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The classifications made by philosophers and psychologists are as if one were to classify clouds by their shape. -Ludwig Wittgenstein, "Philosophical Remarks"
According to the Berlin definition (1), the severity of acute respiratory distress syndrome (ARDS) is classified as mild, moderate, or severe based on the PaO2/FIo2. These mutually exclusive categories are associated with a progressively increasing hospital or 90-day mortality (1).
After its adoption, several studies suggested that the category of "moderate ARDS," that is, patients with a PaO2/FIOj ratio between 100 and 200, is of particular interest because it is the most common form of ARDS (46.5% of all ARDS, compared with 30% for mild and 23.5% for severe ARDS) (2). It is characterized by a very limited use of adjunctive therapeutic measures, despite a hospital mortality rate of 35% (17.4% of patients with moderate ARDS undergo a recruitment maneuver, 18.1% receive neuromuscular blockade, the prone position is used in 5.5% of patients, and less than 50% undergo measurements of end-inspiratory plateau pressure) (2). Furthermore, patients with moderate hypoxemia are often not recognized by physicians as having ARDS because the Fio is considered to be too "low" (i.e., 0.3-0.4), or because of the common belief that fluid overload alone may easily explain a patient's hypoxemia (3).
The Berlin definition retained the P^ /Fio from the prior American European Consensus Committee definition because of its simplicity (1). However, its use to both define and classify the severity of ARDS has been the subject of criticism (4,5). For example, the nonlinear relationship between the Pao /FIo ratio and FIo and the impact of mean airway pressure and position (supine vs. prone) on Pao /FIo suggest the potential for substantial variation for a given degree of hypoxemia (4). Pao /FIo thresholds within the Berlin definition correlate with the severity of intrapulmonary shunt fraction and lung weight, but there is substantial individual variation within these thresholds (1). A recent study showed that lung weight increased significantly only in those patients with a Pao /FIo lower than 150 (6).
As reported in this issue of the Journal, Maiolo and coworkers (pp. 1586-1595) tried to deal with the above-described aspects of the Berlin definition, and hypothesized that using the threshold of 150 ofPaO/FIO2 to divide "moderate ARDS" into "mild-moderate"...