Background: Lung function in children with persistent asthma may be impaired during preschool and school ages. The aim of this study was to describe if some preschool IOS parameters are related to spirometry alterations on reaching school age. Methods: 66 children under six years old diagnosed with persistent asthma were studied prospectively with IOS during their preschool years and spirometry at school age. Results: The mean age was 4.9 years at the first evaluation and 7.9 years at the second evaluation, and 59.1% were male. During preschool, R5, Fres, AX, and D5-20 were found to have diagnostic accuracy (AUC>0.7) for predicting altered spirometry during school age (defined as FEV1 and/or FEV/FVC and/or FVC values below the lower limit of normality according to Quanjer predictive values). AX, D5-20, and R5 had the best LR+ to increase the probability of altered spirometry (50, 10, and 7.1, respectively). R5 was the best IOS parameter for discriminating bronchodilator response (BDR) in schoolchildren (AUC=0.7, LR+ = 3.7). Abnormal IOS increases the risk of having abnormal spirometry (RR=12.7, p= 0.002). This risk is even higher in atopic patients (RR= 21, p=0.003). Conclusion: The findings of this study indicate that some IOS parameters between 3 and 5 years of age are useful for predicting abnormal spirometry and BDR at school age.

Background: Small airway dysfunction in asthma can be measured by impulse oscillometry (IOS), where sometimes the reactance can exhibit an inversion of the curve, and its correction can determine a new value for X5: approximate X5 (X5 approx.). Our hypothesis is that X5 approx. exhibits a closer association with parameters of airway dysfunction in the IOS than X5. Methods: We analyzed 403 children (3-17 years old) who performed IOS (Sentry Suit, Vyntus®) and spirometry, recording R5, AX, X5, X5approx., Fres, D5-20 and FEF25-75. Groups X5 and X5 approx. were compared with respect to the percentage of abnormal IOS parameters, their averages, FEF25-75, and their correlation with each IOS parameter. Also, we explored the correlation between X5 and X5 approx. with each IOS parameter. Results: We found a significant decreasing prevalence of X5 approx. with age (84.6% in preschoolers, 67.2% in schoolchildren, and 36.5% in adolescents, p for trend <0.001). The preschoolers and schoolchildren with X5 approx. exhibited significant (p<0.05) alterations in many other IOS parameters (e.g. R5, Fes, AX, and D5-20) compared with those with X5. Adolescents exhibited a significant (p<0.01) alteration only for D5-20. The means of R5, AX, and D5-20 were significantly (p<0.01) higher in children with X5 approx. than with X5. Finally, in the all the age categories, compared with X5, X5 approx. correlated better with other IOS parameters and FEF25-75. Conclusion: The presence of X5 approx. decreases with age and correlates more closely than X5 with other IOS parameters for the evaluation of small airway dysfunction Inversion of the reactance curve and X5 approx. are better parameters for measuring smallairway dysfunction in asthmatic children*