Objectives Assess pulmonary mechanics in a preterm rabbit model at different time points after antenatal steroids.

Methods Betamethasone 0.05 mg/kg (BM) or normal saline (placebo) was administered to pregnant does on days 25 and 26 of gestation (term 31 days). Does without intervention were normal controls. Fetuses were harvested on days 27 (7 BM, 6 placebo, 7 normal), 28 (14 BM, 12 placebo, 15 normal) or 29 (10 BM, 9 placebo, 9 normal). Ventilation was performed with a Flexivent (Scireq, Montreal); pulmonary mechanics and lung impedance by forced oscillation technique were assessed at 4-minute intervals for 30 minutes.

Results BM caused significant growth retardation at all ages. At 27 days, lung mechanics were not different between the three groups. At 28 days, BM pups had significantly better lung mechanics compared with normal rabbits. Placebo pups showed an equal improvement in lung mechanics. Lung impedance measurements, tissue elastance (H) and tissue damping (G) were better in the placebo group. At 29 days, normal pups had better values than BM or placebo pups.

Conclusion In this model, antenatal administration of steroids had a time-dependent effect on pulmonary mechanics. At 28 days, BM had a positive effect on all parameters; however, an important placebo effect was shown probably secondary to a maternofetal stress response. At 29 days, steroids had a negative effect compared with placebo and normal rabbits. This might be explained by a temporary increase in lung maturation and surfactant release followed by an arrest in lung growth and development.