The ARDS-defining P/F ratio cannot be calculated intraoperatively. The SpO2/FiO2 (S/F ratio) is used as a surrogate. An S/F ratio of 214 most closely corresponds to a P/F ratio of approximately:
- A 100 mmHg (severe ARDS)
- B 300 mmHg (mild ARDS threshold)
- C 200 mmHg (moderate ARDS) ✓
- D 400 mmHg (normal)
Correct answer: C. 200 mmHg (moderate ARDS)
Explanation
The validated S/F to P/F correlation (Khemani 2012) shows: S/F 235 ≈ P/F 200 (moderate ARDS cutoff) and S/F 315 ≈ P/F 300 (mild ARDS cutoff). An S/F of 214 falls below 235, placing it in moderate-to-severe ARDS territory, most closely approximating P/F ~200. These correlations allow non-invasive ARDS categorisation during ventilation when ABG is not immediately available. The relationship is not linear across the full SpO2 range and is most reliable when SpO2 is 80–97%.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.