Background: Assessing the lung function response to postnatal corticosteroids (CS) would allow the objective comparison of different treatment regimens. We hypothesised that assessing the response to treatment combining lung ultrasound (LU) and the forced oscillation technique (FOT) would allow identifying which compartments (e.g., airways or parenchyma) are mostly affected.

Aims: In very preterm infants, we aimed to combine LU and FOT to describe the response to two types of CSs: dexamethasone (DXM) and inhaled budesonide.

Methods: We retrospectively analysed the response to treatment administered to prevent or treat BPD, which led to a clinical improvement. We measured LU score (LUS) and respiratory system reactance (Xrs) by FOT before and after treatment.

Results: We analysed 26 treatments (9 DXM, 17 budesonide) administered to 19 infants. All treatments led to an improvement in either Xrs or LUS. On average, both DXM and budesonide led to a significant improvement in Xrs (p < 0.001, paired t-test) and LUS (p < 0.001). On 20 (77 %) occasions, baseline Xrs was higher than 1.96 z-score and manifested marked improvement with variable change in LUS, suggesting an effect on the airways. On 6 (33 %) occasions, LUS improved, and Xrs did not change or changed within the normal range, suggesting an effect mainly on the parenchymal compartment. The proportion of treatments with DXM and budesonide did not differ between response groups (p = 0.940, chi-squared).

Conclusion: Our data suggest that a clinical improvement following CSs is associated with an improvement in the airways or parenchymal compartment, regardless of the molecule and administration route.