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The volume of pulmonary fluid (PF) in fetus decreases at late gestation and with initiation of labor. In addition there is a massive release of surfactant in PF during labor. In postmaturity the reduction in PF continues till the lungs become virtually fluid-free. Furthermore there is a loss of surface activity with prolongation of gestation. The present study was conducted to determine the role of the volume and surface activity of pulmonary fluid on the opening pressure (OP) in fetal lungs. Fetal (term and post term) and degassed newborn lungs were inflated with air and OP recorded. The lowest OP was noted in lungs of term fetuses delivered after labor. As the lung became dryer, either after birth or during postmaturity, the OP increased. The OP was greater in postmature compared to degassed postnatal lungs. Instillation of minute volumes of isotonic saline in post term to restore the volume of pulmonary fluid to that of term lungs decreased the OP. Thereafter as the volume of intrapulmonary fluid was increased beyond that of term, either in term or post term lungs, the OP was also increased. The angle of deflection from fluid-removed to fluid-instilled segments of the OP-volume curve was more acute in the fetal lung that in the adult. These studies suggest that a reduction in the volume of pulmonary fluid at birth beyond that of term fetus (FRC) may not be beneficial for newborn and that massive release of surfactant in PF during labor serves to enhance opening of air saccules.
The volume of pulmonary fluid (PF) in fetus decreases at late gestation and with initiation of labor. In addition there is a massive release of surfactant in PF during labor. In postmaturity the reduction in PF continues till the lungs become virtually fluid-free. Furthermore there is a loss of surface activity with prolongation of gestation. The present study was conducted to determine the role of the volume and surface activity of pulmonary fluid on the opening pressure (OP) in fetal lungs. Fetal (term and post term) and degassed newborn lungs were inflated with air and OP recorded. The lowest OP was noted in lungs of term fetuses delivered after labor. As the lung became dryer, either after birth or during postmaturity, the OP increased. The OP was greater in postmature compared to degassed postnatal lungs. Instillation of minute volumes of isotonic saline in post term to restore the volume of pulmonary fluid to that of term lungs decreased the OP. Thereafter as the volume of intrapulmonary fluid was increased beyond that of term, either in term or post term lungs, the OP was also increased. The angle of deflection from fluid-removed to fluid-instilled segments of the OP-volume curve was more acute in the fetal lung that in the adult. These studies suggest that a reduction in the volume of pulmonary fluid at birth beyond that of term fetus (FRC) may not be beneficial for newborn and that massive release of surfactant in PF during labor serves to enhance opening of air saccules.