What are the Berlin criteria for diagnosing acute respiratory distress syndrome and how are they applied?

Berlin Criteria for ARDS

Core Diagnostic Requirements

The Berlin Definition requires all four criteria to be present simultaneously: acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms, bilateral opacities on chest imaging not fully explained by effusions/nodules/collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and hypoxemia with PaO₂/FiO₂ ≤300 mmHg measured with minimum PEEP of 5 cmH₂O. 1, 2

Timing Criterion

• Onset must occur within 1 week of a known clinical insult or development of new or worsening respiratory symptoms 2, 3
• This 7-day window distinguishes ARDS from more chronic processes 4

Imaging Criterion

• Bilateral opacities must be visible on chest radiograph or CT scan 1, 2
• These opacities cannot be fully explained by pleural effusions, lobar/lung collapse, or nodules 2, 3
• A reference set of chest radiographs has been developed to enhance inter-observer reliability 3

Origin of Edema Criterion

• Respiratory failure cannot be fully explained by cardiac failure or fluid overload 1, 2
• Critical pitfall: If no clear ARDS risk factor is apparent, objective assessment (such as echocardiography) is required to exclude hydrostatic pulmonary edema 2, 3
• The pulmonary artery wedge pressure criterion from prior definitions was removed; clinical judgment now suffices when a clear risk factor exists 3, 5

Oxygenation and Severity Classification

All severity measurements require minimum PEEP of 5 cmH₂O: 1, 2

• Mild ARDS: 200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg with PEEP ≥5 cmH₂O 1, 2, 4

  • Associated with 27% mortality (95% CI, 24%-30%) 4
  • Median mechanical ventilation duration: 5 days (IQR, 2-11) in survivors 4

• Moderate ARDS: 100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg with PEEP ≥5 cmH₂O 1, 2, 4

  • Associated with 32% mortality (95% CI, 29%-34%) 4
  • Median mechanical ventilation duration: 7 days (IQR, 4-14) in survivors 4

• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg with PEEP ≥5 cmH₂O (some sources suggest PEEP ≥10 cmH₂O for severe) 1, 2, 4

  • Associated with 45% mortality (95% CI, 42%-48%) 4
  • Median mechanical ventilation duration: 9 days (IQR, 5-17) in survivors 4

Essential Diagnostic Workup

• Arterial blood gas is mandatory to determine PaO₂/FiO₂ ratio and assess for metabolic or respiratory acidosis 1
• Chest radiograph or CT showing bilateral opacities is necessary for diagnosis 1
• Echocardiography should be performed if no clear ARDS risk factor exists to exclude cardiogenic pulmonary edema and look for absence of signs suggesting fluid overload 1

Critical Pitfalls to Avoid

• Misattribution of pulmonary edema: The most common error is diagnosing ARDS when cardiac failure is actually present, leading to incorrect treatment 2
• Inadequate PEEP during measurement: PaO₂/FiO₂ ratio must be calculated with patients on at least 5 cmH₂O of PEEP; measurements without adequate PEEP are invalid 1, 6
• Heterogeneity recognition: Only a minority of patients meeting Berlin criteria actually have diffuse alveolar damage on autopsy, highlighting that the definition captures a heterogeneous syndrome 1
• Conditions mimicking ARDS: Diffuse interstitial lung diseases, diffuse pulmonary infections, and drug-induced lung injury can present identically to ARDS and may require specific treatments rather than standard ARDS management 1

Prognostic Considerations

• 24-hour reclassification is more predictive of mortality than initial values, as approximately 16% of patients demonstrate rapid resolution with significantly improved outcomes 1
• Baseline PaO₂/FiO₂ alone has poor predictive accuracy; standardized measurement at 24 hours improves prediction substantially 1
• The Berlin Definition has better predictive validity for mortality compared to the prior American-European Consensus Conference definition, with an area under the receiver operating curve of 0.577 vs 0.536 (P < .001) 4