Chronic respiratory diseases (CRDs) affect the airways and other structures of the lungs and thereby lead to ventilation inhomogeneity. The most common CRDs in children are asthma, bronchiectasis and cystic fibrosis (CF). All three are obstructive diseases. However, while asthma is mostly characterized by obstruction due to bronchoconstriction of the airways, obstruction in bronchiectasis and CF originates primarily from mucus retention due to abnormal airway clearance mechanisms.