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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/systemic and multiorgan dysfunction.
• Seen as a manifestation of trauma, sepsis, tissue damage, other severe illnesses.
• Mild ARDS was once called acute lung injury.
• If PaO₂ < 10 Kpa on FiO₂ = 0.5 then equals 20 Kpa
• Can lead to fibroproliferative phase and lung fibrosis

Aetiology

• Type 1 RF without evidence of significant cardiac dysfunction.
• Loss of Type 2 pneumocytes. Pulmonary inflammation.
• Proteinaceous alveolar oedema
• Neutrophil sequestration in pulmonary capillaries.
• Increased capillary permeability and Protein-rich pulmonary oedema.
• Capillary leakage with flooding and collapse of alveoli.
• Stiff poorly compliant lungs depleted of surfactant with severely impaired gas exchange.

Berlin Criteria and modifications to ARDS definition

Criterion	Description
Timing	ARDS must occur within 1 week of a known clinical insult (e.g., pneumonia, sepsis, trauma) or new or worsening respiratory symptoms.
Chest Imaging	Bilateral opacities must be present on a chest X-ray or CT scan. These opacities should not be fully explained by effusions, lobar/lung collapse, or nodules.
Origin of Edema	Respiratory failure should not be fully explained by cardiac failure or fluid overload. An objective assessment (e.g., echocardiography) should be done to exclude hydrostatic edema if no risk factor is present.
Oxygenation (Severity of ARDS)	Mild ARDS: PaO₂/FiO₂ ratio 200-300 mmHg (26.7-40 kPa) with PEEP or CPAP ≥ 5 cm H₂O Moderate ARDS: PaO₂/FiO₂ ratio 100-200 mmHg (13.3-26.7 kPa) with PEEP ≥ 5 cm H₂O Severe ARDS: PaO₂/FiO₂ ratio < 100 mmHg (< 13.3 kPa) with PEEP ≥ 5 cm H₂O

Clinical

• Seen in context of other severe illness and worsening PaO₂ and CXR changes.
• Breathlessness, cyanosis, basal inspiratory crackles.
• Tachycardia, severe hypoxia in the context of critical illness.

Causes

• Burns and smoke inhalation, pneumonia, pneumonitis from aspiration
• High altitude, fat or air embolism, near drowning, O₂ toxicity.
• Systemic: sepsis, trauma, eclampsia, acute pancreatitis, heroin, barbiturates
• Transfusion-associated lung injury, malignancies, cardiopulmonary bypass.

Differentials

• Congestive cardiac failure
• Bilateral pneumonia (ventilator associated or community acquired)
• Alveolar haemorrhage (WG/Goodpasture's syndrome)
• Acute interstitial pneumonia
• Acute eosinophilic pneumonia or hypersensitivity pneumonitis (need steroids)

Severity levels

Severity Level	PaO₂/FiO₂ Ratio	PEEP (cm H₂O)	Mortality Rate	Clinical Features	Management Strategies
Mild	200-300 mmHg (26.7-40 kPa)	≥ 5	~27%	Hypoxaemia with bilateral infiltrates on chest X-ray	Low tidal volume ventilation, conservative fluid management
Moderate	100-200 mmHg (13.3-26.7 kPa)	≥ 5	~32%	More severe hypoxaemia, bilateral infiltrates on chest X-ray	Low tidal volume ventilation, higher PEEP, prone positioning
Severe	< 100 mmHg (< 13.3 kPa)	≥ 5	~45%	Severe hypoxaemia, diffuse bilateral infiltrates on chest X-ray	Low tidal volume ventilation, higher PEEP, prone positioning, ECMO

Investigations

• ABG: PaO₂/FiO₂=300 mmHg (40 kPa) consistent with ARDS if obtained on PEEP or CPAP> 5 cmH₂O.
• FBC: anaemia, ↑ neutrophils.
• U&E: AKI may be seen, ↑ lactate, ↑ ↑ LFTs.
• CXR: bilateral opacities (oedema).
• Chest CT: bilateral opacities, ground glass appearance.
• Echo: normal LV function. Right heart catheter: PAWP <15-18 mmHg. Blood cultures: sepsis.
• Coagulopathy: DIC may develop with ↑ APTT, PT and ↓ platelets.
• Broncho- alveolar lavage may be useful if undefined infection.

Management

Treatment Strategy	Description	Indication/Notes
Initial	ABC, Treat cause. Death due to progressive multiorgan dysfunction.	Ventilate if needed. Consider High FiO₂ and CPAP
Low Tidal Volume Ventilation	Mechanical ventilation with a tidal volume of 4-6 mL/kg of predicted body weight.	Standard care for all patients with ARDS to minimize ventilator-induced lung injury.
Positive End-Expiratory Pressure (PEEP)	Application of PEEP to prevent alveolar collapse and improve oxygenation.	Typically set at ≥ 5 cm H2O; higher levels may be needed in severe ARDS.
Prone Positioning	Positioning the patient face down to improve ventilation-perfusion matching and oxygenation.	Recommended for patients with moderate to severe ARDS.
Neuromuscular Blockade	Use of neuromuscular blocking agents to reduce patient-ventilator asynchrony.	Considered in early severe ARDS to improve oxygenation and ventilation synchrony.
Conservative Fluid Management	Strategies to avoid fluid overload while maintaining adequate organ perfusion.	Helps reduce pulmonary edema and improve lung function.
Extracorporeal Membrane Oxygenation (ECMO)	Use of an extracorporeal circuit to oxygenate blood outside the body, allowing the lungs to rest.	Considered in severe ARDS when conventional therapies fail.
High-Frequency Oscillatory Ventilation (HFOV)	Mechanical ventilation with very small tidal volumes at a very high rate.	Not routinely used, but may be considered in select cases of severe ARDS.
Inhaled Nitric Oxide	Inhaled gas that selectively dilates pulmonary vessels to improve oxygenation.	Can be used as a rescue therapy, but benefits are limited and may not improve outcomes.
Corticosteroids	Anti-inflammatory drugs to reduce inflammation in the lungs.	May be considered in certain cases, especially if ARDS persists beyond the acute phase.

Complications

• PTX, ventilator-associated pneumonia, multiple organ failure, pulmonary fibrosis, RF. Death 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 using the Delphi technique. J Crit Care, 20:147.