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

Diagnostic Criteria for Acute Respiratory Distress Syndrome (ARDS)

According to the Berlin Definition, ARDS is diagnosed by the presence of four key criteria: onset within 1 week of a known clinical insult, bilateral opacities on chest imaging, respiratory failure not fully explained by cardiac failure or fluid overload, and hypoxemia with PaO2/FiO2 ≤ 300 mmHg with at least 5 cmH2O of PEEP. 1

Core Diagnostic Criteria

1. Timing:

   • Onset within 1 week of a known clinical insult or new/worsening respiratory symptoms 1

2. Imaging:

   • Bilateral opacities on chest radiography or CT scan
   • Not fully explained by effusions, lobar/lung collapse, or nodules 1
   • Note: Radiographic findings may be asymmetric or patchy in some cases 1

3. Origin of Edema:

   • Respiratory failure not fully explained by cardiac failure or fluid overload 1
   • If no risk factor for ARDS is apparent, objective evaluation (e.g., echocardiography) is required to help rule out hydrostatic edema 2

4. Oxygenation Impairment (with minimum PEEP ≥ 5 cmH2O):

   • Mild ARDS: 200 mmHg < PaO2/FiO2 ≤ 300 mmHg
   • Moderate ARDS: 100 mmHg < PaO2/FiO2 ≤ 200 mmHg
   • Severe ARDS: PaO2/FiO2 ≤ 100 mmHg 1, 3

Pathophysiological Basis

ARDS is characterized by:

• Leukocyte infiltration and immune activation
• Alveolar endothelial and epithelial injury
• Increased pulmonary vascular permeability
• Acute pulmonary edema
• Loss of aerated lung tissue 1

The pathological hallmark of ARDS is diffuse alveolar damage (DAD), which evolves through three phases:

1. Exudative phase (days 1-5): Interstitial swelling, proteinaceous alveolar edema, hemorrhage, and fibrin deposition
2. Fibroproliferative phase (days 6-10): Resolution of alveolar edema, less prominent hyaline membranes, mononuclear cell infiltration
3. Fibrotic phase (after 10 days): Fibroblast proliferation and collagen deposition 1

Predictors of Diffuse Alveolar Damage

Four markers independently associated with DAD include:

1. Duration of ARDS ≥ 3 days
2. Severity of hypoxemia (moderate to severe ARDS)
3. Increased dynamic driving pressure
4. Diffuse opacities involving all four quadrants on chest radiography 4

Clinical Considerations

• ARDS is responsible for approximately 25% of mechanically ventilated ICU patients 1
• Mortality rates remain high at 35-40%, increasing with severity of hypoxemia 1
• Common causes include pneumonia, sepsis, and other pulmonary or extrapulmonary insults 5
• Patients may experience severe dyspnea, tachypnea, and persistent hypoxemia 1

Diagnostic Pitfalls to Avoid

1. Misattribution to cardiac causes: While cardiac failure must be excluded, patients with pre-existing cardiac disease can still develop ARDS. Objective assessment (e.g., echocardiography) may be needed in unclear cases 2

2. Delayed recognition: The criteria specify onset within 1 week of a clinical insult, but early recognition is critical for appropriate management 1

3. Inadequate PEEP assessment: Hypoxemia must be assessed with at least 5 cmH2O of PEEP (or CPAP) 1

4. Overlooking radiographic patterns: While bilateral opacities are required, they may be asymmetric or patchy and can be confused with other pulmonary conditions 1

5. Failure to recognize ARDS in patients with ARDS risk factors: Between 28-33% of septic patients meet ARDS criteria when sepsis is first recognized 1

By applying these diagnostic criteria systematically, clinicians can accurately identify ARDS and initiate appropriate management strategies to improve patient outcomes.