Mechanisms of recruitment in oleic acid-injured lungs

Lung recruitment strategies, such as the application of positive end-expira tory pressure (PEEP), are thought to protect the lungs from ventilator-asso ciated injury by reducing the shear stress associated with the repeated ope ning of collapsed peripheral units. Using the parenchymal marker technique, we measured regional lung deformations in 13 oleic acid (OA)-injured dogs during mechanical ventilation in different postures. Whereas OA injury caus ed a marked decrease in the oscillation amplitude of dependent lung regions , even the most dependent regions maintained normal end-expiratory dimensio ns. This is because dependent lung is flooded as opposed to collapsed. PEEP restored oscillation amplitudes only at pressures that raised regional vol umes above preinjury levels. Because the amount of PEEP necessary to promot e dependent lung recruitment increased the end-expiratory dimensions of all lung regions (nondependent AND dependent ones) compared with their preinju ry baseline, the "price" for recruitment is a universal increase in parench ymal stress. We conclude that the mechanics of the OA-injured lung might be more appropriately viewed as a partial liquid ventilation problem and not a shear stress and airway collapse problem and that the mechanisms of PEEP- related lung protection might have to be rethought.