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Background: Monitoring of lung function is necessary to detect irreversible airway obstruction in both asthma and obesity. Impulse oscillometry (IOS), a novo non-invasive equipment, is increasing popularity to measure airway resistance in young children worldwide.

Aims:  To compare IOS parameters among Thai asthmatic children and Thai obese children with and without asthma.

Methods: A cross-sectional study was conducted in 120 participants, aged 4-15 years old. Forty children were in each group (asthma, obesity, and obesity with asthma). All volunteers were consented to measure airway resistance by IOS technique (Jaeger, Germany).

Results: Seventy-three percent of patients were male with the mean age at 8.8 + 2.61 years old. Mean X5 was found normal in childhood obesity (-0.13) when compared to children with asthma (-0.23) and obesity with asthma (-0.19) (p < 0.001 and 0.013 respectively). The cut-off value of X5, according to ROC curve, for predicting asthma in obese patients was -0.16 kPa/L/s with 70% sensitivity, 70% specificity and 70% accuracy (AUC= 0.69). However, with the bronchodilator effect (adjusted by duration of asthma control), we found significant higher percentage change of IOS parameters, including resonant frequency, area of reactance and R5-R20, in asthma (Fres -24.57 + 15.82, AX -58.28 + 13.37, R5-R20 -51.32 + 20.13) than in asthma with obesity (Fres -13.77 + 16.42, AX -43.35 + 21.4, R5-R20 -34.72 + 18.21), (p = 0.014, 0.004, 0.002 respectively).

Conclusions: X5 and percentage changes after bronchodilator of Fres, AX, and R5-R20 are useful parameters to differentiate airway dysfunction in asthmatic children from obese children.