Visual validation of the mechanical stabilizing effects of positive end-expiratory pressure at the alveolar level

Background. Positive end-expiratory pressure (PEEP) reduces ventilator-indu ced lung injury (VILI), presumably by mechanically stabilizing alveoli and decreasing intrapulmonary shear. Although there is indirect support for thi s concept in the literature, direct evidence is lacking. In a surfactant de pletion model of acute lung injury we observed unstable alveolar mechanics referred to as repeated alveolar collapse and expansion (RACE) as measured by changes in alveolar area from inspiration to expiration (I - E-Delta). W e tested the hypothesis that over a range of tidal volumes PEEP would preve nt RACE by mechanically stabilizing alveoli. Materials and methods. Yorkshire pigs were randomized to three groups: cont rol (n = 4), Tween (surfactant-deactivating detergent) (n = 4), and Tween PEEP (7 cm H2O) (n = 4). Using in vivo video microscopy individual alveola r areas were measured with computer image analysis at end inspiration and e xpiration over consecutive increases in tidal volume (7, 10, 15, 20, and 30 cc/kg.) I - E-Delta was calculated for each alveolus. Results. Surfactant deactivation significantly increased I - E-Delta at eve ry tidal volume compared to controls (P < 0.05). PEEP prevented this change , returning I - E-Delta, to control levels over a spectrum of tidal volumes . Conclusions. RACE occurs in our surfactant deactivation model of acute lung injury. PEEP mechanically stabilizes alveoli and prevents RACE over a rang e of tidal volumes. This is the first study to visually document the existe nce of RACE and the mechanical stabilizing effects of PEEP at the alveolar level. The ability of PEEP to stabilize alveoli and reduce shear during mec hanical ventilation has important implications for therapeutic strategies d irected at VILI and acute respiratory distress syndrome. (C) 2001 Academic Press.