Visual Validation of the Mechanical Stabilizing Effects of Positive End-Expiratory Pressure at the Alveolar Level

Background. Positive end-expiratory pressure (PEEP) reduces ventilator-induced lung injury (VILI), presumably by mechanically stabilizing alveoli and decreasing intrapulmonary shear. Although there is indirect support for this concept in the literature, direct evidence is lacking. In a surfactant depletion 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 − EΔ). We tested the hypothesis that over a range of tidal volumes PEEP would prevent RACE by mechanically stabilizing alveoli. Materials and methods. Yorkshire pigs were randomized to three groups: control (n = 4), Tween (surfactant-deactivating detergent) (n = 4), and Tween + PEEP (7 cm H2O) (n = 4). Using in vivo video microscopy individual alveolar areas were measured with computer image analysis at end inspiration and expiration over consecutive increases in tidal volume (7, 10, 15, 20, and 30 cc/kg.) I − EΔ was calculated for each alveolus. Results. Surfactant deactivation significantly increased I − EΔ at every tidal volume compared to controls (P < 0.05). PEEP prevented this change, returning I − EΔ 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 range of tidal volumes. This is the first study to visually document the existence of RACE and the mechanical stabilizing effects of PEEP at the alveolar level. The ability of PEEP to stabilize alveoli and reduce shear during mechanical ventilation has important implications for therapeutic strategies directed at VILI and acute respiratory distress syndrome.