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Acute Respiratory Distress SyndromeThe Berlin DefinitionThe ARDS Definition Task Force*

VALID AND RELIABLE DEFINI-tions are essential to con-duct epidemiological stud-ies successful ly and tofacilitate enrollment of a consistent pa-tient phenotype into clinical trials.1 Cli-nicians also need such definitions toimplement the results of clinical trials,discuss prognosis with families, andplan resource allocation.

Following the initial description ofacute respiratory distress syndrome(ARDS) by Ashbaugh et al2 in 1967,multiple definitions were proposed andused until the 1994 publication of theAmerican-European Consensus Con-ference (AECC) definition.3 The AECCdefined ARDS as the acute onset of hy-poxemia (arterial partial pressure ofoxygen to fraction of inspired oxygen[PaO2/FIO2]200 mm Hg) with bilat-eral infiltrates on frontal chest radio-graph, with no evidence of left atrial hy-pertension. A new overarching entityacute lung injury (ALI)was alsodescribed, using similar criteria but withless severe hypoxemia (PaO2/FIO2300mm Hg).3

The AECC definition was widelyadopted by clinical researchers andclinicians and has advanced theknowledge of ARDS by allowing theacquisition of clinical and epidemio-logical data, which in turn have led toimprovements in the ability to carefor patients with ARDS. However,after 18 years of applied research, anumber of issues regarding variouscriteria of the AECC definition haveemerged, including a lack of explicit

criteria for defining acute, sensitivityof PaO2/FIO2 to different ventilator set-tings, poor reliability of the chestradiograph criterion, and difficultiesdistinguishing hydrostatic edema(TABLE 1).4

*Authors/WritingCommittee and theMembers of theARDS Definition Task Force are listed at the end ofthis article.Corresponding Author: Gordon D. Rubenfeld, MD,MSc, Program in Trauma, Emergency, andCritical Care,Sunnybrook Health Sciences Center, 2075 BayviewAve, Toronto, ON M4N 3M5, Canada ([email protected]).

The acute respiratory distress syndrome (ARDS) was defined in 1994 by theAmerican-European Consensus Conference (AECC); since then, issues regard-ing the reliability and validity of this definition have emerged. Using a con-sensus process, a panel of experts convened in 2011 (an initiative of the Eu-ropean Society of Intensive CareMedicine endorsed by the American ThoracicSociety and the Society of Critical Care Medicine) developed the Berlin Defi-nition, focusing on feasibility, reliability, validity, and objective evaluation ofits performance. A draft definition proposed 3 mutually exclusive categoriesof ARDS based on degree of hypoxemia: mild (200 mm HgPaO2/FIO2300mmHg),moderate (100mmHgPaO2/FIO2200mmHg), and severe (PaO2/FIO2100mmHg) and 4 ancillary variables for severe ARDS: radiographic se-verity, respiratory system compliance (40 mL/cm H2O), positive end-expiratory pressure (10 cm H2O), and corrected expired volume per minute(10L/min). Thedraft BerlinDefinitionwas empirically evaluatedusingpatient-levelmeta-analysis of 4188 patientswith ARDS from4multicenter clinical datasets and269patientswithARDS from3single-center data sets containingphysi-ologic information. The 4 ancillary variables did not contribute to the predic-tive validity of severe ARDS for mortality and were removed from the defini-tion. Using the Berlin Definition, stages of mild, moderate, and severe ARDSwere associatedwith increasedmortality (27%;95%CI, 24%-30%;32%;95%CI, 29%-34%; and 45%; 95% CI, 42%-48%, respectively; P .001) and in-creased median duration of mechanical ventilation in survivors (5 days; inter-quartile [IQR], 2-11; 7 days; IQR, 4-14; and 9 days; IQR, 5-17, respectively;P .001). Compared with the AECC definition, the final Berlin Definition hadbetter predictive validity for mortality, with an area under the receiver operat-ing curve of 0.577 (95% CI, 0.561-0.593) vs 0.536 (95% CI, 0.520-0.553;P.001). This updated and revisedBerlinDefinition forARDSaddresses a num-ber of the limitations of the AECC definition. The approach of combining con-sensus discussions with empirical evaluation may serve as a model to createmore accurate, evidence-based, critical illness syndrome definitions and to bet-ter inform clinical care, research, and health services planning.JAMA. 2012;307(23):2526-2533Published online May 21, 2012. doi:10.1001/jama.2012.5669 www.jama.com

For editorial comment see p 2542.

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For these reasons, and because alldisease definitions should be re-viewed periodically, the European So-ciety of Intensive Care Medicine con-vened an international expert panel torevise the ARDS definition, with en-dorsement from the American Tho-racic Society and the Society of Criti-cal Care Medicine. The objectives wereto update the definition using new data(epidemiological, physiological, andclinical trials) to address the currentlimitations of the AECC definition andexplore other defining variables.

