The New Definition of ARDS

Acute Respiratory Distress Syndrome: the Berlin Definition

ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, et al
JAMA. 2012;307:2526-2533

Summary

The acute respiratory distress syndrome (ARDS) was originally defined as a clinical condition in 1967, although it had been recognized for years prior in soldiers dying of wartime injuries.^[1] Various attempts to better define the condition resulted in the American-European Consensus Conference (AECC) definition, published in 1994.^[2] The AECC definition has served for conducting studies of epidemiology and pathophysiology and randomized treatment trials.^[3,4,5] However, the definition from its inception was somewhat arbitrary and lacked a rationale for each specific component.

This article describes a new definition of ARDS developed by expert consensus, focusing on feasibility, reliability, validity, and objective evaluation of the performance of the definition. Three categories were developed on the basis of hypoxemia: mild (PaO₂/FiO₂ ≤ 300 mm Hg but > 200 mm Hg), moderate (PaO2/FiO₂ ≤ 200 mm Hg but > 100 mm Hg), or severe (PaO₂/FiO₂ ≤ 100 mm Hg). Four ancillary variables were also considered for the definition: radiographic severity, respiratory system compliance, positive end-expiratory pressure, and corrected expired volume per minute.

Data from 4457 patients with ARDS were used to validate the proposed variables. It was found that the 4 ancillary variables did not contribute to the predictive validity of severe ARDS for mortality, and so they were removed from the definition.

Mild, moderate, and severe ARDS were associated with increased mortality rates (27%, 32%, and 45%, respectively; P < .001) and increased median duration of mechanical ventilation in survivors (5, 7, and 9 days; P < .001). Compared with the AECC definition, the Berlin definition better predicted mortality, with an area under the receiver-operating characteristic curve of 0.577 compared with 0.536 (P < .001). The authors concluded that this updated and revised Berlin definition of ARDS addresses some of the limitations of the AECC definition.

Viewpoint

ARDS has long been considered a problematic condition because of the difficulty in making an accurate diagnosis and in finding effective treatments. The 2 problems are intertwined: Failure to timely and accurately identify patients with the condition makes it harder to design studies and test new therapeutics. The strength of this new Berlin definition of ARDS rests on the validation steps used to calibrate the model.

The prior AECC definition was developed by expert consensus, largely without the ability to derive and validate the pieces of the definition. A challenge for the new definition will be to test whether it performs as well in more natural settings and in the real world, compared with the rigorous and homogeneous environment of controlled clinical studies.

Another important and interesting point is that the new validated Berlin definition is remarkably similar to the earlier AECC definition. It requires an "at risk" diagnosis (such as sepsis, trauma, aspiration, or another accepted cause of ARDS), as well as bilateral infiltrates on chest radiography (one of the most subjective and variable parts of the definition)^[6] and hypoxemia measured by the PaO₂/FiO₂ ratio. ARDS experts previously argued that hypoxemia was not predictive of subsequent mortality and thus clinical trials should not be powered for this outcome. But now, after all of the data have been reviewed and assembled by expert consensus, it turns out that hypoxemia is the most relevant prognostic variable we have.

Abstract