Developmental changes in respiratory mechanics in the neonatal rat.

The rat lung undergoes rapid changes during the first several weeks of life. Respiratory function on neonatal rats was performed to determine if physiologic alterations coincides with classically described lung growth, and morphologic and morphometric changes. The authors examined respiratory mechanics during normal postnatal lung development in the rat utilizing both a forced oscillation technique, which partitions the mechanical properties of the lung tissue from the airways, as well as pressure-volume (PV) curves to assess lung elasticity. Respiratory function testing was performed on each postnatal days of life 10 to 20. The PV curves were characterized by a leftward shift over days 10 to 16, followed by a shift back to the right over days 17 to 20. Both tissue damping (G) and elastance (H) decreased from days of life 10 to 15, and then equilibrated from days 15 to 21. Hysteresivity (=G/H) at positive end-expiratory pressure (PEEP) = 1 and 3 cm H2O declined significantly with age. The effect of PEEP was significantly greater in the younger animals than in the older animals. The respiratory function assessments coincide well with the classically described periods of lung growth. These studies provide a normal baseline for correlating changes in structural with respiratory function.