MethodsConsensus Process. Three co-chairswere appointed by the European Soci-ety of Intensive Care Medicine, who inturn selected panelists based on theirwork in the area of ARDS and to ensuregeographic representation from both Eu-rope and North America. An overviewof the consensus process used by thepanel is outlined in the FIGURE. In re-vising the definition of ARDS, the panelemphasized feasibility, reliability, facevalidity (ie, how clinicians recognizeARDS), and predictive validity (ie, abil-ity to predict response to therapy, out-comes, or both). In addition, the paneldetermined that any revision of the defi-nition should be compatible with theAECC definition to facilitate interpre-tation of previous studies. After initialpreparations and an in-person consen-sus discussion, a draft definition wasproposed,13 which underwent empiri-cal evaluation. The definition was fur-ther refined through consensus discus-sion informed by these empirical data.Empirical Evaluation of Draft

Definition.Cohort Assembly. Through the reviewof the literature presented at the con-sensus meeting, discussions with otherexperts, and review of personal files, thepanel identified studies that met the fol-lowing eligibility criteria: (1) large, mul-ticenter prospective cohorts, includ-ing consecutive patients or randomizedtrials, or smaller, single-center prospec-tive studies with unique radiological orphysiological data that enrolled adultpatients with ALI as defined by AECC;

(2) studies collected granular data nec-essary to apply the individual criteriaof both the draft Berlin Definition andthe AECC definition; and (3) authorsof these original studies were willing toshare data and collaborate. The panelidentified 7 distinct data sets (4 mul-ticenter clinical studies for the clinicaldatabase14-17 and 3 single-center physi-ological studies for the physiological da-tabase18-20) that met these criteria. Fur-ther details of these studies are includedin the eMethods (http://www.jama.com).

Variables. Studies provided data onhospital or 90-day mortality. Ventilator-free days at 28 days after the diagnosisof ALI were calculated as a compositemeasure of mortality and durationof mechanical ventilation. Duration ofmechanical ventilation in survivors wasselected as an indirect marker of sever-ity of lung injury because this outcomeis not biased by mortality or decisions

related to the withdrawal of life-sustaining treatments.21 Progressionof se-verity of ARDS within 7 days was as-sessed using the longitudinal datacollected within each cohort. We distin-guished patients with more extensive in-volvement on the frontal chest radio-graph (3 or 4 quadrants) from those withthe minimal criterion of bilateral opaci-ties (2 quadrants).

Static compliance of the respiratorysystem (CRS) was calculated as tidal vol-ume (mL) divided by plateau pressure(cm H2O) minus posi t ive end-expiratory pressure (PEEP) (cm H2O).The corrected expired volume per min-ute (VECORR) was calculated as the mea-sured minute ventilation multiplied bythe arterial partial pressure of carbondioxide (PaCO2) divided by 40 mm Hg.22

Total lung weight was estimated fromquantitative computed tomography(CT) images.23 Shunt was calculated atone site as previously reported.24

Table 1. The AECCDefinition3Limitations andMethods to Address These in the Berlin Definition

AECC Definition AECC LimitationsAddressed in

Berlin Definition

Timing Acute onset No definition of acute4 Acute time framespecified

ALI category All patients with PaO2/FIO2 300 mm Hg

Misinterpreted asPaO2/FIO2 = 201-300,leading to confusingALI/ARDS term

3 Mutually exclusivesubgroups ofARDS by severity

ALI term removed

Oxygenation PaO2/FIO2 300mm Hg (regard-less of PEEP)

Inconsistency of PaO2/FIO2 ratio due to theeffect of PEEP and/orFIO25-7

Minimal PEEP leveladded acrosssubgroups

FIO2 effect lessrelevant in severeARDS group

Chest radiograph Bilateral infiltrates ob-served on frontalchest radiograph

Poor interobserverreliability of chestradiographinterpretation8,9

Chest radiographcriteria clarified

Example radiographscreateda

PAWP PAWP 18 mm Hgwhen measured orno clinical evi-dence of left atrialhypertension

High PAWP and ARDSmay coexist10,11

Poor interobserverreliability of PAWP andclinical assesments ofleft atrialhypertension12

PAWP requirementremoved

Hydrostatic edemanot the primarycause ofrespiratory failure

Clinical vignettescreateda to helpexcludehydrostatic edema

Risk factor None Not formally included indefinition4

IncludedWhen none

identified, need toobjectively rule outhydrostatic edema

Abbreviations: AECC, American-European Consensus Conference; ALI, acute lung injury; ARDS, acute respiratory dis-tress syndrome; FIO2, fraction of inspired oxygen; PaO2, arterial partial pressure of oxygen; PAWP, pulmonary arterywedge pressure; PEEP, positive end-expiratory pressure.

aAvailable on request.

THE BERLIN DEFINITION OF ACUTE RESPIRATORY DISTRESS SYNDROME

2012 American Medical Association. All rights reserved. JAMA, June 20, 2012Vol 307, No. 23 2527


Analytic Framework and StatisticalMethods. The analytic framework forevaluating the draft Berlin ARDS Defi-nition was to (1) determine the distri-bution of patient characteristics acrossthe defined severity categories; (2)evaluate the value of proposed ancil-lary variables (more severe radio-graphic criterion, higher PEEP levels,static respiratory compliance, andVECORR) in defining the severe ARDSsubgroup in the draft definition; (3) de-termine the predictive validity for mor-tality of the final Berlin Definition; and(4) compare the final Berlin Defini-

tion to the AECC definition. In addi-tion, in a post hoc analysis, we soughtthresholds for CRS and VECORR thatwould identify a severe group of pa-tients with ARDS who had more than50% mortality and include more than10% of the study population.

