Comparative respiratory mechanics in newborn mammals

The static mechanical properties of the respiratory system have been studied in newborn mammals (body size from rat to pig) and in animals (rats and rabbits) during growth. The elastic recoil pressure at functional residual capacity of the newborn is less than the adult value. The dry lung weight (LW)/body weight (BW) ratio is larger in the newborns (NB) than the adults (A). Functional residual capacity (FRC) is directly related to BW in the newborn. In general the FRC/BW ratio decreases with age while the FRC/LW increases. Respiratory system, lung and chest wall compliance are interspecific constants among newborn mammals. Examination of the NB and A allometric functions suggests that in general changes of respiratory variables are maturation dependent rather than size dependent. From the measurements of respiratory frequency and tracheal dimensions and assuming the calculated tracheal volume to be proportional to dead space the allometric relationship for dead space ventilation may be obtained. By employing the known allometric function for minute ventilation calculation suggests a constant alveolar P(,CO(,2)) and P(,O(,2)) for newborn mammals.Studies of the onset of breathing and the pattern of breathing at 10 min, 60 min, 90 min, and a few days after birth have been made in infants delivered by cesarean section and vaginally. The pattern of breathing at birth is irregular. The largest contribution to the formation of functional residual capacity in the first 30 seconds of life is made by the first breath. The first breath is deeper and slower with a longer expiration than subsequent breaths. The irregularity of the breathing pattern improves from 10 to 90 min after delivery. Breathing frequency remains high in this period (10-90 min) compared to a few days and tidal volume is increased at a few days compared to the early period of life