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Anaesthesia / Critical Care Respiratory Emergency Medicine Strong — JAMA 2012 (Berlin Definition)

P/F Ratio (Horowitz Index)

PaO₂/FiO₂ ratio. Quantifies oxygenation efficiency and is the cornerstone of the Berlin ARDS Definition. Normal >400. Enter PaO₂ from ABG and FiO₂ as a percentage. ARDS also needs onset within 1 week, bilateral opacities not fully explained by effusion, collapse or nodules, respiratory failure not fully explained by cardiac failure or fluid overload, and PEEP/CPAP ≥ 5 cmH₂O — the ratio alone does not diagnose ARDS.

Arterial partial pressure of oxygen from ABG

Fraction of inspired oxygen as a percentage. Room air = 21, pure O₂ = 100

Score interpretation

P/F > 300 — Above the ARDS Range ≥ 300.01

P/F Ratio >300: Normal oxygenation. ARDS criteria not met.

→ Routine monitoring. Continue current respiratory support. No ARDS classification required.

P/F 201–300 — Mild ARDS Range 200.01–300

P/F Ratio > 200 and ≤ 300 with PEEP/CPAP ≥ 5 cmH₂O: Mild ARDS (Berlin Definition). Mortality ~27%.

→ Lung-protective ventilation: Vt 6 ml/kg IBW, plateau pressure <30 cmH₂O. Treat underlying cause (sepsis, aspiration, pneumonia). Optimise PEEP. Reassess in 12–24 hours.

P/F 101–200 — Moderate ARDS Range 100.01–200

P/F Ratio > 100 and ≤ 200 with PEEP ≥ 5 cmH₂O: Moderate ARDS (Berlin Definition). Mortality ~32%.

→ Lung-protective ventilation (Vt 6 ml/kg IBW). Higher PEEP strategy. Prone positioning for ≥16 hours/day. Neuromuscular blockade (48 hours) if severe dyssynchrony. Consider ECMO referral if deteriorating.

P/F ≤ 100 — Severe ARDS Range 0–100

P/F Ratio ≤100 with PEEP ≥5 cmH₂O: Severe ARDS (Berlin Definition). Mortality ~45%.

→ Mandatory prone positioning (≥16 hours/day). High PEEP strategy. Neuromuscular blockade. Early ECMO referral to specialist centre if P/F <80 or refractory to conventional therapy. Specialist intensivist + respiratory physician. Goals of care discussion.

Interpretation bands for the P/F Ratio. Apply clinical judgement and local guidance.

References

Related

Curated clinical cross-links plus same-class fallbacks.

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