We did not evaluate other PaO2/FIO2cutoffs or the requirement of a mini-mum PEEP level (5 cm H2O) as theywere selected by the panel using facevalidity criteria and to ensure compat-ibility with prior definitions. Simi-larly, we did not explore other vari-ables that might improve predictivevalidity, such as age and severity of non-pulmonary organ failure, because theywere not specific to the definition ofARDS.25

To compare the predictive validity ofthe AECC definition and the BerlinDefinition, we used the area under thereceiver operating curve (AUROC or Cstatistic) in logistic regression modelsof mortality with a dummy variable forthe ARDS definition categories.26 Be-cause this technique requires indepen-dent categories to create the dummyvariable and the AECC definition forARDS is a subset of ALI, we could notcompare the AECC definition as speci-fied. Therefore, we modified the AECCdefinition and divided ALI into the in-dependent categories of ALI non-ARDS (200 mm HgPaO2/FIO2300mm Hg) and ARDS alone (PaO2/FIO2200 mm Hg). Although the cat-egory of ALI non-ARDS is not explic-itly described by the AECC, it has beenused by many investigators.27,28

P values for categorical variables werecalculated with the 2 test; P values forcontinuous variables were estimatedwith the t test, Mann-Whitney, analy-sis of variance, or Kruskal-Wallis, de-pending on the distribution and num-ber of variables. The receiver operatingcurve statistical analyses were per-formed by using MedCalc for Win-dows version 12.1.4.0 (MedCalc Soft-ware) and other statistical tests wereperformed with SAS/STAT for Win-dows version 9.2 (SAS Institute Inc).Statistical significance was assessed atthe 2-sided P .05 level.

Results

Draft Consensus Definition.

The ARDSConceptualModel.The panelagreed that ARDS is a type of acute dif-fuse, inflammatory lung injury, lead-ing to increased pulmonary vascularpermeability, increased lung weight, andloss of aerated lung tissue. The clini-cal hallmarks are hypoxemia and bilat-eral radiographic opacities, associatedwith increased venous admixture, in-creased physiological dead space, anddecreased lung compliance. The mor-phological hallmark of the acute phaseis diffuse alveolar damage (ie, edema,inflammation, hyaline membrane, orhemorrhage).29

Draft Definition Criteria. Following 2days of consensus discussions, the panelproposed a draft definition with 3 mu-tually exclusive severity categories (mild,moderate, and severe) of ARDS. A set ofancillary variables was proposed to fur-ther characterize severe ARDS and thesewere explicitly specified for further em-pirical evaluation.13

Timing.Most patients with ARDS areidentified within 72 hours of recogni-tion of the underlying risk factor, withnearly all patients with ARDS identi-fied within 7 days.30 Accordingly, for apatient to be defined as having ARDS,the onset must be within 1 week of aknown clinical insult or new or wors-ening respiratory symptoms.

Chest Imaging.The panel retained bi-lateral opacities consistent with pul-monary edema on the chest radio-graph as defining criteria for ARDS, butalso explicitly recognized that thesefindings could be demonstrated on CTscan instead of chest radiograph. Moreextensive opacities (ie, 3 or 4 quad-rants on chest radiograph) were pro-posed as part of the severe ARDScategory and identified for furtherevaluation.

Origin of Edema. Given the declin-ing use of pulmonary artery cathetersand because hydrostatic edema in theform of cardiac failure or fluid over-load may coexist with ARDS,10,11 thepulmonary artery wedge pressure cri-terion was removed from the defini-

Figure. Outline of Consensus Process

Premeeting preparations(May to September 2011)

Selection of panelists by chairs

Precirculation of key topics for discussion

Preparation of background material by panelists

In-person discussions(September 30 to October 2, 2011, Berlin, Germany)

Presentations of key background material

Development of the conceptual model of ARDS

Draft of Berlin Definition based on informal consensus discussions

Empirical evaluation of draft definition(October 2011 to January 2012)

Assembling clinical and physiologic cohorts

Demonstration of patient characteristics and distribution according to definition categories

Evaluation of impact of ancillary variables for severe ARDS subgroup

Follow-up of consensus discussions and analysis(February 2012 by multiple teleconferences)

Presentation of empirical evaluation

Final definition created based on further informal consensus discussions

Decision to present the results of a post hoc higher-risk subset

Testing of predictive validity

ARDS indicates acute respiratory distress syndrome.




tion. Patients may qualify as havingARDS as long as they have respiratoryfailure not fully explained by cardiacfailure or fluid overload as judged bythe treating physician using all avail-able data. If no ARDS risk factor (eTable1) is apparent, some objective evalua-tion (eg, with echocardiography) is re-quired to help eliminate the possibil-ity of hydrostatic edema.

Oxygenation. The term acute lung in-jury as defined by the AECC was re-moved, due to the perception that clini-cians were misusing this term to refer toa subset of patients with less severe hy-poxemia rather than its intended use asan inclusive term for all patients with thesyndrome. Positive end-expiratory pres-sure can markedly affect PaO2/FIO25,6;therefore, a minimum level of PEEP (5cm H2O), which can be delivered non-invasively in mild ARDS, was includedin the draft definition of ARDS. A mini-mum PEEP level of 10 cm H2O was pro-posed and empirically evaluated for thesevere ARDS category.

Additional Physiologic Measurements.Compliance of the respiratory systemlargely reflects the degree of lung vol-ume loss.2 Increased dead space is com-mon in patients with ARDS and is asso-ciated with increased mortality.24

However, because the measurement ofdeadspace is challenging, thepanel choseminute ventilation standardized at aPaCO2 of 40 mm Hg (VECORR=minuteventilationPaCO2/40) as a surro-gate.22 The draft definition of severeARDS included the requirement of either

a low respiratory system compliance(40 mL/cm H2O), a high VECORR (10L/min), or both. These variables wereidentified for further study during theevaluation phase.

The panel considered a number ofadditional measures to improve speci-ficity and face validity for the in-creased pulmonary vascular permeabil-ity and loss of aerated lung tissue thatare the hallmarks of ARDS, includingCT scanning, and inflammatory or ge-netic markers (eTable 2). The mostcommon reasons for exclusion of thesemeasures were lack of routine avail-ability, lack of safety of the measure incritically ill patients, or a lack of dem-onstrated sensitivity, specificity, or bothfor use as a defining characteristic forARDS.Empirical Evaluation of the Draft

Definition.Patients.A total of 4188 patients in theclinical database had sufficient data toclassify as having ARDS by the AECCdefinition. Of these patients, 518 (12%)could not be classified by the draft Ber-lin Definition because PEEP was miss-ing or was less than 5 cm H2O. Pa-tients who could not be classified by thedraft Berlin Definition had a mortalityrate of 35% (95% CI, 31%-39%), a me-dian (interquartile range [IQR]) of 19(1-25) ventilator-free days, and a me-dian (IQR) duration of mechanical ven-tilation in survivors of 4 (2-8) days.These patients were excluded fromanalyses of the draft Berlin Definitionand comparisons between the AECC

definit ion and the draft Berl inDefinition.

