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Clinical year in review IV: Acute respiratory distress syndrome, radiology in the intensive care unit, nonpulmonary critical care, and pulmonary infections in the immunocompromised host.

Although a plateau pressure–limited low tidal volume ventilatory strategy reduces mortality in acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) (1), there is still uncertainty regarding the ideal level of positive endexpiratory pressure (PEEP) and the role of recruitment maneuvers. Two large randomized clinical trials of different levels of PEEP in ARDS were published in 2008. Meade and colleagues from the Lung Open Ventilation Study (2) randomized 983 patients with ALI and PaO2/FIO2 , 250 to two different ventilator strategies. The open lung ventilation group received low tidal volume (6 ml/kg predicted body weight), high levels of PEEP titrated based on FIO2, and recruitment maneuvers with a goal plateau pressure of 40 cm H2O or less. The control ventilation group was ventilated using the ARDS Network (ARDSNet) low tidal volume protocol (6 ml/kg predicted body weight) with a goal plateau pressure of 30 cm H2O or less, PEEP titration by FIO2 level as published in the ARDSNet protocol, and no recruitment maneuvers. After randomization, both groups received similar tidal volumes, but PEEP levels and plateau pressures were approximately 4–5 cm H2O higher in the open lung group. Mortality was not different between the two groups (36.4% in the open lung group vs. 40.4% in the control group; relative risk [RR], 0.90; 95% confidence interval [CI], 0.77–1.05; P 5 0.19). The duration of mechanical ventilation among survivors was also not different between groups. Patients in the open lung ventilation group were less likely to require rescue therapies for or to die with refractory hypoxemia. Mercat and colleagues from the Expiratory Pressure Study Group (EXPRESS) (3) took a different approach to maximizing lung recruitment in patients with ARDS, randomizing 767 patients with ALI to an increased recruitment or a minimal distension ventilatory strategy. The increased recruitment group received low tidal volume ventilation (6 ml/kg predicted body weight) with PEEP titrated to maintain the plateau pressure between 28 and 30 cm H2O. The minimal distension group also received low tidal volume ventilation, but PEEP was set at very low levels (5–9 cm H2O). Recruitment maneuvers were discouraged in both groups. PEEP weaning trials were used in both groups to determine when PEEP could safely be reduced. After randomization, both PEEP and plateau pressures were approximately 6 to 7 cm H2O higher in the increased recruitment group. Hospital mortality was not different between the two groups. However, the increased recruitment group had significantly more ventilator-free days (median, 7 vs. 3; P 5 0.04) and organ failure–free days (median, 6 vs. 2; P 5 0.04). As in Meade and colleagues’ study, the increased recruitment group required fewer rescue therapies for refractory hypoxemia. Considering the Mercat and Meade studies together with the ARDSNet study of higher versus lower PEEP (4), we now have data from over 2,000 patients suggesting that, although safe, the routine application of higher levels of PEEP in all patients with ARDS does not have a major impact on mortality in ARDS. There are several potential reasons for the failure of higher levels of PEEP to have a major impact on mortality. It is possible that higher PEEP levels may have less of an effect when a low tidal volume strategy is used routinely as was the case in these three studies. In addition, the rise in plateau pressure when higher PEEP levels are used may lead to alveolar hyperinflation. Of note, in all three studies, the higher PEEP groups had better oxygenation, which may have led to fewer rescue therapies and earlier qualification for weaning. However, as has been the case in many clinical trials in ARDS, better oxygenation was not a good surrogate for improved mortality. The greater number of ventilator-free days in Mercat and coworkers’ study compared with the very low PEEP group and the trend toward lower mortality in all three studies suggests that there are some patients with ARDS who may benefit from higher PEEP. Identifying patients who can derive benefit from higher levels of PEEP through lung recruitment and differentiating these patients from those in whom higher PEEP leads to alveolar hyperinflation is a major challenge that requires further study. Grasso and colleagues (5) hypothesized that application of PEEP guided by the ARDSNet PEEP/FIO2 guidelines in patients with ‘‘focal’’ ARDS (as defined by focal loss of aeration on chest computed tomography [CT]) would produce alveolar hyperinflation as compared with PEEP titration based on an individualized approach with breath-to-breath monitoring of the stress index, a tool to quantify tidal alveolar hyperinflation and recruiting/derecruiting. The stress index is measured by computerized analysis of the contour of the inspiratory pressure curve during constant flow conditions and reflects changes in lung elastance. Fifteen patients were studied during two randomly ordered 12-hour periods of ventilation, with PEEP titrated by either the ARDSNet protocol or by targeting a stress index of 0.9 to 1.1. The stress index arm was designed to minimize alveolar hyperinflation. All 15 patients had evidence of alveolar hyperinflation when PEEP was titrated using the ARDSNet protocol. PEEP levels were lower when titrated by stress index, as were lung elastance, and serum levels of IL-6, IL-8, and soluble tumor necrosis factor receptor 1. This study suggests that application of even modest levels of PEEP can lead to alveolar hyperinflation, at least in the subgroup of patients with ARDS with focal loss of aeration (Received in original form June 17, 2008; accepted in final form June 23, 2008)