Effects of gestation and antenatal steroid on airway and tissue mechanics in newborn lambs.

The aim of this study was to partition airway and parenchymal mechanics in newborn lambs at different gestations and following variable exposure to antenatal maternal betamethasone using the forced oscillation technique (FOT). Pulmonary impedance data were collected in 37 sedated and intubated apneic lambs with the FOT between 0.5 and 20 Hz and fitted by a model to estimate airway resistance (Raw) and inertance (Iaw) and the coefficients of tissue resistance (GL) and elastance (HL). Total respiratory resistance (Rrs) was also determined during tidal ventilation by using the multiple linear regression technique. Advancing gestation or increasing antenatal steroid exposure had no clinically significant effect on the values of Raw and Iaw, whereas Rrs and both GL and HL decreased markedly. There was a decrease in tissue hysteresivity (GL/HL) with repeated antenatal steroid exposure. Partitioning of lung mechanics highlights the dominant contribution of the tissues to the total respiratory resistance in the immature ovine lung. Clinically relevant changes in lung mechanics associated with structural and functional maturation of the immature ovine lung are primarily confined to the tissue compartment.