Compared with patients from thepopulation-based cohorts, patients fromclinical trials and the academic cen-ters cohorts were younger, had more se-vere hypoxemia, and had more opaci-ties on chest radiographs. The cohortof patients from the clinical trials hadthe lowest mortality, likely reflecting theinclusion and exclusion criteria of thetrials.31 The cohort of patients from aca-demic centers had the highest mortal-ity and the lowest percentage of traumapatients, reflecting the referral popu-lation (eTable 3).

There were 269 patients in the physi-ological database with sufficient data toclassify ARDS by the AECC defini-tion, although the numbers of pa-tients in each cohort were small. Pa-tients in the Turin cohort had worsePaO2/FIO2 ratios and had higher mor-tality than the other studies (eTable 4).

Evaluation of Ancillary Variables.Thedraft Berlin Definition for severe ARDSthat included a PaO2/FIO2 of 100 mm Hgor less, chest radiograph with 3 or 4quadrants with opacities, PEEP of at least10 cm H2O, and either a CRS of 40 mL/cmH2O or less or a VECORR of at least 10L/min identified a smaller set of pa-tients with identical mortality to the sim-pler severe ARDS category of PaO2/FIO2of 100 mm Hg or less (TABLE 2). To ad-dress the possibility that the CRS andVECORR thresholds might be different inpatients with higher body weight, weevaluated weight-adjusted cutoffs for

Table 2. Exploration of Proposed Variables to Define Severe ARDSa

Severe ARDS Definition

Mild Moderate Severe

No. (%) ofPatients

% Mortality(95% CI)

No. (%) ofPatients

% Mortality(95% CI)

No. (%) ofPatients

% Mortality(95% CI)

Consensus panel draftPaO2/FIO2 100 mm Hg chest

radiograph of 3 or 4 quadrants PEEP 10 cm H2O (CRS 40 mL/cmH2O or VECORR 10 L/min)

220 (22) 27 (24-30) 2344 (64) 35 (33-36) 507 (14) 45 (40-49)b

Consensus panel finalPaO2/FIO2 100 mm Hg 220 (22) 27 (24-30) 1820 (50) 32 (29-34) 1031 (28) 45 (42-48)b,c

Abbreviations: ARDS, acute respiratory distress syndrome; CRS, compliance of the respiratory system; FIO2, fraction of inspired oxygen; PaO2, arterial partial pressure of oxygen;PEEP, positive end-expiratory pressure; VECORR, corrected expired volume per minute.

aThe moderate group includes patients with PaO2/FIO2200 mm Hg and patients with PaO2/FIO2100 mm Hg who do not meet the additional criteria for severe ARDS in the draftdefinition. All patients are receiving at least 5 cm H2O PEEP and have bilateral infiltrates on chest radiograph.

bP .001 comparing mortality across stages of ARDS (mild, moderate, severe) for draft and final definitions.cP=.97 comparing mortality in consensus draft severe ARDS to consensus final severe ARDS definitions.




these variables in one of the cohorts.There was no significant difference in thepredictive validity of the weight-adjusted criteria. The consensus panel re-viewed these results and considered thelack of evidence for predictive validity ofthese ancillary variables and their po-tential contribution to face validity andconstruct validity and decided to use thesimpler definition for severe ARDS thatrelied on oxygenation alone.The Berlin Definition.The final Ber-

lin Definition of ARDS is shown inTABLE 3. Twenty-two percent (95% CI,21%-24%) of patients met criteria formild ARDS (which is comparable withthe ALI non-ARDS category of theAECC definition; TABLE 4), 50% (95%CI, 48%-51%) of patients met criteriafor moderate ARDS, and 28% (95% CI,

27%-30%) of patients met criteria forsevere ARDS. Mortality increased withstages of ARDS from mild (27%; 95%CI, 24%-30%) to moderate (32%; 95%CI, 29%-34%) to severe (45%; 95% CI,42%-48%). Median (IQR) ventilator-free days declined with stages of ARDSfrom mild (20 [1-25] days) to moder-ate (16 [0-23] days) to severe (1 [0-20] day). Median (IQR) duration of me-chanical ventilation in survivorsincreased with stages of ARDS frommild (5 [2-11] days) to moderate (7 [4-14] days) to severe (9 [5-17] days).

Using the Berlin Definition, 29% (95%CI, 26%-32%) of patients with mildARDS at baseline progressed to moder-ate ARDS and 4% (95% CI, 3%-6%) pro-gressed to severe ARDS within 7 days;and 13% (95% CI, 11%-14%) of pa-

tients with moderate ARDS at baselineprogressed to severe ARDS within 7 days.All differences between outcome vari-ables across categories of modified AECC(ALI non-ARDS and ARDS alone) andacross categories of Berlin Definition(mild, moderate, and severe) were sta-tistically significant (P .001).

Compared with the AECC defini-tion, the final Berlin Definition had bet-ter predictive validity for mortality withan AUROC of 0.577 (95% CI, 0.561-0.593) vs 0.536 (95% CI, 0.520-0.553; P .001), with the difference inAUROC of 0.041 (95% CI, 0.030-0.050). To ensure that missing PEEPdata in one of the cohorts did not biasthe results, the regression analysis wasrepeated without this cohort andyielded similar results.

