Ventilation strategies for acute lung injury and acute respiratory distress syndrome.

To the Editor: Limiting plateau pressures in the respiratory system of patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS) to 28 to 30 cm H2O may help guarantee lung protection. In the large multicenter Express trial, Dr Mercat and colleagues set positive end-expiratory pressure (PEEP) as high as possible to avoid plateau pressure above 28 to 30 cm H2O (mean, 27.5 cm H2O). In the lower PEEP (minimal distention) group in the Express trial, plateau pressure was kept as low as possible to maintain oxygenation targets (mean, 21 cm H2O). There was no difference in mortality between the 2 groups, but the higher PEEP/plateau pressure (increased recruitment) group showed a greater number of ventilator-free and organ failure–free days. Plateau pressure in the increased recruitment group dropped to 24 cm H2O within the first week. Two smaller trials (n=53 and n=103) showed that higher PEEP levels, comparable with the Express trial (14-16 cm H2O), can reduce mortality in ARDS, despite using plateau pressures that have been considered unsafe. Both studies started out with a plateau pressure of 32 and 31 cm H2O, respectively, but within 1 week they only needed a plateau pressure of 24 to 26 cm H2O. There are 2 main differences between these studies and the Express trial. In the smaller trials, plateau pressure before lung protective ventilation was 30 to 32 cm H2O compared with 23 cm H2O in the Express study, at comparable PEEP levels (8 cm H2O) and tidal volumes. Also, both smaller studies used standardized ventilator settings with the patients to see whether ARDS criteria (ratio of partial pressure of arterial oxygen over fraction of inspired oxygen [PaO2:FIO2] 200 mm Hg) were maintained over a set time period. Alterations in chest wall mechanics may also result in marked differences in the real transpulmonary pressure, leading to progressive lung derecruitment and ventilatorinduced lung injury (VILI). Recruitment maneuvers may improve lung function by allowing ventilation on the deflation limb of the pressure-volume curve, resulting in higher end-expiratory lung volumes at similar airway pressures and potentially minimizing VILI. Arbitrarily limiting plateau pressure without individualized settings, with potential resultant progressive lung derecruitment, may prevent advances in lung protective ventilation. An individually titrated recruitment maneuver leading to an early short-term increase in plateau pressure, especially in more severely hypoxemic patients with altered chest wall mechanics, may result in better oxygenation, rapid lowering of plateau pressure in the first week, and possibly improved outcome. Jack J. Haitsma, MD, PhD [email protected] Interdepartmental Division of Critical Care Medicine St Michael’s Hospital Toronto, Ontario, Canada Paolo Pelosi, MD Department of Ambient Health and Safety University of Insubria Varese, Italy