Hysteresis in saturated phospholipid films and its potential relevance for lung surfactant function in vivo

The surface pressure-area (pi-A) hysteresis in films of dipalmitoyl phosphatidylcholine (DPPC) is investigated under dynamic cycling at 22 degrees C. The dynamic pi-a hysteresis of such films increases with extent of film compression, becoming maximal when compression is carried out past the point of generalized monolayer collapse based on fatty acid chain limiting areas. Several characteristics of the pi-A hysteresis found in the dynamic cycling of pure DPPC films are shown to be present in mixed surface films containing this saturated phospholipid, including binary DPPC:dioleoyl phosphatidylcholine (DOPC) films and multicomponent lung extract films at body temperature. Specific hysteresis characteristics, particularly a rapid surface pressure fall over a small area change at the beginning of film expansion, may have direct importance for lung surfactant function in vivo, and a possible model for this is presented. A major consequence stressed in terms of pulmonary mechanics concerns the role of lung surfactant pi-A hysteresis characteristics upon alveolar recruitment and increased inflation uniformity during inspiration. An attempt is made to reconcile lung surfactant surface behavior with recent concepts of alveolar area and shape changes during breathing, as well as with known clinical findings in states of lung surfactant deficiency. Some possible numerical parameters that may be useful in evaluating surfactant "activity" are presented.