Acute Respiratory Distress Syndrome Definitions


Definitions of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) have varied over time

  • ARDS was first described by Ashbaugh and Petty in 1967 in a case series of 12 ICU patients who shared the common features of unusually persistent tachypnea and hypoxemia accompanied by opacification on chest radiographs and poor lung compliance, despite different underlying causes
  • for more than 20 years, there was no common definition of ARDS
  • inconsistent definitions led to the published prevalence in ICU ranging from 10 to 90% of patients
  • The 1994 AECC definition became globally accepted, but had limitations

The current definition is the ‘Berlin Definition’ published in 2013, which was created by a consensus panel of experts convened in 2011 (an initiative of the European Society of Intensive Care Medicine endorsed by the American Thoracic Society and the Society of Critical Care Medicine)


ARDS is an acute diffuse, inflammatory lung injury, leading to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue…[with] hypoxemia and bilateral radiographic opacities, associated with increased venous admixture, increased physiological dead space and decreased lung compliance.

Key components

  1. acute, meaning onset over 1 week or less
  2. bilateral opacities consistent with pulmonary edema must be present and may be detected on CT or chest radiograph
  3. PF ratio <300mmHg with a minimum of 5 cmH20 PEEP (or CPAP)
  4. “must not be fully explained by cardiac failure or fluid overload,” in the physician’s best estimation using available information — an “objective assessment“ (e.g. echocardiogram) should be performed in most cases if there is no clear cause such as trauma or sepsis.


  • ARDS is categorized as being mild, moderate, or severe:
 ARDS Severity  PaO2/FiO2*  Mortality** 
Mild200 – 30027%
Moderate100 – 20032%
Severe< 10045%
 *on PEEP 5+;  **observed in cohort

Changes from the 1994 AECC definition

  • the term acute lung injury was abandoned
  • measurement of the PaO2/FIO2 ratio was changed to require a specific minimum amount of PEEP
  • 3 categories of ARDS were proposed (mild, moderate, and severe) based on the PaO2/FIO2 ratio
  • Radiographic criteria were changed to improve interrater reliability
  • PCWP criterion was removed and additional clarity was added to improve the ability to exclude cardiac causes of bilateral infiltrates

Issues with the Berlin definition

  • ability to predict mortality is still poor, but slightly better (based on meta-analysis of 4188 patients): Berlin ROC AUC = 0.577 compared to 0.536 for AECC
  • 4 ancillary variables for severe ARDS were assessed but did not have additional predictive value, so were not included in the definition:
    • radiographic severity
    • respiratory system compliance (≤40 mL/cm H2O)
    • positive end-expiratory pressure (≥10 cm H2O), and
    • corrected expired volume per minute (≥10 L/min)
  • Berlin definition doesn’t include underlying aetiology and lacks a direct measure of lung injury
  • use of vasopressors at the time of diagnosis of ARDS is associated with a much higher mortality regardless of the PF ratio (not accounted for in the Berlin definition)
  • Does not allow early identification of patients who may be amenable to therapies before ARDS becomes established
  • unclear how the Berlin definition will affect diagnosis and management in the real world
  • Berlin definition still allows CXR to be used for diagnosis, which compared poorly with CT chest when studied by Figueroa-Casa et al, 2013:
    • Sensitivity 0.73; specificity, 0.70; positive and negative predictive values 0.88 and 0.47
  • The Berlin definition has low sensitivity when compared to autopsy findings:
    • Thille et al (2013) found that the Berlin Definition had a sensitivity of 89% and specificity of 63% to identify ARDS, based on autopsies of 356 patients with clinical criteria for ARDS using evidence of diffuse alveolar damage as the gold standard


Now obsolete

Four key components must be present for the diagnosis of ARDS:

  • the syndrome must present acutely
  • hypoxemia, measured as PaO2/FIO2 ratio <200 (the ratio is >450 in healthy persons)
  •  bilateral infiltrates on chest radiograph
  • cannot be due to cardiac failure (elevated left atrial pressure), as evidenced by either clinical examination or a PCWP >18 cm H2O

The AECC also introduced the concept of acute lung injury:

  • defined similarly to ARDS, except that the PaO2/FIO2 ratio needed only be <300


  • cited by thousands of papers
  • defined the entry criteria into the practice changing ARDsnet ARMA trial that led to the widespread adoption of protective lung ventilation
  • incorporated into practice bundles


  • other definitions such as the Lung Injury Score and the Delphi definition have a greater sensitivity when matched against autopsy evidence
  • acute is ill defined
  • PF ratio can be manipulated by adjusting PEEP
  • CXR interpretation is unreliable
  • PACs are rarely used
  • PCWP may oscillate above and below the cut-off and may be elevated for reasons other than heart failure
  • ALI  was used inconsistently, just PF ratio 200 to 300, or all patients <300 including ARDS?

These cons led to the development of the 2012 Berlin definition

CCC Ventilation Series

Journal articles

  • ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, Thompson BT, Ferguson ND, Caldwell E, Fan E, Camporota L, Slutsky AS. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012 Jun 20;307(23):2526-33. PMID: 22797452.
  • Angus DC. The acute respiratory distress syndrome: what’s in a name? JAMA. 2012 Jun 20;307(23):2542-4. doi: 10.1001/jama.2012.6761. PMID: 22797455.
  • Ashbaugh DG, Bigelow DB, Petty TL, Levine BE. Acute respiratory distress in adults. Lancet (London, England). 2(7511):319-23. 1967. [pubmed]
  • Figueroa-Casas. Accuracy of the chest radiograph to identify bilateral pulmonary infiltrates consistent with the diagnosis of acute respiratory distress syndrome using computed tomography as reference standard. J Crit Care 2013;  [Article Link]
  • Phillips CR. The Berlin definition: real change or the emperor’s new clothes? Crit Care. 2013 Aug 1;17(4):174. PMC4057493.
  • Thille AW, Esteban A, Fernández-Segoviano P, Rodriguez JM, Aramburu JA, Peñuelas O, Cortés-Puch I, Cardinal-Fernández P, Lorente JA, Frutos-Vivar F. Comparison of the Berlin definition for acute respiratory distress syndrome with autopsy. Am J Respir Crit Care Med. 2013 Apr 1;187(7):761-7. PMID: 23370917. [Article Link]

FOAM and web resources

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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