PEEP in ARDS--how much is enough?

In 1967, Ashbaugh et al. 1 introduced the use of positive end-expiratory pressure (PEEP) during mechanical ventilation to treat refractory hypoxemia in patients with the acute respiratory distress syndrome (ARDS). Almost 40 years later, the question of how much PEEP is enough remains relevant. Controversy regarding the optimal level of PEEP has persisted despite years of investigation into this question. An increase in our understanding of the pathophysiology of ARDS and ventilator-induced lung injury has led to a renewed interest in the debate. Studies in animals, 2-4 designed to illuminate the cause of ventilator-induced lung injury, show that two primary mechanistic factors may contribute to the evolution of ventilator-induced lung injury: overdistention of the alveoli by high transpulmonary pressures and shear-stress forces produced by repetitive alveolar recruitment and derecruitment (collapse) in patients with ARDS who are receiving mechanical ventilation. The first proposed mechanism was addressed by the initial ARDS Clinical Trials Network study. 5 This randomized trial demonstrated a significant survival benefit among patients who received a low tidal volume (6 ml per kilogram of body weight) rather than the once widely accepted higher tidal volume of 12 ml per kilogram. This milestone study convincingly illustrated how lung-protective strategies of ventilation could improve the outcome of ARDS. In this issue of the Journal, 6 the ARDS Clinical Trials Network investigators examine the second potential mechanism of ventilator-induced lung injury. In patients with ARDS, the qualitative and quantitative surfactant defect leads to considerable endexpiratory alveolar collapse. During inspiration, 7