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

Diagnostic Criteria for Acute Respiratory Distress Syndrome (ARDS)

ARDS is diagnosed by the presence of bilateral opacities on chest radiography indicative of pulmonary edema within 1 week of a known clinical insult, in combination with impaired oxygenation (PaO₂/FiO₂ ≤ 300 mmHg) despite at least 5 cmH₂O of positive end-expiratory pressure (PEEP), where cardiac failure does not fully explain the radiographic and clinical abnormalities. 1, 2

Essential Diagnostic Criteria (Berlin Definition)

• Timing: Onset within 1 week of a known clinical insult or new/worsening respiratory symptoms 2, 3
• Chest Imaging: Bilateral opacities on chest radiography not fully explained by effusions, lung collapse, or nodules 1, 3
• Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload 1, 2
• Oxygenation: PaO₂/FiO₂ ≤ 300 mmHg with PEEP or CPAP ≥ 5 cmH₂O 2, 3

ARDS Severity Classification

ARDS is classified based on the degree of hypoxemia:

• Mild ARDS: 200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg with PEEP ≥ 5 cmH₂O 2, 3
• Moderate ARDS: 100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg with PEEP ≥ 5 cmH₂O 2, 3
• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg with PEEP ≥ 5 cmH₂O 2, 3

Clinical Presentation

• Patients typically present with severe dyspnea, tachypnea, and unremitting hypoxemia 1
• Increased work of breathing due to decreased thoracic compliance, increased airway resistance, and ventilation-perfusion mismatching 1
• Both increased physiological dead-space ventilation and intrapulmonary shunting contribute to the elevated minute ventilation required for effective CO₂ excretion 1
• Patients may also have altered mental status or extrapulmonary organ failure complicating their respiratory dysfunction 1

Differential Diagnosis Considerations

• Cardiac Failure: While the pulmonary artery wedge pressure criterion has been removed from the Berlin Definition, objective evaluation (e.g., echocardiography) is required to rule out hydrostatic edema when no clear ARDS risk factor is present 4, 3
• Pneumonia: ARDS often must be differentiated from pneumonia, though pneumonia can be both a cause and complication of ARDS 5
• Nosocomial Tracheobronchitis: Consider in patients with fever, leukocytosis, or purulent secretions without a new lung infiltrate 1

Common Pitfalls in ARDS Diagnosis

• Underrecognition: Poor reliability of some criteria may contribute to clinicians missing the diagnosis 6
• Radiographic Interpretation: Standard chest radiographs are poor predictors of the severity of oxygenation defect or clinical outcome 1
• Timing Assessment: Failure to identify the onset within one week of a clinical insult can lead to missed diagnoses 1
• Cardiac Contribution: Failing to properly assess for cardiac causes of pulmonary edema can lead to misdiagnosis 3
• PEEP Requirement: Ensuring minimum PEEP of 5 cmH₂O when assessing oxygenation is critical for accurate classification 2

Special Considerations

• For patients with ARDS who also have ARDS risk factors, at least one of three clinical criteria (fever, leukocytosis, or purulent secretions) or other signs of pneumonia (hemodynamic instability or deterioration of blood gases) should lead to more diagnostic testing 1
• Lung ultrasound may be a useful adjunct tool for diagnosis in resource-limited settings where arterial blood gas testing and chest radiography are not readily available 1
• The Berlin Definition has significantly greater predictive validity for mortality than the prior American-European Consensus Conference definition 3

By following these diagnostic criteria systematically, clinicians can accurately identify ARDS and appropriately classify its severity to guide management decisions that impact morbidity and mortality.