Diagnostics

ARDS is a diagnosis of exclusion (see the Berlin criteria for ARDS). Consider ARDS in patients with rapid-onset respiratory failure and a potential trigger.

The Berlin criteria are the criteria most commonly used to define ARDS.

All four of the following conditions must be met:

• Acute onset: respiratory failure within one week of a known predisposing factor (e.g., sepsis, pneumonia) or worsening respiratory symptoms

• Bilateral opacities (on chest x-ray or CT)

  • Similar appearance to pulmonary edema

  • Not sufficiently explained by pleural effusions, lobar or lung collapse, or nodules

• Hypoxemia: PaO2/FiO2 ≤ 300 mm Hg (measured with a minimum of 5 cm H2O PEEP) [9]

  • Mild ARDS: PaO2/FiO2 = 201–300 mm Hg

  • Moderate ARDS: PaO2/FiO2 = 101–200 mm Hg

  • Severe ARDS: PaO2/FiO2 ≤ 100 mm Hg

• Respiratory failure cannot be fully accounted for by heart failure or fluid overload.

Chest x-ray is usually sufficient for diagnosis. However, distinguishing between ARDS and CHF can be challenging. In these cases, correlation with other tests (e.g., CT chest, lung ultrasound, echocardiogram) may be useful.

• Indications: all patients suspected of having ARDS

• Acute findings (1–7 days)

  • Often normal in the first 24 hours

  • Diffuse bilateral symmetrical infiltrates

  • In severe cases: bilateral attenuations that make the lung appear white on x-ray (“white lung”)

  • Air bronchograms may be visible.

• Intermediate (8–14 days) to late (> 15 days) findings

  • Typical course: Acute features remain stable, then resolve.

  • Fibrotic course: Reticular opacities begin to appear and may become permanent.

• Findings supportive of ARDS rather than CHF

  • Predominantly peripheral opacities

  • Small or absent pleural effusions

  • No cardiomegaly or septal lines

• Indications: may be used if chest x-ray findings are insufficient or to further investigate for underlying causes or complications

• Acute findings (1–7 days)

  • Symmetrical ground-glass opacities are the most important finding.

  • Gravity-dependent density gradient

    • The lungs may appear normal in nondependent regions.

    • Dense consolidation in dependent regions

  • Bronchial dilatation may be visible.

  • Additional findings may include small pleural effusions, air bronchograms (see “Chest x-ray” above).

• Intermediate (8–14 days) to late (> 15 days) findings: a phase of stability is followed either by resolution or progressive development of fibrosis

  • Mixed findings may be seen.

  • Potential long-term persistence of ground-glass opacities

  • Cysts and bullae may develop.

• Indications: may be helpful in differentiating between cardiogenic pulmonary edema and ARDS

• Key findings

  • Bilateral B pattern

  • C pattern (consolidation)

  • Abnormal pleural line (thickening, irregular pattern, and/or alterations in lung sliding)

• Arterial blood gas

  • Hypoxemic respiratory failure (↓ PaO2) and, initially, respiratory alkalosis (↑ pH)

    • PaO2/FiO2 ≤ 300 mm Hg (see “Definition” above)

    • Increased A-a gradient

  • With disease progression, hypercapnic respiratory failure (↑ PaCO2; ↓ pH) may develop due to respiratory exhaustion.

• Underlying causes/triggers

  • CBC: leukocytosis in sepsis or pneumonia

  • Lipase: elevated in pancreatitis

  • Blood cultures: to identify bacteremia

  • Sputum gram stain and culture: to identify bacterial pneumonia

  • Advanced tests: urine antigen testing, serologic tests (see “Diagnostics” in pneumonia)

• Differential diagnoses

  • BNP: to evaluate for heart failure [11]

  • D-dimer: to evaluate for pulmonary embolism

  • Troponin: to evaluate for cardiac ischemia

• Complications: See diagnostics in acute kidney injury, sepsis, and DIC.

• ECG: Signs of STEMI, LVH, or cardiac arrhythmias may indicate CHF.

• Echocardiography: to exclude or assess the degree of heart failure

• Cardiogenic pulmonary edema

• Acute exacerbations of interstitial lung diseases

• Transfusion-related acute lung injury (TRALI)

• Transfusion-associated circulatory overload (TACO)

• See also differential diagnoses of dyspnea.