High Positive End-expiratory Pressure and Mortality in Acute Respiratory Distress Syndrome

Abstract

Servicio de Anestesia-Reanimación, Residencia General, Hospital Universitario La Paz, Madrid, Spain. dpestana.hulp@salud.madrid.orgIn acute lung injury and acute respiratory distress syndrome (ARDS), the aim of positive end-expiratory pressure (PEEP) is to recruit lung tissue preventing the cyclic opening and closing of alveoli (atelectrauma).1However, PEEP is associated to deleterious pulmonary (overdistension of healthy tissue) and hemodynamic (decreased venous return, abnormalities in organ blood flow) effects.2–4In recent years, several studies have attempted to answer the question of which PEEP should be used in acute lung injury and ARDS. Two strategies may be used: the setting of a “low” PEEP to minimize its secondary effects or a “high” PEEP to maximize lung recruitment and gas exchange (open lung strategy). In their recent meta-analysis, Phoenix et al. 5observed that, in ARDS patients, the use of a high-PEEP strategy showed a trend toward improved mortality and increased risk of barotrauma, although these changes were not statistically significant. However, the authors stated that “the benefits [of this strategy] far outweigh potential risks” and considered that “current evidence supports the use of high PEEP in unselected groups of patients.”A major limitation in these studies is the lack of definition of high PEEP. Protocols include two strategies in which one of the groups is randomly assigned to receive a higher level of PEEP than the other. The selection of the PEEP level is rather arbitrary, based on oxygenation criteria, and always limiting the plateau pressure. The PEEP is never individualized according to the primary cause (pulmonary vs. extrapulmonary) or severity of ARDS. Results are not conclusive, because every group includes patients who require different levels of PEEP. Therefore, the potential benefits of a specific strategy in some of the patients in a group are likely neutralized by the deleterious effects on the rest of the patients. Another reason that may explain the lack of conclusive results is the limitation of the plateau pressure in all patients, which plays a major role in outcome and may be more important that the level of PEEP in unselected cases.The results from the meta-analysis are in accord with recent literature questioning the decrease in mortality in ARDS in the past decade despite the implementation of new ventilatory strategies.6–8In the ARDSNet trial,9a significant reduction in mortality was observed when a “protective strategy,” based on a low tidal volume (6 ml/kg), was used. But we may speculate that patients were actually being protected from an “aggressive strategy” (tidal volume 12 ml/kg in the control arm). It is likely that the application of a high PEEP in the initial phase of severe ARDS, with an expected important lung edema and inflammation, is justified.10Even accepting this approach, it remains unanswered for how long the PEEP should be “high.” The lack of clear benefits in unselected patients is probably related to the absence of objective tests that help in the individual titration of the ventilatory parameters. Several techniques have been proposed, such as the plotting of pressure–volume curves,11the stress index12(actually a sort of dynamic pressure–volume curve), or the electrical impedance tomography.13Interestingly, in the three smaller studies included in the meta-analysis by Phoenix et al. 5but finally excluded for the conclusions, PEEP was set according to the pressure–volume curve. Of note is the apparent major benefit observed with a high PEEP level in these studies. Until recruitment/derecruitment and hyperinflation are not estimated repeatedly in individual ARDS patients as their lung injury evolves, it is unlikely that any attempt to demonstrate the superiority of a ventilatory strategy will be conclusive.Servicio de Anestesia-Reanimación, Residencia General, Hospital Universitario La Paz, Madrid, Spain. dpestana.hulp@salud.madrid.org