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

Diagnostic Criteria for ARDS

ARDS is diagnosed when all four of the following criteria are met: (1) acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms, (2) bilateral pulmonary opacities on chest imaging not fully explained by effusions, lobar/lung collapse, or nodules, (3) respiratory failure not fully explained by cardiac failure or fluid overload, and (4) impaired oxygenation with PaO₂/FiO₂ ≤ 300 mmHg with minimum PEEP of 5 cmH₂O. 1, 2

Core Diagnostic Components

Timing Criterion

• Symptoms must develop within 1 week of a known clinical insult or show new or worsening respiratory symptoms 1, 2
• This temporal requirement distinguishes ARDS from chronic lung conditions 3

Imaging Criterion

• Bilateral opacities must be present on chest radiograph or CT scan 1, 2
• These opacities should be patchy and diffuse, though they may appear asymmetric or focal in early stages 4
• The infiltrates cannot be fully explained by pleural effusions, lobar/lung collapse, or nodules 2, 3

Origin of Edema Criterion

• Respiratory failure must not be fully explained by cardiac failure or fluid overload 1, 2
• If risk factors for hydrostatic edema are present, objective assessment (such as echocardiography) may be needed to exclude cardiogenic pulmonary edema 3
• This distinguishes ARDS from congestive heart failure, which typically shows signs of fluid overload including increased vascular pedicle width or cardiothoracic ratio 4

Oxygenation Criterion

• PaO₂/FiO₂ ratio ≤ 300 mmHg with minimum PEEP of 5 cmH₂O 1, 2
• The severity classification based on oxygenation is:
  • Mild ARDS: 200 < PaO₂/FiO₂ ≤ 300 mmHg 1, 5
  • Moderate ARDS: 100 < PaO₂/FiO₂ ≤ 200 mmHg 1, 5
  • Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg 1, 5

Underlying Pathophysiology

• The pathological hallmark is diffuse alveolar damage with interstitial swelling, proteinaceous alveolar edema, hemorrhage, fibrin deposition, and hyaline membrane formation 6
• The syndrome involves alveolar epithelial inflammation, airspace flooding with plasma proteins, surfactant depletion, and extensive right-to-left intrapulmonary shunting causing hypoxemia 1
• Only a minority of patients meeting clinical ARDS criteria actually show diffuse alveolar damage on pathological examination, highlighting the syndrome's heterogeneity 4

Clinical Presentation

Early Manifestations

• Patients present with severe dyspnea, tachypnea, and unremitting hypoxemia 4
• Between 28-33% of septic patients meet ARDS criteria when clinical manifestations first appear 4
• Additional findings include increased work of breathing, dead space ventilation, and decreased thoracic compliance 4

Associated Conditions

• Most cases are associated with pneumonia or sepsis as the inciting event 2
• Other risk factors include aspiration, trauma, pancreatitis, and transfusion-related acute lung injury 3

Critical Diagnostic Pitfalls

• Do not diagnose ARDS based on radiographic infiltrates alone without meeting all four criteria, as chest X-rays are poor predictors of oxygenation severity 4
• Exclude ARDS-mimics such as diffuse interstitial acute lung diseases, diffuse pulmonary infections, and drug/chemical-induced lung disease, which may require specific treatments like immunosuppressants 4
• Avoid treating simple colonization with antibiotics; tracheal aspirate cultures showing organisms do not confirm pneumonia without meeting full diagnostic criteria 4
• For patients with suspected pneumonia complicating ARDS, obtain lower respiratory tract cultures before changing antibiotics, as sterile cultures (without recent antibiotic changes) strongly suggest pneumonia is not present 4, 7
• In pediatric patients, apply the Pediatric Acute Lung Injury Consensus Conference (PALICC) criteria for P-ARDS rather than adult criteria 4