Air opening pressure in fluid-filled lungs.

The present study was conducted to gain understanding of the role of pulmonary fluid and of concentration of surfactant in the pulmonary fluid on the air pressure-volume characteristics and in particular on the opening pressure (OP). The excised rat lungs were degassed, inflated with isotonic saline and subsequently deflated to a predetermined volume. These lungs were then inflated with air and OP recorded. As the lung saline volume decreased from about 90% to about 20% of maximal lung saline volume, the OP decreased from 22 to 14 cm H2O, respectively, in parallel to the elastic recoil (saline deflation P-V curve) of the lung. This suggests that the terminal airways are patent, and fully dilated at a transpulmonary pressure of 14 cm H2O. With further reduction in lung saline volume, however, the OP increased and approached the OP of fluid-free degassed lungs at 22 cm H2O, indicating closure of terminal airways at low lung volumes. The OP was reduced by increasing the concentration of surface active material in isotonic saline prior to instillation in the lung. These studies suggest that pulmonary fluid and excess surfactant reduce the OP and facilitate uniform distribution of inspired air.