Respiratory mechanics in small newborn mammals

The passive mechanical properties of the lung and respiratory system have been measured in newborn opossums, hamsters and mice, which, by ranging in size between 0.3 and 3 are among the smallest newborn mammals. Compared to newborns of larger species, resistances are not increased as much as compliances are decreased. This indicates that the geometry of the lung changes in the smallest species, and by protecting airway resistance limits the energetic losses determined by breathing at high frequencies. In the 0.3 g newborn opossum the lung is disproportionately large, possibly indicating the existence of a lower limit in lung size beyond which the design of the mammalian lung would not be functionally efficient. Finally, passive time constants were 40-60 msec (without the upper airway resistance). These values are shorter than in larger species, and several considerations indicate that they could be even shorter during spontaneous breathing. However, they are not compatible with the extremely brief inspiratory times and large tidal volumes which would have been required to sustain the high oxygen requirements expected for mammals of such a small size. This therefore supports the hypothesis that in these small animals the mechanical properties of the respiratory system can pose a constraint to the rate of resting ventilation and, possibly, to that of oxygen consumption.