Ontogeny of respiratory control and pulmonary mechanics in newborn rabbits

Maturation of the respiratory pattern and the active and passive mechanical properties of the respiratory system were assessed in 19 tracheotomized rabbits (postnatal age range: 1-26 days) placed in a body plethysmograph. With maturation both minute ventilation and tidal volume significantly increased, whereas respiratory frequency decreased. When normalized for body weight (kg) both the passive (Rrs X kg) and active (R'rs X kg) resistances of the respiratory system significantly increased with age, whereas the corresponding passive (Crs X kg-1) and active (C'rs X kg-1) compliances significantly decreased. At any given age R'rs X kg only slightly exceeded Rrs X kg, whereas C'rs X kg-1 was significantly lower than Crs X kg-1. Moreover, the maturational increases in Rrs X kg and R'rs X kg exceeded the corresponding decreases in Crs X kg-1 and C'rs X kg-1, resulting in significant age-related increases in both the passive (tau rs) and active (tau'rs) time constants of the respiratory system. Due to the age-related increases in tau'rs, producing a delayed volume response to any given inspiratory driving pressure, the relative volume loss obtained at any time during inspiration was greater in the maturing rabbit. On the other hand, because of concomitant compensatory changes in respiratory pattern, evidenced by increases in inspiratory duration with age, the end-inspiratory tidal volume loss in the maturing animal was maintained generally less than 10% at all postnatal ages. Thus maturational changes in respiratory pattern appear coupled to changes in the active mechanical properties of the respiratory system. The latter coupling serves to optimize the transduction of inspiratory pressure into volume change in a manner consistent with establishing the minimum inspiratory work of breathing during postnatal development.