An investigation of pulmonary surfactant physicochemical behavior under airway reopening conditions.

Airway reopening mechanics depend on surfactant physicochemical properties. During reopening, the progression of a finger of air down an airway creates an interface that is continually expanding into the bulk fluid. Conventional surfactometers are not capable of evaluating physicochemical behavior under these conditions. To study these aspects, we investigated the pressure required to push a semi-infinite bubble of air down a fluid-filled cylindrical capillary of radius R. The ionic surfactant SDS and pulmonary surfactant analogs L-alpha-dipalmitoylphosphatidylcholine and Infasurf were investigated. We found that the nonequilibrium adsorption of surfactant can create a large nonequilibrium normal stress and a surface shear stress (Marangoni stress) that increase the bubble pressure. The nonphysiological surfactant SDS is capable of eliminating the normal stress and partially reducing the Marangoni stress. The main component of pulmonary surfactant, L-alpha-dipalmitoylphosphatidylcholine, is not capable of reducing either stress, demonstrating slow adsorption properties. The clinically relevant surfactant Infasurf is shown to have intermediate adsorption properties, such that the nonequilibrium normal stress is reduced but the Marangoni stress remains large. Infasurf's behavior suggests that an optimal surfactant solution will have sorption properties that are fast enough to reduce the reopening pressure that may damage airway wall epithelial cells but slow enough to maintain the Marangoni stress that enhances airway stability.