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Acute Respiratory Distress Syndrome (ARDS)

  • ARDS defined as
    • Respiratory distress
    • Stiff lungs – ↓compliance, resulting in ↑inflation pressures
    • CXR – infiltrates
    • Cardiac – no apparent cause of pulmonary oedema
    • Gas exchange abnormalities
      • Mild – PaO2 / FiO2 = 300-200mmHg
      • Moderate – PaO2/FiO2 = 200-100mmHg
      • Severe – PaO2/FiO2 <100mmHg

Etiology

Direct lung injury

  • Pneumonia
  • Aspiration of gastric contents

Indirect lung injury

  • Sepsis (MCC)
  • Trauma
  • Drug overdose – aspirin/heroin

Pathophysiology

  • ARDS is an early manifestation of a generalised inflammatory response with endothelial dysfunction
    • Frequently associated with MODS

Non-cardiogenic pulmonary oedema – cardinal feature of ARDS

  • First clinical sign of generalised ↑vascular permeability – neutrophils play a key role
  • Damage of pulmonary epithelium → ↓surfactant production → predisposes for alveolar collapse

Pulmonary hypertension

  • Interstitial oedema → vascular compression → mechanical obstruction of pulmonary circulation
  • Activation of coagulation cascade → thrombosis + obstruction of pulmonary microvasculature
  • ↑ANS activity → ↑catecholamines + thromboxanes → pulmonary vasoconstriction

Haemorrhagic intra-alveolar exudates

  • Exudates is rich in platelets, fibrin, fibrinogen, clotting factors – inactivates surfactant + stimulates inflam
  • Promotes hyaline membrane formation
  • Promotes fibroblast migration into alveoli

Resolution, fibrosis, repair

  • Within a few days formation of a new epithelial lining begins
  • Activated fibroblasts accumulate in the interstitial spaces – can lead to progressive interstitial fibrosis

Physiological changes

  • ↑dead space and shunts
  • ↓lung compliance – airflow limitation

Clinical features

  • Unexplained tachypnoea
  • Central cyanosis (lips and membranes) + dyspnoea – as hypoxaemia increases
  • Chest pain
  • Fine crackles

Diagnosis

  • ABGs – low oxygen
  • CXR – bilateral, diffuse infiltrates
  • Swan-Ganz catheter – measures PCWP (indirectly measures LAP)
    • Has to be <19mmHg to consider a diagnosis of ARDS

Treatment

  • Treatment underlying cause
  • Mechanical ventilation – CPAP (if PaO2 remains <8kPa then intubate and give mechanical ventilation)
  • Limit pulmonary oedema – fluid restriction, diuretics (furosemide)
  • Prone position – improves gas exchange by redistribution of lung densities and perfusion
  • Inhaled NO – increases perfusion of ventilated lung areas, reduces pulmonary hypertension
  • Prostacyclin – similar effect to NO, but better at monitoring
  • Aerosolised surfactant

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