or 4, laryngomalacia, subglottic stenosis, vocal cord paralysis, any other laryngeal obstacle); congenital or acquired heart disease.