ARDS is a clinical syndrome of acute respiratory failure characterized by hypoxemia and bilateral pulmonary infiltrates that cannot be fully accounted for by heart failure or fluid overload.
Epidemiology
Etiology
- Systemic causes
- Sepsis (most common cause), e.g., secondary to trauma, infection or peritonitis
- Trauma
- Shock
- Massive transfusion
- Acute pancreatitis
- Primary damage to the lungs
- Pneumonia
- Aspiration
- Inhaled toxins
- Drowning incidents
Tip
Sepsis is the most common cause of ARDS.
Pathophysiology
- Tissue damage (pulmonary or extrapulmonary) → release of inflammatory mediators (e.g., interleukin-1) → inflammatory reaction → migration of neutrophils into alveoli → excessive release of neutrophilic mediators (e.g., cytokines, proteases, reactive oxygen species) → injury to alveolar capillaries and endothelial cells (diffuse alveolar damage, DAD) leading to:
- Exudative phase: excess fluid in interstitium and on alveolar surface → pulmonary edema with normal pulmonary capillary wedge pressure (noncardiogenic pulmonary edema) → decreased lung compliance and respiratory distress
- Hyaline membrane formation: exudation of neutrophils and protein-rich fluid into the alveolar space → formation of alveolar hyaline membranes → impaired gas exchange → hypoxemia
- Hypoxemia → compensation through hyperventilation → respiratory alkalosis
- Hypoxemia → chronic hypoxic pulmonary vasoconstriction → pulmonary hypertension and right-to-left pulmonary shunt (increased shunt fraction)
- Damage to type I and type II pneumocytes → decrease in surfactant → alveolar collapse → intrapulmonary shunting
- Organizing phase (late stage): proliferation of type II pneumocytes and infiltration of fibroblasts → progressive interstitial fibrosis
Clinical features
- Acute dyspnea
- Tachypnea and tachycardia
- Cyanosis
- Diffuse crackles
Diagnostics
Berlin criteria for ARDS
- 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)
- 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.
Mnemonic
ARDS diagnostic criteria include: Abnormal x-ray, Respiratory failure < 1 week after a known or suspected trigger, Decreased PaO2/FiO2, Should exclude CHF or fluid overload as a potential cause of respiratory distress.