Respiratory mechanics and lung development in the rat from early age to adulthood

The purpose of the present study was to establish how the dependence of res piratory mechanics on lung inflation changes during development. We studied seven groups of rats from 10 days to 3 mo of age at five levels of positiv e end-expiratory pressure (PEEP) from 0 to 7 hPa (1 hPa = 0.1 kPa approxima te to 1 cmH(2)O). At each PEEP level, we measured respiratory system resist ance and elastance at both 0.9 and 4.8 Hz to partition the mechanical prope rties into its airway and tissue components. Elastance increased more rapid ly with PEEP in the younger animals, which we interpret as reflecting a mor e pronounced strain stiffening of the younger parenchyma. However, the decr ease in airway resistance with PEEP was more pronounced in the older animal s. Morphometric analysis showed that mean tissue density decreased and tota l alveolar surface area increased with age. Our data suggest that the mecha nical interdependence between airways and parenchyma is weaker in very youn g animals compared with mature animals. This may play a role in the hyperre sponsiveness of immaturity.