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Acute Respiratory Distress Syndrome (ARDS)

BASICS

  • Definition: Acute hypoxemia (within 7 days of a known insult or new/worsening symptoms) + bilateral opacities on imaging (patchy, diffuse, or homogenous) not fully explained by cardiac failure or fluid overload.
  • Diagnosis of exclusion.
  • Severity by PaO₂/FiO₂ ratio (at PEEP/CPAP ≥5 cm H₂O):
  • Mild: 200 < PaO₂/FiO₂ ≤ 300 mm Hg
  • Moderate: 100 < PaO₂/FiO₂ ≤ 200 mm Hg
  • Severe: PaO₂/FiO₂ ≤ 100 mm Hg
  • Other names: Acute lung injury, increased-permeability pulmonary edema, noncardiac pulmonary edema

EPIDEMIOLOGY

  • Incidence: 64–79/100,000 person-years (US)
  • 10–15% of ICU, ~23% of ventilated patients meet ARDS criteria
  • 25% mild, 75% moderate-severe at diagnosis
  • Female sex is a risk factor

ETIOLOGY & PATHOPHYSIOLOGY

  • Direct alveolar injury: Pneumonia, aspiration, near drowning, pulmonary contusion, inhalation injury
  • Indirect alveolar injury: Sepsis, shock, transfusion, burns, trauma, drug overdose, cardiopulmonary bypass, reperfusion
  • Phases:
  • Exudative: Alveolar macrophage activation, neutrophil influx, epithelial-endothelial barrier loss, fluid flooding, hyaline membranes, alveolar collapse
  • Proliferative: Fibroblast and alveolar epithelial repair, matrix and cellular junction formation, reabsorption of fluid

  • Fibrotic: (not always) Prolonged ventilation, fibrosis, higher mortality

  • Genetics: No single gene for clinical use

RISK FACTORS

  • Advanced age, female sex, smoking, alcohol, cardiac/vascular surgery, TBI
  • Systemic inflammation markers correlate with worse outcomes

PREVENTION

  • No proven prevention; mitigate risk factors
  • Early lung protective ventilation & sepsis management improve outcomes

ASSOCIATED CONDITIONS

  • 85% of ARDS: Pneumonia, sepsis, aspiration
  • Others: Trauma, burns, cardiac surgery, pancreatitis, inhalational injury, TRALI, shock, drug toxicity

DIAGNOSIS

HISTORY

  • Abrupt onset of respiratory distress/hypoxemia after known insult

PHYSICAL EXAM

  • Tachypnea, tachycardia (early)
  • Increased O₂ requirements
  • Decreased breath sounds, ± rales

DIFFERENTIAL

  • Bilateral pneumonia (inc. COVID-19), CHF, interstitial/airway disease, hypersensitivity pneumonitis, TB, alveolar hemorrhage, veno-occlusive disease, mitral stenosis, drug-induced lung disease

TESTS

  • Labs: CBC, CMP, ABG (hypoxemia)
  • ECG: Sinus tachycardia, nonspecific ST-T changes
  • Imaging: CXR (bilateral opacities), CT (diffuse interstitial)
  • Cultures: Blood, sputum, COVID-19 PCR
  • TTE: If cardiac failure not excluded
  • Bronchoscopy: If unclear etiology
  • Berlin Criteria: PaO₂/FiO₂ ≤ 300 + new bilateral infiltrates

Special Testing

  • Esophageal manometry to estimate pleural pressure and optimize PEEP, especially in obesity

TREATMENT

GENERAL

  • Identify/treat underlying cause
  • Lung protective ventilation: Tidal volume 6 mL/kg predicted body weight, plateau pressure ≤30 cm H₂O
  • May decrease to 4 mL/kg if plateau >30 cm H₂O
  • Permissive hypercapnia: Allowed if pH >7.30
  • PEEP: No single guideline; high PEEP may help in moderate/severe ARDS
  • Neuromuscular blockade: Early use (<48h) in moderate-severe ARDS (PaO₂/FiO₂ <150)
  • Prone positioning: For moderate-severe ARDS (PaO₂/FiO₂ <150, PEEP >5) if intubated <36h, for 16h/day, up to 28 days or until PaO₂/FiO₂ >150
  • ECMO: Reserved for severe ARDS not improving with conventional therapy

FLUID MANAGEMENT

  • Conservative strategy (target low CVP) = more ventilator-free days, shorter ICU stay

MEDICATION

  • No specific drug improves ARDS outcomes
  • Corticosteroids: Improve oxygenation/airway pressure, unclear effect on mortality; avoid starting after day 14
  • Inhaled NO, surfactant, statins, NSAIDs, antioxidants, albuterol, elastase inhibitors: Not beneficial

ADDITIONAL THERAPIES

  • High-frequency oscillation ventilation: Not recommended (increased mortality)
  • Airway pressure release ventilation: Improves oxygenation, no mortality benefit
  • Noninvasive ventilation: Not recommended in severe hypoxemia
  • Nutrition: Enteral preferred; parenteral may be harmful

PREGNANCY

  • Supportive care, consider fetal well-being, delivery, and pregnancy physiology; always involve OB

ADMISSION/ICU CARE

  • ICU management with lung protective ventilation, PEEP, prone positioning/paralysis as indicated
  • Early ECMO referral if failing initial support
  • Vasopressors for refractory shock
  • Nursing: Skin/eye/oral care, DVT/stress ulcer prophylaxis, tube suction, position changes, sedation, trach care

DISCHARGE

  • Improved underlying cause, respiratory status, oxygen requirement

ONGOING CARE

  • Monitor: Driving pressure, static compliance, daily labs
  • Imaging as needed (not daily by default)

PROGNOSIS

  • Mortality up to 45%; rises with severity (mild 35%, moderate 40%, severe 46%)
  • Long-term: Pulmonary dysfunction, decreased QoL, persistent imaging changes, neuropsychological disability

COMPLICATIONS

  • Barotrauma, nosocomial infection, delirium, line infection, DVT, GI bleed, nutrition, MODS, death

PATIENT EDUCATION

ICD-10

  • J80 Acute respiratory distress syndrome

CLINICAL PEARLS

  • ARDS = acute hypoxemia, bilateral infiltrates, PaO₂/FiO₂ ≤300
  • Treat underlying cause
  • Lung protective ventilation decreases mortality
  • Consider proning and paralytics if PaO₂/FiO₂ <150