Makindo Medical Notes

Related Subjects: Asthma |Acute Severe Asthma |Exacerbation of COPD |Pulmonary Embolism |Cardiogenic Pulmonary Oedema |Pneumothorax |Tension Pneumothorax |Fat embolism |Hyperventilation Syndrome |Acute Respiratory Distress Syndrome (ARDS) |Respiratory Failure |Non invasive ventilation (NIV) |Intubation and Mechanical Ventilation

ARDS must occur within 1 week of a clinical insult or worsening respiratory symptoms. Chest Imaging: Bilateral opacities should be visible on chest X-ray or CT scan. Oedema: ARDS should not be fully explained by cardiac failure or fluid overload. There is progressive hypoxia.

📖 About

• ARDS is a systemic disease with acute hypoxia and multi-organ dysfunction.
• Seen as a manifestation of trauma, sepsis, tissue damage, or other severe illnesses.
• Mild ARDS was once called “acute lung injury.”
• PaO₂ < 10 kPa on FiO₂ = 0.5 corresponds to ~20 kPa.
• May progress to a fibroproliferative phase with lung fibrosis.

🧬 Aetiology & Pathophysiology

• Type 1 respiratory failure without cardiac dysfunction.
• Loss of type II pneumocytes → ↓ surfactant.
• Protein-rich alveolar oedema due to ↑ capillary permeability.
• Neutrophil sequestration → inflammatory damage.
• Capillary leak → alveolar flooding and collapse.
• Lungs become stiff, poorly compliant → impaired gas exchange.

📝 Berlin Criteria

• Timing: Within 1 week of known insult or worsening symptoms.
• Chest imaging: Bilateral opacities not fully explained by effusions, collapse, or nodules.
• Origin of oedema: Not explained by cardiac failure or fluid overload (echo if uncertain).
• Oxygenation severity:
  • Mild: PaO₂/FiO₂ 200–300 mmHg (26.7–40 kPa) with PEEP ≥ 5 cmH₂O.
  • Moderate: PaO₂/FiO₂ 100–200 mmHg (13.3–26.7 kPa) with PEEP ≥ 5 cmH₂O.
  • Severe: PaO₂/FiO₂ < 100 mmHg (<13.3 kPa) with PEEP ≥ 5 cmH₂O.

👩‍⚕️ Clinical Features

• Occurs in context of severe illness (e.g. sepsis, trauma, pneumonia).
• Progressive breathlessness, cyanosis, basal crackles.
• Tachycardia, refractory hypoxaemia.

🧾 Causes

• Direct lung injury: burns, smoke inhalation, pneumonia, aspiration, high altitude, near drowning, O₂ toxicity, fat/air embolism.
• Indirect/systemic: sepsis, trauma, eclampsia, pancreatitis, heroin/barbiturates, transfusion-associated lung injury, cardiopulmonary bypass, malignancy.

🔍 Differentials

• Congestive cardiac failure.
• Bilateral pneumonia (VAP, community-acquired).
• Alveolar haemorrhage (e.g. Wegener’s, Goodpasture’s).
• Acute interstitial pneumonia.
• Acute eosinophilic pneumonia or hypersensitivity pneumonitis.

📊 Severity Levels

• Mild: PaO₂/FiO₂ 200–300 mmHg, mortality ~27%, managed with low tidal volume ventilation + conservative fluids.
• Moderate: PaO₂/FiO₂ 100–200 mmHg, mortality ~32%, requires higher PEEP, prone positioning.
• Severe: PaO₂/FiO₂ <100 mmHg, mortality ~45%, may need ECMO, advanced ventilation strategies.

🧪 Investigations

• ABG: PaO₂/FiO₂ ratio confirms severity (on PEEP ≥5).
• FBC: anaemia, ↑ neutrophils.
• U&E: AKI, ↑ lactate, ↑ LFTs.
• CXR/CT: bilateral opacities, ground glass changes.
• Echo: exclude LV dysfunction.
• Blood cultures: look for sepsis.
• Coagulopathy: DIC may develop (↑ APTT/PT, ↓ platelets).
• BAL: useful in unclear infection.

💊 Management

• Initial: ABCs, treat underlying cause (e.g. sepsis, trauma).
• Ventilation: Low tidal volume (4–6 mL/kg), avoid barotrauma.
• PEEP: Prevent alveolar collapse, titrate as needed.
• Prone positioning: Improves V/Q matching in moderate–severe ARDS.
• Neuromuscular blockade: Early severe ARDS to improve synchrony.
• Fluid management: Conservative strategy to reduce oedema.
• Rescue therapies: ECMO if refractory; inhaled NO as temporary bridge.
• Steroids: Sometimes used if ARDS persists beyond acute phase.

⚠️ Complications

• Pneumothorax.
• Ventilator-associated pneumonia.
• Multi-organ failure.
• Pulmonary fibrosis.
• Mortality ~30–50%.

📖 References

• De Haro et al. (2013) ARDS: prevention and early recognition. Annals Int Care, 3:11.
• Ferguson et al. (2005) Development of a clinical definition for ARDS. J Crit Care, 20:147.