Conventional spirometry has long been considered the primary test for assessing respiratory function deficits in children and adults. However, the need to perform forced breathing maneuvers can make it challenging to perform these examinations in an uncooperative child. For these reasons, techniques such as gas dilutions and the Multiple-Breath-Washout (MBW) method have been implemented because they allow for early assessment of damage to the small airways. These methods permit the determination of the possible presence of ventilatory inhomogeneity in the lungs by analyzing the clearance of an inert gas used as a tracer. The equipment consists of a mass spectrometer combined with a flow meter. The Lung Clearance Index (LCI) is most often used to evaluate ventilatory inhomogeneity. This narrative review aimed to review the literature on technical and practical aspects of the MBW test and evaluate the clinical implications of the LCI in pediatric respiratory disease.

Background: Treatment with recombinant human growth hormone (rhGH) is approved for children born small for gestational age (SGA) who fail to show postnatal catch-up growth; however, optimizing its efficacy remains a challenge. Aim: to evaluate the impact of rhGH therapy on growth trajectory (GT) and adult height (AH) in SGA children and to identify factors influencing height gain (HG). Methods: A total of 49 SGA children (24 males, 25 females) without postnatal growth recovery and treated with rhGH were enrolled. Clinical and anthropometric data were collected at treatment initiation (T0), after 1 (T1) and 2 years (T2) of therapy, at pubertal onset (P0), during the first (P1) and second year (P2) of puberty, and at attainment of AH. Parameters included age, bone age, H, weight, BMI (all expressed as SDS), HG, and the difference between H and target height (Δ H-TH). Results: a significant increase in HG at all evaluated stages was observed (p < 0.05). The H–TH difference progressively decreased from T0, particularly until the first two years of puberty. Nevertheless, mean AH was −1.75 ± 0.63 SDS, and it was found to fall within the TH range in 86% of cases. Univariate and multivariate regression analysis revealed that age and H at T0 were independent predictors of HG. Conclusions: rhGH treatment has a positive impact on GT in children born SGA. Pubertal growth has a limited contribution in influencing AH of these patients. H and timing of treatment initiation significantly influence HG in SGA children. Early selection of patients for rhGH therapy could further improve their GT.