The Berlin Definition performed simi-larly in the physiological database as inthe clinical database (TABLE 5, eFigure1, and eFigure 2). Twenty-five percent(95% CI, 20%-30%) of patients metcriteria for mild ARDS, 59% (95% CI,54%-66%) of patients met criteria formoderate ARDS, and 16% (95% CI, 11%-21%) of patients met criteria for severeARDS. Mortality increased with stages ofARDS from mild (20%; 95% CI, 11%-31%) to moderate (41%; 95% CI, 33%-49%) to severe (52%; 95% CI, 36%-68%), with P=.001 for differences inmortality across stages of ARDS. Me-dian (IQR) ventilator-free days de-clined with stages of ARDS from mild

Table 3. The Berlin Definition of Acute Respiratory Distress Syndrome

Acute Respiratory Distress Syndrome

Timing Within 1 week of a known clinical insult or new or worsening respiratorysymptoms

Chest imaginga Bilateral opacitiesnot fully explained by effusions, lobar/lung collapse, ornodules

Origin of edema Respiratory failure not fully explained by cardiac failure or fluid overloadNeed objective assessment (eg, echocardiography) to exclude hydrostatic

edema if no risk factor present

OxygenationbMild 200 mm Hg PaO2/FIO2 300 mm Hg with PEEP or CPAP 5 cm H2Oc

Moderate 100 mm Hg PaO2/FIO2 200 mm Hg with PEEP 5 cm H2O

Severe PaO2/FIO2 100 mm Hg with PEEP 5 cm H2OAbbreviations: CPAP, continuous positive airway pressure; FIO2, fraction of inspired oxygen; PaO2, partial pressure of

arterial oxygen; PEEP, positive end-expiratory pressure.aChest radiograph or computed tomography scan.b If altitude is higher than 1000m, the correction factor should be calculated as follows: [PaO2/FIO2 (barometric pressure/

760)].cThis may be delivered noninvasively in the mild acute respiratory distress syndrome group.

Table 4. Predictive Validity of ARDS Definitions in the Clinical Database

Modified AECC Definitiona Berlin Definition ARDSa

ALI Non-ARDS ARDS Mild Moderate Severe

No. (%) [95% CI] of patients 1001 (24) [23-25] 3187 (76) [75-77] 819 (22) [21-24] 1820 (50) [48-51] 1031 (28) [27-30]

Progression in 7 d from mild,No. (%) [95% CI]

336 (34) [31-37] 234 (29) [26-32] 33 (4) [3-6]

Progression in 7 d from moderate,No. (%) [95% CI]

230 (13) [11-14]

Mortality, No. (%) [95% CI]b 263 (26) [23-29] 1173 (37) [35-38] 220 (27) [24-30] 575 (32) [29-34] 461 (45) [42-48]

Ventilator-free days, median (IQR)b 20 (2-25) 12 (0-22) 20 (1-25) 16 (0-23) 1 (0-20)

Duration of mechanical ventilation insurvivors, median (IQR), db

5 (2-10) 7 (4-14) 5 (2-11) 7 (4-14) 9 (5-17)

Abbreviations: AECC, American-European Consensus Conference; ALI, acute lung injury; ARDS, acute respiratory distress syndrome; FIO2, fraction of inspired oxygen; IQR, inter-quartile range; PaO2, arterial partial pressure of oxygen; PEEP, positive end-expiratory pressure.

aThe definitions are the following for ALI non-ARDS (200 mm HgPaO2/FIO2300 mm Hg, regardless of PEEP), ARDS (PaO2/FIO2200 mm Hg, regardless of PEEP), mild Ber-lin Definition (200 mm HgPaO2/FIO2300 mm Hg with PEEP 5 cm H2O), moderate Berlin Definition (100 mm HgPaO2/FIO2200 mm Hg with PEEP 5 cm H2O), andsevere Berlin Definition (PaO2/FIO2100 mm Hg with PEEP 5 cm H2O).

bComparisons of mortality, ventilator-free days, and duration of mechanical ventilation in survivors across categories of modified AECC (ALI non-ARDS and ARDS) and acrosscategories of Berlin Definition (mild, moderate, and severe) are all statistically significant (P .001).




(8.5 [0-23.5] days) to moderate (0 [0-16.5] days) to severe (0 [0-6.5] days),withP=.003 for differences in ventilator-free days across stages of ARDS. Me-dian (IQR) duration of mechanical ven-tilation in survivors increased with stagesof ARDS from mild (6.0 [3.3-20.8] days)to moderate (12.0 [5.0-19.3] days) to se-vere (19.0 [9.0-48.0] days), withP=.045for differences in duration of mechani-cal ventilation in survivors across stagesof ARDS.

Using the Berlin Definition, stages ofmild, moderate, and severe ARDS had in-creased mean lung weight by CT scan(1371 mg; 95% CI, 1268-1473; 1556 mg;95% CI, 1474-1638; and 1828 mg; 95%CI, 1573-2082; respectively) and in-creased mean shunt (21%; 95% CI, 16%-26%; 29%; 95% CI, 26%-32%; and 40%;95% CI, 31%-48%; respectively). Com-parisons of lung weight and shunt (fromthe single site providing these data)across categories of modified AECC (ALInon-ARDS and ARDS alone) and acrosscategories of Berlin Definition (mild,moderate, and severe) were statisticallysignificant (P .001) (Table 5, eFigure3, and eFigure 4).

