ARDS and refractory hypoxaemia

Defining and diagnosing ARDS

Reviewed Nov 2026·3 references
New· views rounded down, updated weekly

ARDS is acute (≤ 1 week) hypoxaemic respiratory failure with bilateral opacities not fully explained by cardiac failure or fluid overload. The 2024 global definition adds non-intubated patients on HFNC ≥ 30 L/min, SpO2/FiO2 criteria, and lung ultrasound for resource-limited settings.

Definition
Global definition 2024: risk factor, onset within 1 week, bilateral opacities (CXR, CT or ultrasound), not fully explained by cardiac failure or fluid overload, and hypoxaemia: non-intubated P/F ≤ 300 or S/F ≤ 315 (SpO2 ≤ 97%) on HFNO ≥ 30 L/min or NIV/CPAP ≥ 5 cmH2O; intubated graded by P/F (or S/F) with PEEP ≥ 5.

At the bedside

Timed steps
  1. 1

    Risk factor

    Suspect

    Pneumonia (bacterial, viral, tropical such as leptospirosis, scrub typhus, malaria, dengue), sepsis, aspiration, pancreatitis, trauma, transfusion (TRALI), drowning, burns.

  2. 2

    Timing and imaging

    Confirm

    Onset within 1 week of insult or new/worsening symptoms; bilateral opacities on CXR/CT, or bilateral B-lines/consolidations on ultrasound by a trained operator.

  3. 3

    Cardiogenic oedema

    Exclude

    Echo (LV function, valves, filling pressures), BNP, fluid balance. ARDS can coexist with heart failure.

  4. 4

    Severity (intubated)

    Grade

    Mild P/F 201-300 (S/F 236-315); moderate 101-200 (S/F 149-235); severe ≤ 100 (S/F ≤ 148), PEEP ≥ 5.

  5. 5

    Treat it

    Find the cause

    Cultures, respiratory PCR, tropical serology (module 25), procalcitonin; CT if unclear; bronchoscopy/BAL in immunocompromised.

Global ARDS definition 2024: oxygenation categories

SettingCriterion
Non-intubatedP/F ≤ 300 or S/F ≤ 315 (SpO2 ≤ 97%) on HFNO ≥ 30 L/min or NIV/CPAP ≥ 5 cmH2O
Intubated, mild200 < P/F ≤ 300 or 235 < S/F ≤ 315
Intubated, moderate100 < P/F ≤ 200 or 148 < S/F ≤ 235
Intubated, severeP/F ≤ 100 or S/F ≤ 148
Resource-limitedS/F ≤ 315 (SpO2 ≤ 97%); no PEEP or flow minimum; ultrasound acceptable for imaging
Red flag · call your senior now
P/F < 150 on PEEP ≥ 5: prone positioning indicated (next topics).
Red flag
P/F < 80 for > 6 h despite optimisation: refer for ECMO now.
Pitfall
Calling every bilateral infiltrate ARDS without an echo.
Pitfall
Using S/F when SpO2 is 98-100% (flat part of the curve; invalid).
Pearl
Leptospirosis pulmonary haemorrhage and falciparum malaria are important ARDS causes in Indian monsoon months.

Calculators for this topic

PaO₂/FiO₂ ratio

Oxygenation ratio with ARDS severity bands (Berlin; PEEP ≥ 5 cmH₂O).

SpO₂/FiO₂ ratio

Non-invasive oxygenation ratio; valid only when SpO₂ ≤ 97%. Global ARDS definition uses S/F ≤ 315.

Oxygenation index

OI = FiO₂(%) × mean airway pressure ÷ PaO₂.

References

  1. 1

    Matthay MA, Arabi Y, Arroliga AC, et al. A new global definition of acute respiratory distress syndrome. Am J Respir Crit Care Med. 2024;209(1):37-47

  2. 2

    ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526-2533

  3. 3

    Grasselli G, Calfee CS, Camporota L, et al. ESICM guidelines on acute respiratory distress syndrome: definition, phenotyping and respiratory support strategies. Intensive Care Med. 2023;49(7):727-759

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Lung-protective ventilation and PEEP

Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.