Respiratory mechanics during initial lung aeration at birth in the preterm lamb

Despite recent insights into the dynamic processes during lung aeration at birth, several aspects remain poorly understood. We aimed to characterize changes in lung mechanics during the first inflation at birth and their relationship to changes in lung volume. Intubated preterm lambs (gestational age, 124-127 days; n = 17) were studied at birth. Lung volume changes were measured by electrical impedance tomography (V_L). Respiratory system resistance (R₅) and oscillatory compliance (C_x5) were monitored with the forced oscillation technique at 5 Hz. Lambs received 3-7 s of 8 cmH₂O of continuous distending pressure (CDP) before delivery of a sustained inflation (SI) of 40 cmH₂O. The SI was then applied until either C_x5 or the V_L or the airway opening volume was stable. CDP was resumed for 3-7 s before commencement of mechanical ventilation. The exponential increases with time of C_x5 and V_L from commencement of the SI were characterized by estimating their time constants (τC_x5 and τV_L, respectively). During SI, a fast decrease in R₅ and an exponential increase in C_x5 and V_L were observed. C_x5 and V_L provided comparable information on the dynamics of lung aeration in all lambs, with τC_x5 and τV_L being highly linearly correlated (r² = 0.87, P < 0.001). C_x5 and V_L decreased immediately after SI. Despite the standardization of the animal model, changes in C_x5 and R₅ both during and after SI were highly variable. Lung aeration at birth is characterized by a fast reduction in resistance and a slower increase in oscillatory compliance, the latter being a direct reflection of the amount of lung aeration.