In a post hoc analysis, combining aPaO2/FIO2 of 100 mm Hg or less witheither a Crs of 20 mL/cm H2O or less ora VECORR of at least 13 L/min identifieda higher-risk subgroup among pa-

tients with severe ARDS that included15% of the entire ARDS population andhad a mortality of 52% (95% CI, 48%-56%). Patients with severe ARDS whodid not meet the higher-risk subset cri-teria included 13% of the entire ARDSpopulation and had a mortality rate of37% (95% CI, 33%-41%). The differ-ence between the mortality of patientswith higher-risk severe ARDS and pa-tients with severe ARDS who did notmeet these criteria was statistically sig-nificant (P .001).

CommentDeveloping and disseminating formaldefinitions for clinical syndromes incritically ill patients are essential for re-search and clinical practice. Althoughprevious proposals have relied solely onthe consensus process, this is to ourknowledge the first attempt in criticalcare to link an international consen-sus panel endorsed by professional so-cieties with an empirical evaluation.

The draft Berlin Definition classi-fied patients with ARDS into 3 inde-pendent categories but relied on ancil-lary variables (severity of chestradiograph, PEEP 10 cm H2O, CRS40 mL/cm H2O, and VECORR 10L/min) in addition to oxygenation to de-fine the severe ARDS group. When theancillary variables selected by the panel

were subjected to evaluation, theseparameters did not identify a group ofpatients with higher mortality and wereexcluded from the final Berlin Defini-tion after further consensus discus-sion. Without this evaluation, a need-lessly complex ARDS definition wouldhave been proposed. However, static re-spiratory system compliance and an un-derstanding of minute ventilation areimportant variables for clinicians toconsider in managing patients withARDS, even though those variables werenot included as part of the defini-tion.32

The Berlin Definition addresses someof the limitations of the AECC defini-tion, including clarification of the ex-clusion of hydrostatic edema and add-ing minimum ventilator settings, andprovides slight improvement in pre-dictive validity. Our study presents dataon the outcomes of patients with ARDSdefined according to the Berlin Defini-tion in a large heterogeneous cohort ofpatients including patients managedwith modern approaches to lung pro-tective ventilation. Estimates of theprevalence and clinical outcomes ofmild, moderate, and severe ARDS canbe assessed from this database for re-search and health services planning.

Acute respiratory distress syndrome isa heterogeneous syndrome with com-

Table 5. Predictive Validity of ARDS Definitions in the Physiologic Database

Modified AECC Definitiona Berlin Definition ARDSa

ALI Non-ARDS ARDS Mild Moderate Severe

No. (%) [95% CI] of patients 66 (25) [19-30] 203 (75) [70-80] 66 (25) [20-30] 161 (59) [54-66] 42 (16) [11-21]

Mortality, No. (%) [95% CI]b 13 (20) [11-31] 84 (43) [36-50] 13 (20) [11-31] 62 (41) [33-49] 22 (52) [36-68]

Ventilator-free daysMedian (IQR) 8.5 (0-23.5) 0 (0-16.0) 8.5 (0-23.5) 0 (0-16.5) 0 (0-6.5)

Missing, No. 10 26 10 25 1

Duration of mechanical ventilation insurvivors, median (IQR), d

6.0 (3.3-20.8) 13.0 (5.0-25.5) 6.0 (3.3-20.8) 12.0 (5.0-19.3) 19.0 (9.0-48.0)

Lung weight, mgcMean (SD) 1371 (360.4) 1602 (508.1) 1371 (360.4) 1556 (469.7) 1828 (630.2)

Missing, No. 16 48 16 32 16

Shunt, mean (SD), %c,d 21 (21) 32 (13) 21 (12) 29 (11) 40 (16)Abbreviations: AECC, American-European Consensus Conference; ALI, acute lung injury; ARDS, acute respiratory distress syndrome; FIO2, fraction of inspired oxygen; IQR, inter-

quartile range; PaO2, arterial partial pressure of oxygen; PEEP, positive end-expiratory pressure.aThe definitions are the following for ALI non-ARDS (200 mm HgPaO2/FIO2300 mm Hg, regardless of PEEP), ARDS (PaO2/FIO2200 mm Hg, regardless of PEEP), mild Ber-

lin Definition (200 mm HgPaO2/FIO2300 mm Hg with PEEP 5 cm H2O), moderate Berlin Definition (100 mm HgPaO2/FIO2200 mm Hg with PEEP 5 cm H2O), andsevere Berlin Definition (PaO2/FIO2100 mm Hg with PEEP 5 cm H2O).

bEight patients are missing in the moderate Berlin Definition ARDS group. P=.001 for difference in mortality across Berlin stages of ARDS.cComparisons of lung weight and shunt across categories of modified AECC (ALI non-ARDS and ARDS) and across categories of Berlin Definition (mild, moderate, and severe)

are statistically significant (P .001).dOnly available at 1 site.




plex pathology and mechanisms. Theproposed definition does not resolve thisproblem. Investigators may choose to de-sign future trials using 1 or more of theARDS subgroups as a base study popu-lation, which may be further refinedusing criteria specific to the putativemechanism of action of the interven-tion (eg, IL-6 levels for an antiIL-6 trialor more stringent hypoxemia criteria fora study on extracorporeal membraneoxygenation). Furthermore, some vari-ables that were excluded from the Ber-lin Definition because of current feasi-bility and lack of data on operationalcharacteristics may become more use-ful in the future. We anticipate that clini-cal research using our model of defini-tion development will be used to revisethe definition in the future.

