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 (VLEIT). Respiratory system resistance (R5) and oscillatory compliance (Cx5) 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 Cx5 or the VLEIT 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 Cx5 and VLEIT from commencement of the SI were characterized by estimating their time constants (τCx5 and τVLEIT, respectively). During SI, a fast decrease in R5 and an exponential increase in Cx5 and VLEIT were observed. Cx5 and VLEIT provided comparable information on the dynamics of lung aeration in all lambs, with τCx5 and τVLEIT being highly linearly correlated (r2 = 0.87, P < 0.001). Cx5 and VLEIT decreased immediately after SI. Despite the standardization of the animal model, changes in Cx5 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.