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

Criteria for Calculating Acute Respiratory Distress Syndrome (ARDS)

The diagnosis of ARDS is based on the Berlin Definition criteria, which requires the presence of acute hypoxemia (PaO₂/FiO₂ ≤ 300 mmHg) with a minimum requirement of 5 cmH₂O of positive end-expiratory pressure (PEEP), along with bilateral radiographic opacities not fully explained by cardiac failure or fluid overload, appearing within one week of a known clinical insult. 1

Berlin Definition Criteria for ARDS

1. Timing: Within 1 week of a known clinical insult or new/worsening respiratory symptoms 2

2. Chest Imaging: Bilateral opacities on chest radiograph or CT scan not fully explained by effusions, lung collapse, or nodules 2

   • Ultrasound can be used as an alternative imaging modality, especially in resource-limited settings 3
   • The Radiographic Assessment of Lung Edema (RALE) score can be used to quantify the extent of opacities with excellent diagnostic accuracy (cutoff ≥10 has 100% sensitivity and 71% specificity) 4

3. Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload 2

   • Note: The pulmonary artery wedge pressure criterion has been removed from the definition 5
   • If no risk factor for ARDS is apparent, objective evaluation (e.g., echocardiography) is required to rule out hydrostatic edema 5

4. Oxygenation: Hypoxemia defined by PaO₂/FiO₂ ratio ≤ 300 mmHg with PEEP ≥ 5 cmH₂O 2

   • Alternative: SpO₂/FiO₂ ≤ 315 (if SpO₂ ≤ 97%) can be used when arterial blood gas analysis is unavailable 3

ARDS Severity Classification

ARDS is categorized into three severity levels based on oxygenation:

Severity	PaO₂/FiO₂	Mortality	Recommended Management
Mild	201-300 mmHg	27%	Lower PEEP (5-10 cmH₂O) [1,2]
Moderate	101-200 mmHg	32%	Higher titrated PEEP [1,2]
Severe	≤100 mmHg	45%	Higher titrated PEEP, consider prone positioning and neuromuscular blockade [1,2]

Pathophysiological Considerations

• The principal cause of hypoxemia in ARDS is extensive right-to-left intrapulmonary shunting of blood flow 6
• Intrapulmonary shunting can exceed 25% of total cardiac output in ARDS patients, compared to <5% normally 6
• Decrements in lung compliance related to small airway and alveolar collapse are nearly universal in ARDS 6
• The pathological hallmark of ARDS is diffuse alveolar damage (DAD), which is found in 45% of patients with clinical ARDS and correlates with severity (present in 58% of severe ARDS cases) 7

Recent Updates to ARDS Definition

The most recent global definition of ARDS has expanded to include:

• High-flow nasal oxygen with a minimum flow rate of ≥30 L/min as an acceptable respiratory support method 3
• Use of pulse oximetry (SpO₂/FiO₂ ≤ 315) when arterial blood gas analysis is unavailable 3
• Ultrasound as an alternative imaging modality for identifying bilateral opacities 3
• Modified criteria for resource-limited settings that do not require specific PEEP levels, oxygen flow rates, or specific respiratory support devices 3

Clinical Implications

• Diffuse alveolar damage changes dynamically as ARDS evolves through three phases: exudative (days 1-5), fibroproliferative (days 6-10), and fibrotic (after 10 days) 6
• The diagnosis of ARDS using the Berlin Definition has better predictive validity for mortality than previous definitions 2
• Severe ARDS lasting more than 72 hours represents a more homogeneous group with a high proportion (69%) of diffuse alveolar damage on autopsy 7

By correctly applying these criteria, clinicians can accurately diagnose ARDS, determine its severity, and implement appropriate management strategies to improve patient outcomes.