There are limitations to our ap-proach. First, although the Berlin Defi-nition had statistically significantly su-perior predictive validity for mortalitycompared with the modified AECC defi-nition, the magnitude of this differenceand the absolute values of the AUROCare small and would be clinically unim-portant if the Berlin Definition was de-signed as a clinical prediction tool. How-ever, predictive validity for outcome isonly one criterion for evaluating a syn-drome definition and the purpose of theBerlin Definition is not a prognostica-tion tool.33 Although the Berlin Defini-tion was developed with a frameworkincluding these criteria, we did not em-pirically evaluate face validity, contentvalidity, reliability, feasibility, or suc-cessat identifyingpatients forclinical trialenrollment.

Second, it is possible that our resultsare not generalizable because of the datasets we studied. This seems unlikely be-cause patients from a broad range ofpopulations, including clinical trials, aca-demic centers, and community pa-tients, were included in the analyses.

Third, some variables (eg, CRS andPEEP) were missing in some patients inthe data sets we used, either due to themode of mechanical ventilation that pre-cluded their measurement or the prac-ticalities of population-based research.However, bias due to cohort selection or

missing data seem unlikely because ourresults were robust to sensitivity analy-ses that excluded individual cohorts.

Fourth, it ispossible that theancillaryvariables did not identify a higher-risksubsetbecause thenumberofquadrantson the chest radiograph cannot be as-sessed reliably, PEEP was not used in apredictable fashion, or CRS and VECORRwerenotaccuratelymeasured.However,if this is true, it is likely also to be true infuture studiesand inclinicalpracticebe-causethestudydatabasewasconstructedfrom clinical trial, academic, and com-munitysitesreflectingpractice intherealworldofclinicalresearch. Inaddition,weevaluated PEEP and CRS as used by cli-nicians inpracticeandnotasa testofpre-specified ventilator settings that may bebetterthanthevariablesevaluatedherein,but may not be practical, particularly inobservational cohort studies.5,6

Fifth, because our study was not anexercise in developing a prognosticmodel for ARDS, we only consideredthe variables and cutoffs proposed bythe consensus panel. We could notcompare this definition directly to theAECC definition because the catego-ries of that definition overlap. It is pos-sible that the outcomes as well as therelative proportion of patients withineach category of ARDS will change ifthe underlying epidemiology of the syn-drome evolves due to changes in clini-cal practice or risk factors.34 This isparticularly true for the post hoc higher-risk subset reported, for which the cutpoints were derived from the data sets.

ConclusionIn conclusion, we developed a consen-sus draft definition for ARDS with an in-ternational panel using a framework thatfocused on feasibility, reliability, and va-lidity. We tested that definition using em-pirical data on clinical outcome, radio-graphic findings, and physiologicalmeasures from 2 large databases con-structed from 7 contributing sources toassess the predictive value of ancillaryvariables, refine the draft definition, andcompare the predictive validity of thedefinition to the existing AECC defini-tion. This approach for developing the

Berlin Definition for ARDS may serve asan example for linking consensus defi-nition activities with empirical researchto better inform clinical care, research,and health services planning.

Published Online: May 21, 2012. doi:10.1001/jama.2012.5669Authors/Writing Committee: V. Marco Ranieri, MD(Department of Anesthesia and Intensive CareMedi-cine, University of Turin, Turin, Italy); Gordon D.Rubenfeld, MD, MSc (Program in Trauma, Emer-gency, and Critical Care, Sunnybrook Health Sci-ences Center, and Interdepartmental Division of Criti-cal Care Medicine, University of Toronto, Toronto,Ontario, Canada); B. Taylor Thompson, MD (Depart-ment ofMedicine,Massachusetts General Hospital andHarvard Medical School, Boston); Niall D. Ferguson,MD,MSc (Department ofMedicine, University HealthNetwork and Mount Sinai Hospital, and Interdepart-mental Division of Critical Care Medicine, Universityof Toronto, Toronto,Ontario, Canada); Ellen Caldwell,MS (Division of Pulmonary and Critical Care Medi-cine, University ofWashington, Seattle); Eddy Fan,MD(Department of Medicine, University Health Net-work and Mount Sinai Hospital, University of To-ronto, Toronto, Ontario, Canada); Luigi Camporota,MD (Department of Critical Care, Guys and St.Thomas NHS Foundation Trust, Kings Health Part-ners, London, England); and Arthur S. Slutsky, MD(Keenan Research Center of the Li Ka Shing Knowl-edge Institute of St. Michaels Hospital; Interdepart-mental Division of Critical Care Medicine, Universityof Toronto, Toronto, Ontario, Canada).Author Contributions:Dr Rubenfeld andMs Caldwellhad full access to all of the data in the study and takeresponsibility for the integrity of the data and the ac-curacy of the data analysis.Study concept and design: Ranieri, Rubenfeld,Thompson, Ferguson, Caldwell, Camporota.Acquisition of data: Ranieri, Rubenfeld, Thompson.Analysis and interpretation of data: Rubenfeld,Thompson, Ferguson, Caldwell, Fan, Slutsky.Drafting of the manuscript: Rubenfeld, Ferguson,Caldwell, Slutsky.Critical revision of the manuscript for important in-tellectual content: Ranieri, Rubenfeld, Thompson,Ferguson, Caldwell, Fan, Camporota, Slutsky.Statistical analysis: Rubenfeld, Caldwell, Slutsky.Obtained funding: Ranieri.Administrative, technical, or material support:Rubenfeld, Thompson, Fan, Camporota.Study supervision: Ranieri, Rubenfeld, Thompson,Slutsky.Conflict of Interest Disclosures:All authors have com-pleted and submitted the ICMJE Form for Disclosureof Potential Conflicts of Interest. Dr Ranieri reportedreceiving consulting fees or honoraria from Maquetand Hemodec and board membership from Faron. DrRubenfeld reported receiving consulting fees or hono-raria from Ikaria, Faron, and Cerus. Dr Thompson re-ported receiving support for travel from European So-ciety of Intensive Care Medicine; being an advisoryboard member of Hemodec and AstraZeneca; receiv-ing consultancy fees from US Biotest, Sirius Genetics,sanofi-aventis, Immunetrics, Abbott, and Eli Lilly; andreceiving grants from the National Heart, Lung, andBlood Institute. Dr Slutsky reported receiving sup-port for travel expenses from European Society of In-tensive CareMedicine; boardmembership from Ikaria;receiving consultancy fees from GlaxoSmithKline andTarix; having stock/stock options with Apeiron andTarix; and sitting on advisory boards forMaquetMedi-cal and NovaLung and steering committees forHemoDec and Eli Lilly. No other authors reported anyfinancial disclosures.Members of theARDSDefinition Task Force:V.Marco




