In an adult mechanically ventilated patient with acute respiratory distress syndrome, at what PaO2/FiO2 ratio and FiO2 level should prone positioning be initiated while using lung‑protective ventilation?

Prone Positioning Threshold in ARDS

Initiate prone positioning when the PaO₂/FiO₂ ratio is <150 mmHg in mechanically ventilated ARDS patients receiving lung-protective ventilation. 1

PaO₂/FiO₂ Threshold for Proning

The most recent and authoritative guidelines establish a clear threshold:

• Strong recommendation: Begin prone positioning at PaO₂/FiO₂ <150 mmHg with moderate-quality evidence supporting mortality reduction 1
• The 2024 Intensive Care Medicine positioning guideline reaffirms this <150 mmHg threshold as the standard for initiating prone positioning 1
• Earlier Surviving Sepsis Campaign guidelines (2016) suggested a more conservative threshold of PaO₂/FiO₂ ≤100 mmHg, but this has been superseded by the broader <150 mmHg recommendation 1

FiO₂ Requirements

No specific FiO₂ threshold is mandated, but prone positioning should be considered when:

• FiO₂ ≥0.6 (60%) is required to maintain adequate oxygenation 2, 3
• PEEP ≥5 cmH₂O is being used 2, 3
• These ventilator settings should be present for 12-24 hours after ARDS onset to confirm severity before proning 2

Timing and Implementation

Early implementation is critical:

• Initiate prone positioning as soon as the PaO₂/FiO₂ <150 mmHg threshold is met rather than waiting for further deterioration 1
• The mortality benefit is strongest when proning begins within 24-48 hours of meeting severity criteria 2, 4
• Duration: Maintain prone position for at least 12 hours, preferably 16 hours per session 1

Prerequisites Before Proning

Before initiating prone positioning, ensure:

• Lung-protective ventilation is optimized: tidal volume 6 mL/kg predicted body weight, plateau pressure ≤30 cmH₂O 1, 5
• Hemodynamic stabilization with volume resuscitation completed, though vasopressor use is not a contraindication 1
• Higher PEEP strategy (≥10 cmH₂O) is employed for moderate-to-severe ARDS 1, 5

Response Assessment and Duration

Discontinuation criteria:

• Stop prone positioning if oxygenation improvement persists in supine position: PaO₂/FiO₂ ≥150 mmHg measured 4 hours after returning supine, with PEEP ≤10 cmH₂O and FiO₂ ≤0.6 1
• Discontinue after two unsuccessful positioning attempts (no oxygenation improvement) 1
• Repeat daily prone sessions until improvement criteria are met 4, 6

Evidence Strength

The recommendation is based on:

• Strong evidence from the PROSEVA trial showing 28-day mortality reduction from 32.8% to 16% (P<0.001) in severe ARDS patients with PaO₂/FiO₂ <150 mmHg 2
• Meta-analyses consistently demonstrate mortality benefit at the PaO₂/FiO₂ threshold of 100-150 mmHg 2, 7
• The 2024 guideline provides Level 1 evidence supporting prone positioning at PaO₂/FiO₂ <150 mmHg 1

Common Pitfalls to Avoid

• Do not delay proning while attempting other interventions—early implementation improves outcomes 1, 2
• Do not use incomplete prone positioning (less than 180° rotation)—complete proning has stronger oxygenation effects 1
• Do not prone for short durations—sessions <12 hours are inadequate 1
• Monitor pressure ulcer risk carefully during prone positioning, as this complication is more frequent 1, 7