Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is acute diffuse inflammatory lung injury causing increased vascular permeability, pulmonary oedema, low compliance and severe hypoxaemia.

Berlin definition

• Onset within 1 week of a known insult or new or worsening respiratory symptoms.
• Bilateral opacities on chest imaging not fully explained by effusions, collapse or nodules.
• Respiratory failure not fully explained by cardiac failure or fluid overload.
• Oxygenation with PEEP or CPAP of at least 5 cmH2O: mild PaO2/FiO2 200 to 300, moderate 100 to 200, severe 100 or less.
• Causes: pneumonia (commonest, including COVID 19, influenza), sepsis, aspiration, trauma, pancreatitis, transfusion related acute lung injury, burns, near drowning, drugs, inhalation injury, fat embolism.
• Incidence about 10 per 100,000 per year; around 10% of ICU admissions; mortality 35 to 45% (mild 27%, severe 45%).
• Phases: exudative, proliferative, fibrotic. Long term sequelae include weakness, cognitive impairment, post traumatic stress.

• Rapid onset dyspnoea, tachypnoea, hypoxaemia refractory to oxygen, increased work of breathing, bilateral crackles.
• Signs of precipitating condition (sepsis, pneumonia).
• Respiratory failure requiring ventilation, with low static compliance.

• Chest radiograph and CT: bilateral infiltrates, dependent consolidation, ground glass, reduced aerated lung ("baby lung").
• Arterial blood gas: low PaO2/FiO2, respiratory acidosis in later phases.
• Echocardiography/BNP to exclude cardiogenic pulmonary oedema; pulmonary artery catheter rarely.
• Identify cause: cultures, procalcitonin, viral PCR, bronchoalveolar lavage, lipase, toxicology.
• Monitoring: plateau pressure, driving pressure, compliance, lung ultrasound.

Lung protective ventilation (cornerstone)

• Tidal volume 6 mL/kg predicted body weight, plateau pressure 30 cmH2O or less, driving pressure under 15 cmH2O, respiratory rate up to 35 to avoid severe hypercapnia (pH over 7.20 tolerated).
• PEEP titrated with FiO2 (higher PEEP table in moderate to severe disease); recruitment manoeuvres selectively.
• Prone positioning for at least 12 to 16 hours daily in moderate to severe ARDS (PaO2/FiO2 under 150) reduces mortality by about 16 percentage points.
• Neuromuscular blockade (cisatracurium 15 mg bolus then 37.5 mg/h) for up to 48 hours in early severe ARDS if ventilator dyssynchrony.
• Conservative fluid strategy once shock resolved.
• Corticosteroids: dexamethasone 20 mg daily for 5 days then 10 mg daily for 5 days (DEXA ARDS), or methylprednisolone 1 mg/kg, in moderate to severe ARDS.
• VV ECMO for severe refractory hypoxaemia (PaO2/FiO2 under 80 for over 6 hours or under 50 for over 3 hours, or pH under 7.25 with PaCO2 over 60) in experienced centres.
• Inhaled nitric oxide or epoprostenol as rescue (no mortality benefit).
• Treat cause: antibiotics, source control.
• Supportive: light sedation, early mobilisation, thromboprophylaxis (enoxaparin 40 mg daily), stress ulcer prophylaxis, nutrition enterally, glycaemic control, avoid unnecessary transfusion.
• Post ICU rehabilitation, screening for PTSD and cognitive decline.