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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Acute onset (≤1 week) bilateral lung infiltrates + severe hypoxaemia NOT fully explained by cardiac failure
- Berlin criteria severity by PaO2/FiO2 ratio (P/F): mild 200–300, moderate 100–200, severe <100 mmHg
- Common causes: pneumonia (most common), sepsis, aspiration, pancreatitis, trauma, blood transfusion (TRALI)
- Management: lung-protective ventilation (low tidal volume 6 mL/kg IBW, plateau pressure <30 cmH2O)
- Prone positioning for 16+ hours/day improves survival in moderate-severe ARDS
Overview
ARDS is a syndrome of acute, diffuse, inflammatory lung injury characterised by increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue. The Berlin definition (2012) requires: acute onset within 1 week of a known clinical insult, bilateral opacities on CXR/CT not fully explained by effusions, lobar/lung collapse, or nodules, respiratory failure not fully explained by cardiac failure or fluid overload, and a P/F ratio ≤300 mmHg with PEEP ≥5 cmH2O. The pathophysiology involves an exudative phase (alveolar flooding), a proliferative phase (organisation), and a fibrotic phase.
Epidemiology
UK ICU incidence is approximately 30–40 per 100,000 adults per year. Mortality is 35–45% overall (mild 27%, moderate 32%, severe 45%). Pneumonia is the commonest precipitant. Risk factors include sepsis, aspiration, pancreatitis, major trauma, massive blood transfusion, and near-drowning. COVID-19 was a major cause of ARDS during the pandemic.
Clinical Features
Symptoms
Acute severe breathlessness (rapid onset over hours to days)
Symptoms of underlying cause (fever, cough in pneumonia; abdominal pain in pancreatitis)
Signs
Severe hypoxaemia refractory to supplemental oxygen
Tachypnoea and increased work of breathing
Bilateral crackles on auscultation
Cyanosis
Signs of the precipitating cause
Multi-organ failure in severe cases
Investigations
First-line
ABGSevere hypoxaemia — calculate P/F ratio (PaO2 in mmHg / FiO2). <300 with PEEP ≥5 = ARDS. (Convert kPa to mmHg: multiply by 7.5)
Chest X-rayBilateral diffuse opacities ("white-out") not fully explained by effusions or atelectasis
EchocardiogramExclude cardiogenic pulmonary oedema (normal LA pressure/function)
Second-line
CT thoraxBetter characterisation of infiltrates; identifies complications (pneumothorax, abscess)
Blood cultures, sputum culture, procalcitoninIdentify infectious precipitant
Serum amylase/lipaseIf pancreatitis suspected
Specialist
Bronchoalveolar lavageIf atypical infection suspected or to exclude pulmonary haemorrhage, eosinophilic pneumonia
1
Lung-protective ventilation (cornerstone)
- Low tidal volume: 6 mL/kg IDEAL body weight (NOT actual weight)
- Plateau pressure <30 cmH2O
- Moderate-high PEEP (titrate to oxygenation)
- Permissive hypercapnia is acceptable
- Target SpO2 88–95%
2
Prone positioning
- Prone for ≥16 hours/day in moderate-severe ARDS (P/F <150)
- Proven mortality benefit (PROSEVA trial)
3
Adjunctive therapies
- Conservative fluid management (avoid fluid overload)
- Neuromuscular blockade (cisatracurium) may be considered in severe ARDS for 48 hours
- Treat underlying cause aggressively (antibiotics for sepsis/pneumonia, source control)
4
Refractory ARDS
- ECMO (extracorporeal membrane oxygenation) — consider referral to an ECMO centre for severe refractory hypoxaemia
- Inhaled nitric oxide or prostacyclin as rescue therapies (limited evidence)
Complications
- Multi-organ failure: ARDS rarely occurs in isolation — renal failure, liver failure, DIC are common
- Ventilator-associated pneumonia (VAP): Secondary infection from prolonged ventilation
- Barotrauma: Pneumothorax, pneumomediastinum from positive pressure ventilation
- ICU-acquired weakness: Critical illness polyneuropathy/myopathy
- Long-term sequelae: Persistent lung function impairment, psychological morbidity (PTSD, depression, cognitive dysfunction)
UKMLA Exam Tips
- 1Berlin criteria: acute onset + bilateral infiltrates + P/F ratio ≤300 + not cardiac failure
- 2Lung-protective ventilation: 6 mL/kg IDEAL body weight, plateau <30 cmH2O — reduces mortality
- 3Prone positioning reduces mortality in moderate-severe ARDS — PROSEVA trial
- 4Most common cause = pneumonia. Most common extrapulmonary cause = sepsis
- 5TRALI (transfusion-related acute lung injury) = ARDS within 6 hours of blood transfusion
- 6P/F ratio: PaO2 of 8 kPa = 60 mmHg. On FiO2 0.4 = P/F 150 = moderate ARDS
practicetest your knowledge on ardsApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — respiratory and beyond.
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