ARDS is an acute, diffuse inflammatory lung injury characterised by:

• Poor oxygenation

• Difficult ventilation

• Diffuse pulmonary infiltrates

• Acute onset

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Microscopically, it is associated with:

• Capillary endothelial damage

• Diffuse alveolar damage

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Berlin Definition

1. Timing

   • Acute onset; occurs over 1 week or less​

2. Chest imaging

   • Bilateral radiographic changes consistent with pulmonary oedema​

3. Aetiology

   • Respiratory failure not fully explained by cardiac failure or fluid overload​

   • Objective assessment may be required (Eg. TTE) in patient without risk-factors for ARDS

4. Deranged oxygenation

   • Mild​

     • PF Ratio 200 - 300 with PEEP/CPAP > 5 cmH2O​

   • Moderate

     • PF Ratio 100 - 200 with PEEP/CPAP > 5 cmH2O​

   • Severe

     • PF Ratio < 100 with PEEP/CPAP > 5 cmH2O​

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Challenges in ARDS

• Oxygenation

  • Increased V/Q mismatch --> difficulty oxygenating​

  • Prolonged exposure to high levels of inspired oxygen (> 60%) may cause parenchymal lung injury and oxygen toxicity

• Lung Mechanics

  • Heterogenous lung parenchymal changes​

    • Collapsed/consolidated dependent lung with low lung compliance

    • Relatively spared non-dependent lung with normal lung compliance

  • Normal tidal volumes --> over-distension of non-dependent (normally-compliant) lung​

    • Gas flow follows path of least resistance

    • Excessive non-dependent lung volumes and pressures

      • Risk of volutrauma and barotrauma​

  • High inspiratory pressures required to ventilate collapsed/consolidated lung --> risk of barotrauma

  • Collapse / Re-expansion cycle of dependent alveoli --> risk of alveolar shear injury (atelectatrauma)

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Ventilation Strategies

• ​Protective lung ventilation

  • Low tidal volume strategy​

    • 6 - 8 ml/kg ideal body weight

  • Minimises risk of volutrauma and barotrauma to non-dependent, aerated lung

    • Aim plateau pressures < 30 cmH2O​

  • Permissive hypercapnoea

    • PCO2 will inevitably rise with low tidal volumes, but this may be accepted within certain limits​

    • Respiratory rate should be manipulated to alter minute ventilation (do not adjust tidal volume)

• Open lung ventilation

  • High PEEP approach to maintain alveolar recruitment and prevent atelectatrauma​

• FiO2 and PEEP titrated to oxygen saturation target (> 88%)

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Additional Management Strategies

• Fluid restrict

• Diuresis

• Sedation / Paralysis

• Rescue Therapies

  • Call early for help in the deteriorating patient!​

  • Rescue strategies include:

    • Recruitment manoeuvres​

    • Proning

    • Inhaled pulmonary vasodilators

    • ECMO