Statics of the respiratory system in newborn mammals.

The static mechanical properties of the respiratory system have been studied in newborn mammals within the first two weeks of life, ranging in body size from rat to piglet, and compared to the corresponding adult. Animals were supine, paralyzed, and passively ventilated. The pressure-volume curves of the respiratory system and of the lung have been constructed by changing volume a known amount and measuring the tracheal pressure before and after opening the rib cage. In the newborns, the mean elastic recoil pressure of the lung at functional residual capacity (PLFRC) is 1.9 cm H2O with no clear correlation with body weight. The dry lung weight (LW)/body weight (BW) ratio is greater in newborns than in adults and progressively decreases with age. FRC (measured directly with the saline displacement method) is proportional to LW1.09 and BW1.03. In newborns, lung compliance per lung weight (CL/LW) is similar between species (Cl alpha LW1.07). Chest wall compliance per body weight is also constant (CW alpha BW1.00). By comparing the newborns with the corresponding adults we found that PLFRC is 35-57% of the adult value. FRC/LW in the newborn is less than or equal to the adult value, while FRC/BW is generally larger in the newborn. CL/LW is usually smaller in the newborn than in the adult while CW/BW is larger.