What are the diagnosis criteria for Acute Respiratory Distress Syndrome (ARDS)?

ARDS Diagnostic Criteria

ARDS diagnosis requires all four components of the Berlin Definition 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

Core Diagnostic Components

Timing Requirement

• Acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms must be documented 1, 2

Imaging Criteria

• Bilateral opacities must be visible on chest radiograph or CT scan 1, 3
• These opacities cannot be fully explained by effusions, lobar/lung collapse, or nodules 1, 3
• Asymmetric or patchy focal patterns are common and do not exclude the diagnosis 3
• Standard chest radiographs are poor predictors of severity but remain necessary for diagnosis 1, 3

Oxygenation Requirement

• PaO₂/FiO₂ ratio ≤300 mmHg with minimum PEEP of 5 cmH₂O is mandatory 1
• Arterial blood gas is required to determine this ratio and assess for metabolic or respiratory acidosis 1
• The measurement must be performed while the patient is on at least 5 cmH₂O PEEP to avoid misclassification 1

Exclusion of Cardiac Causes

• Respiratory failure must not be fully explained by cardiac failure or fluid overload 1, 2
• Echocardiography should be performed if no clear ARDS risk factor exists to exclude cardiogenic pulmonary edema 1
• Look for absence of signs suggesting fluid overload (cardiomegaly, vascular congestion, pleural effusions) 3

Severity Classification

Once ARDS is diagnosed, classify severity based on degree of hypoxemia (all measured with minimum PEEP 5 cmH₂O): 1

• Mild ARDS: 200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg
• Moderate ARDS: 100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg
• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg

Essential Diagnostic Workup

Mandatory Tests

• Arterial blood gas to calculate PaO₂/FiO₂ ratio 1
• Chest radiograph or CT to document bilateral opacities 1
• Echocardiography when no clear risk factor exists or cardiac causes cannot be clinically excluded 1

Adjunctive Imaging

• Lung ultrasound demonstrates bilateral diffuse areas of reduced lung aeration with interstitial syndrome, consolidations, pleural line abnormalities, and decreased lung sliding 3
• Ultrasound is helpful for semi-quantitative evaluation of lung aeration and can guide respiratory intervention management 3
• CT has higher sensitivity than chest radiography for detecting ARDS-related changes 3

Critical Pitfalls to Avoid

Common Diagnostic Errors

• Do not exclude ARDS based on asymmetric or focal infiltrates – these patterns occur commonly and do not rule out the diagnosis 3
• Do not rely on chest radiography alone to assess severity – it correlates poorly with oxygenation defects 3
• Always measure PaO₂/FiO₂ with at least 5 cmH₂O PEEP to avoid misclassification of severity 1

ARDS Mimics Requiring Specific Treatment

• Diffuse interstitial lung diseases, diffuse pulmonary infections, and drug-induced lung injury can present identically to ARDS and fall within the syndrome definition 1
• These conditions may require specific treatments rather than standard ARDS management 1
• Only a minority of patients meeting Berlin criteria actually have diffuse alveolar damage on autopsy, highlighting the heterogeneity captured by clinical diagnostic criteria 1

Differentiating from Cardiogenic Pulmonary Edema

• Cardiogenic edema typically shows pulmonary venous congestion with increased vascular markings, vascular redistribution to upper lobes, cardiomegaly, and pleural effusions 3
• Lung ultrasound demonstrates superior sensitivity compared to chest radiographs for differentiation 3
• In cardiogenic pulmonary edema, B-lines are directly proportional to congestion severity and respond rapidly (within minutes to hours) to diuretic therapy 3
• Patchy, heterogeneous distribution with spared areas favors ARDS over cardiogenic edema 3

Prognostic Considerations

• Reclassification at 24 hours 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, while standardized measurement at 24 hours improves prediction substantially 1