Critical Care Update
Berlin Definition: An “Evolution” of the ARDS Concept
2012;17(7):15A, 16.

Berlin Definition: An “Evolution” of the ARDS Concept

The Berlin Definition, a new characterization of ARDS, improves upon the American-European Consensus Conference definition adopted 18 years ago, according to researchers.

SAN FRANCISCO—A new definition of ARDS was presented at the American Thoracic Society (ATS) 2012 International Conference.

The Berlin Definition, as the new definition is known, is the result of a consensus process initiated by the European Society of Intensive Care Medicine and endorsed by the American Thoracic Society and the Society of Critical Care Medicine.

Characterizing the new definition was an evolution rather than a revolution in the understanding of ARDS, said Niall D. Ferguson, MD, MSc, of the Department od Medicine, University Health Network and Mount Sinai Hospital, and the Interdepartmental Division of Critical Care Medicine, University of Toronto.

“We specifically set out to tweak our existing definition and to make some improvements to the American-European Consensus Conference (AECC) definition—which we have been using for the last 18 years. But we also wanted to maintain links to that definition so that we would be able to interpret previous research.”

Co-presenting at the ATS conference was Gordon D. Rubenfeld, MD, MSc, of the Interdepartmental Division of Critical Care Medicine at the University of Toronto. The Task Force’s findings also were published in the June 20 JAMA.

The Need for an Update
Widely used by clinical researchers and clinicians, the AECC definition had helped advance the field of both research and clinical care in ARDS. However, over time, a number of issues became evident. Among these were the lack of a precise definition of the term acute, the sensitivity of PaO2 /fraction of inspired oxygen (FiO2) to different ventilator settings, inconsistencies in the interpretation of chest radiographs based on the AECC criterion, and problems differentiating hydrostatic edema.

“We tried to keep these issues in mind as we were updating the definition and I think we made a number of improvements in some of these areas,” Ferguson said. “We now have a specific definition of acute onset. The term acute lung injury has been eliminated, and we have three mutually exclusive categories of mild, moderate, and severe ARDS,” he added.

The draft Berlin Definition was evaluated empirically via a meta-analysis of four large multicenter cohorts of patients with a total of 4,188 ARDS patients classified by the AECC definition and three single center cohorts with 269 patients.

“It was a very heterogeneous group reflective of a variety of different clinical settings,” said Rubenfeld. “The data were collected over a fairly long period of time, and patients had a mean age of 54 years, with a predominance of males. Pulmonary sepsis was the number one cause of ARDS, followed by nonpulmonary sepsis, trauma, and other causes. Overall mortality was about 34%.”

The analysis also examined four ancillary variables for severe ARDS: radiographic severity, respiratory system compliance (≤ 40 mL/cm H2O), PEEP (≥ 10 cm H2O), and corrected expired volume per minute (≥ 10 L/minute). These variables did not increase predictive validity for mortality and therefore were not included in the definition.

“But we absolutely believe these are important physiologic variables for clinicians to understand as they’re managing patients,” Rubenfeld said.

Final Draft
In the final draft, a minimum level of PEEP or continuous positive’ airway pressure (CPAP) of 5 cm H2O or higher was established across all levels to mitigate the Pao2 /Fio2 inconsistencies that occur with various ventilator settings. Acute onset was defined as occurring within one week of a known clinical insult or new or worsening respiratory symptoms, and the chest imaging criteria were defined as bilateral opacities that are not fully explained by effusions, lobar/lung collapse, or nodules. The origin of edema was defined as respiratory failure not fully explained by cardiac failure or fluid overload, with a need for an objective assessment with echocardiography to exclude hydrostatic edema if no risk factor is present. ARDS severity was defined as follows:
• Mild—200 mm Hg < PaO2 /FiO2 ≤ 300 mm Hg with PEEP or CPAP ≥ 5 cm H2O
• Moderate—100 mm Hg < PaO2 /FiO2 ≤ 200 mm Hg with PEEP ≥ 5 cm H2O
• Severe—PaO2 /FiO2 ≤ 100 mm Hg with PEEP ≥ 5 cm H2O.

The final Berlin Definition has a better predictive validity for mortality than the AECC definition, the researchers said. Furthermore the Task Force’s combining of consensus discussion with empirical evaluation may serve as a paradigm for the creation of more accurate, evidence-based critical illness syndrome definitions in the future.

“This is the first study we are aware of that combined an a priori consensus building process with an empiric evaluation and patient-level meta-analysis in a single iterative process,” noted Rubenfeld.

—Laurel McKee Ranger

Suggested Reading
The ARDS Definition Task Force. Acute respiratory distress syndrome: The Berlin Definition. JAMA. 2012;307(23):2526-2533.

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