Reference Values for Respiratory Impedance in Bulgarian Children Aged 2–8 Years Using the Forced Oscillation Technique (FOT)

Background/Objectives: The forced oscillation technique (FOT) is a non-invasive, effort-independent method for assessing respiratory mechanics and is particularly suited for young children who cannot reliably perform spirometry. This study aimed to evaluate the main anthropometric determinants of respiratory impedance parameters—resistance (Rrs) and reactance (Xrs)—in healthy Bulgarian children aged 2 to 8 years. Methods: A total of 100 healthy children were evaluated using a commercially available device at oscillation frequencies of 5, 11, and 19 Hz. Anthropometric data were collected, and FOT measurements were conducted following ATS/ERS guidelines. Stepwise multiple linear regression was applied to identify predictors of Rrs and Xrs. Results: Height (mean height of the children: 113.89 ± 8.46 cm) emerged as the most significant determinant of both Rrs and Xrs across all frequencies with a moderate inverse correlation observed between Rrs at 5 Hz and height (r = −0.446; p < 0.001). Weight additionally influenced reactance at 5 Hz. The mean R5–19 was 0.55, but no significant associations with height or weight were found. Stepwise regression confirmed height as the sole consistent predictor, while sex and age had no significant effect. Conclusions: This study is the first to present the determinants of key FOT parameters in a population of Bulgarian children. Height was identified as the strongest predictor of respiratory impedance and should be prioritized in the development of reference values for children under 8 years old. These findings reinforce the clinical utility of FOT in early childhood.