• Giorgia Maiolo, Francesca Collino, Francesco Vasques, Francesca Rapetti, Tommaso Tonetti, Federica Romitti, Massimo Cressoni, Davide Chiumello, Onnen Moerer, Peter Herrmann, Tim Friede, Michael Quintel, and Luciano Gattinoni.
• 1 Department of Anesthesiology, Emergency and Intensive Care Medicine, University of Göttingen, Göttingen, Germany.
• Am. J. Respir. Crit. Care Med. 2018 Jun 15; 197 (12): 1586-1595.

RationaleThe ratio of PaO2 to FiO2 (P/F) defines acute respiratory distress syndrome (ARDS) severity and suggests appropriate therapies.ObjectivesWe investigated 1) whether a 150-mm-Hg P/F threshold within the range of moderate ARDS (100-200 mm Hg) would define two subgroups that were more homogeneous; and 2) which criteria led the clinicians to apply extracorporeal membrane oxygenation (ECMO) in severe ARDS.MethodsAt the 150-mm-Hg P/F threshold, moderate patients were split into mild-moderate (n = 50) and moderate-severe (n = 55) groups. Patients with severe ARDS (FiO2 not available in three patients) were split into higher (n = 63) and lower (n = 18) FiO2 groups at an 80% FiO2 threshold.Measurements And Main ResultsCompared with mild-moderate ARDS, patients with moderate-severe ARDS had higher peak pressures, PaCO2, and pH. They also had heavier lungs, greater inhomogeneity, more noninflated tissue, and greater lung recruitability. Within 84 patients with severe ARDS (P/F < 100 mm Hg), 75% belonged to the higher FiO2 subgroup. They differed from the patients with severe ARDS with lower FiO2 only in PaCO2 and lung weight. Forty-one of 46 patients treated with ECMO belonged to the higher FiO2 group. Within this group, the patients receiving ECMO had higher PaCO2 than the 22 non-ECMO patients. The inhomogeneity ratio, total lung weight, and noninflated tissue were also significantly higher.ConclusionsUsing the 150-mm-Hg P/F threshold gave a more homogeneous distribution of patients with ARDS across the severity subgroups and identified two populations that differed in their anatomical and physiological characteristics. The patients treated with ECMO belonged to the severe ARDS group, and almost 90% of them belonged to the higher FiO2 subgroup.

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