Ranieri, MD (Department of Anesthesia and Inten-sive Care Medicine, University of Turin, Turin, Italy);Gordon D. Rubenfeld, MD,MSc (Program in Trauma,Emergency, and Critical Care, Sunnybrook Health Sci-ences Center and Interdepartmental Division of Criti-cal CareMedicine, University of Toronto, Toronto,On-tario, Canada); B. Taylor Thompson,MD (DepartmentofMedicine,Massachusetts General Hospital andHar-vardMedical School, Boston); Niall D. Ferguson, MD,MSc (Department ofMedicine, University Health Net-work andMount Sinai Hospital, and Interdepartmen-tal Division of Critical CareMedicine, University of To-ronto, Toronto, Ontario, Canada); Ellen Caldwell, MS(Division of Pulmonary andCritical CareMedicine, Uni-versity of Washington, Seattle); Eddy Fan, MD (De-partment ofMedicine, University Health Network andMount Sinai Hospital, University of Toronto, Toronto,Ontario, Canada); Luigi Camporota, MD (Depart-ment of Critical Care, Guys and St. Thomas NHSFoundation Trust, Kings Health Partners, London, En-gland); and Arthur S. Slutsky, MD (Keenan ResearchCenter of the Li Ka Shing Knowledge Institute of St.Michaels Hospital; Interdepartmental Division of Criti-cal Care Medicine, University of Toronto, Toronto,Ontario, Canada); Massimo Antonelli, MD (Diparti-mento di Anestesia e Rianimazione,Universita Cattolicadel Sacro Cuore, Rome, Italy); Antonio Anzueto, MD(Department of Pulmonary/Critical Care, Universityof Texas Health Sciences Center, SanAntonio); Richard

Beale, MBBS (Department of Critical Care, Guys andSt. Thomas NHS Foundation Trust, Kings Health Part-ners, London, England); Laurent Brochard,MD (Medi-cal-Surgical Intensive Care Unit, Hopitaux Universi-taires de Geneve, Geneva, Switzerland); Roy Brower,MD (Division of Pulmonary and Critical Care Medi-cine, Johns Hopkins University, Baltimore, Mary-land); Andres Esteban, MD, PhD (Servicio de Cuida-dos Intensivos, Hospital Universitario de Getafe,CIBERES, Madrid, Spain); Luciano Gattinoni, MD (Is-tituto di Anestesiologia e Rianimazione, Universita de-gli Studi di Milano,Milan, Italy); Andrew Rhodes, MD(Department of Intensive CareMedicine, St. GeorgesHealthcare NHS Trust, London, England); Jean-LouisVincent, MD (Department of Intensive Care, ErasmeUniversity, Brussels, Belgium); Provided data for theempiric evaluation of the definition but were not partof the consensus development: Andrew Bersten, MD(Department of Critical Care Medicine, Flinders Uni-versity, Adelaide, SouthAustralia); DaleNeedham,MD,PhD (OutcomesAfter Critical Illness and SurgeryGroup[OACIS], Division of Pulmonary andCritical CareMedi-cine and Department of Physical Medicine and Re-habilitation, JohnsHopkinsUniversity, Baltimore,Mary-land); and Antonio Pesenti, MD (Department ofAnesthesia and Critical Care, Ospedale San Gerardo,Monza, Italy; and Department of Experimental Medi-cine, University of Milano Bicocca, Milan, Italy).Funding/Support: This workwas supported by the Eu-

ropean Society of Intensive Care Medicine and grantR01HL067939 from the National Institutes of Health(Dr Rubenfeld). Dr Ferguson is supported by a Cana-dian Institutes of Health Research New InvestigatorAward (Ottawa, Canada).Role of the Sponsors: The European Society of Inten-sive Care Medicine, the National Institutes of Health,the Canadian Institutes of Health Research, and the en-dorsing professional societies had no role in the designand conduct of the study, in the collection, manage-ment, analysis, and interpretation of the data, or in thepreparation, review, or approval of the manuscript.Online-Only Material: The eMethods, eReferences,eTables 1 through4, and eFigures 1 through4 are avail-able at http://www.jama.com.Additional Contributions: Salvatore Maggiore, MD,PhD (Department ofAnesthesiology and IntensiveCare,AgostinoGemelliUniversityHospital,UniversitCattolicadel SacroCuore, Rome, Italy), andAnders Larsson,MD,PhD (Department of Surgical Sciences, Anesthesiol-ogy and Critical Care Medicine, Uppsala University,Uppsala, Sweden), attended the roundtable as repre-sentatives of the European Society of Intensive CareMedicine. Drs Maggiore and Larsson received nocompensation for their roles. Karen Pickett, MB BCh(Department of Intensive Care, Erasme Hospital, Uni-versite Libre de Bruxelles, Brussels, Belgium), providedtechnical assistance. Dr Pickett received compensa-tion for her role in the conference.

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