IOS measurements
IOS measurements were performed using a Vyntus® APS/IOS (Vyaire GmbH, Höchberg, Germany), calibrated daily with a 3 L syringe as directed by the manufacturer, and analysed using the SentrySuite® software package (version 3.10). According to current international recommendations, measurements were taken before spirometry to avoid the effect of forced spirometry manoeuvres on bronchial tone [15]. IOS measurements were performed in accordance with ERS guidelines [28]. Prior to testing, each child was familiarized with the procedure. During the measurement, the patient remained seated with the head in a neutral or slightly extended position, breathing through a mouthpiece and wearing a nose clip; moreover, the patient’s cheeks were firmly maintained by the hands of the investigator or caregiver to minimize pressure losses. Only triplicate artefact-free IOS measurements lasting ≥20 s with a coherence value ≥ 0.8 in R5 and ≥ 0.9 in R20 and a coefficient of variation ≤ 15% were selected.