The value of positive end-expiratory pressure and FIO2 criteria in the definition of the acute respiratory distress syndrome

Objectives: The criteria that define acute lung injury and the acute respiratory distress syndrome include PaO2/FIO2 but not positive end-expiratory pressure or FIO2. PaO2/FIO2 ratios of some patients increase substantially after mechanical ventilation with positive end-expiratory pressure of 5-10 cm H2O, and the mortality of these patients may be lower than those whose PaO2/FIO2 ratios remain <200. Also, PaO2/FIO2 may increase when FIO2 is raised from moderate to high levels, suggesting that patients with similar PaO2/FIO2 ratios but different FIO2 levels have different risks of mortality. The primary purpose of this study was to assess the value of adding baseline positive end-expiratory pressure and FIO2 to PaO2/FIO2 for predicting mortality of acute lung injury/acute respiratory distress syndrome patients enrolled in Acute Respiratory Distress Syndrome Network clinical trials. We also assessed effects of two study interventions on clinical outcomes in subsets of patients with mild and severe hypoxemia as defined by PaO2/FIO2. Design: Analysis of baseline physiologic data and outcomes of patients previously enrolled in clinical trials conducted by the National Institutes of Health Acute Respiratory Distress Syndrome Network. Setting: Intensive care units of 40 hospitals in North America. Patients: Two thousand three hundred and twelve patients with acute lung injury/acute respiratory distress syndrome. Interventions: None. Measurements and Main Results: Only 1.3% of patients enrolled in Acute Respiratory Distress Syndrome Network trials had baseline positive end-expiratory pressure <5 cm H2O, and 50% had baseline positive end-expiratory pressure >= 10 cm H2O. Baseline PaO2/FIO2 predicted mortality, but after controlling for PaO2/FIO2, baseline positive end-expiratory pressure did not predict mortality. In contrast, after controlling for baseline PaO2/FIO2, baseline FIO2 did predict mortality. Effects of two study interventions (lower tidal volumes and fluid-conservative hemodynamic management) were similar in mild and severe hypoxemia subsets as defined by PaO2/FIO2 ratios. Conclusion: At Acute Respiratory Distress Syndrome Network hospitals, the addition of baseline positive end-expiratory pressure would not have increased the value of PaO2/FIO2 for predicting mortality of acute lung injury/acute respiratory distress syndrome patients. In contrast, the addition of baseline FIO2 to PaO2/FIO2 could be used to identify subsets of patients with low or high mortality. (Crit Care Med 2011; 39: 2025-2030)