The Current Berlin Definition

The Berlin definition aimed to provide a better clinical definition and to classify patients according to severity. An expanded rationale was then published by the expert panel to propose treatments and ventilatory management according to the degree of hypoxemia [9]. The changes proposed in the Berlin definition to address the major limitations of AECC definition are the following:

First, the “acute onset” of ARDS has been specified, and respiratory symptoms have to be present within 7 days after a clinical insult (Table 1.1). Timing accuracy enables elimination of mimickers of ARDS who develop respiratory symptoms over several weeks such as idiopathic pulmonary fibrosis, nonspecific interstitial pneumonitis, cryptogenic organizing pneumonia, granulomatosis with polyangiitis, or drug-induced lung disease [10].

Second, patients have been stratified according to their severity in terms of hypoxemia and classified as mild, moderate, and severe ARDS when PaO₂/FiO₂ ratio is between 201 and 300, between 101 and 200, and equal to or below 100 mmHg, respectively [8]. By including patients with a PaO₂/FiO₂ ratio up to 300 mmHg, the Berlin definition now encompasses the patients with mild ARDS that was formerly named acute lung injury. Oxygenation criteria were well correlated to severity with mortality of 27, 32, and 45% in mild, moderate, and severe ARDS, respectively. As a major limitation of the AECC definition was the

Table 1.1 Risk factors of acute respiratory distress syndrome adapted from [9]

assessment of PaO₂/FiO₂ ratio regardless of the PEEP level used, the Berlin definition stated that PaO₂/FiO₂ ratio had to be measured with a PEEP level of at least 5 cmH2O [8].

Third, the AECC definition considered that pulmonary arterial wedge pressure should not exceed 18 mmHg in ARDS [6]. However, high values of pulmonary wedge pressure are commonly observed in patients with ARDS [11, 12], and routine use of pulmonary artery catheter is pointless for hemodynamic management [13]. Therefore, pulmonary artery wedge pressure requirement was removed from the Berlin definition, and it was stated that respiratory failure must not be fully explained by cardiac failure of fluid overload as judged by the clinician or confirmed by echocardiography, if needed, to rule out cardiogenic pulmonary edema [8].

Fourth, the Berlin definition considered radiological findings as bilateral opacities on chest X-ray but also on CT scan, which were not fully explained by effusions, lobar or lung collapse, or nodules [8].

Four ancillary variables were assessed for severe ARDS, including more extensive opacities on chest radiograph, i.e., at least three quadrants, a high PEEP level >10 cmH₂O, low respiratory system compliance <40 ml/cm H₂O, and a corrected expired volume >10 L/min. However, these criteria were not included in the Berlin definition because they did not help to discriminate patients with severe ARDS [8].