PaO₂/FiO₂ (P/F) Ratio: A Quick Guide for Healthcare Providers
What is the P/F Ratio?
The PaO₂/FiO₂ (P/F) ratio measures how well the lungs transfer oxygen into the blood. It compares arterial oxygen pressure (PaO₂ from ABG) to the inspired oxygen fraction (FiO₂).
Formula:
P/F Ratio =
PaO₂ (mmHg)
FiO₂
Key Definitions:
• P/F Ratio: PaO₂/FiO₂ ratio (Horowitz Index)
• PaO₂: Arterial oxygen partial pressure (mmHg)
• FiO₂: Fraction of inspired oxygen (0.21-1.0)
• Normal300-500 mmHg
• Critical value
• PaO₂: Arterial oxygen partial pressure (mmHg)
• FiO₂: Fraction of inspired oxygen (0.21-1.0)
• Normal300-500 mmHg
• Critical value
Clinical Significance:
• Primary metric for ARDS severity
• Assesses oxygenation efficiency
• Independent of PEEP level
• Used in Berlin Criteria for ARDS diagnosis
• Guides ventilator management decisions
• Assesses oxygenation efficiency
• Independent of PEEP level
• Used in Berlin Criteria for ARDS diagnosis
• Guides ventilator management decisions
Calculation Examples:
Room air (FiO₂=0.21), PaO₂=90 mmHg:
P/F = 90/0.21 = 428 mmHg (normal)
50% O₂ (FiO₂=0.5), PaO₂=80 mmHg:
P/F = 80/0.5 = 160 mmHg (moderate ARDS)
100% O₂ (FiO₂=1.0), PaO₂=85 mmHg:
P/F = 85/1.0 = 85 mmHg (severe ARDS)
P/F = 90/0.21 = 428 mmHg (normal)
50% O₂ (FiO₂=0.5), PaO₂=80 mmHg:
P/F = 80/0.5 = 160 mmHg (moderate ARDS)
100% O₂ (FiO₂=1.0), PaO₂=85 mmHg:
P/F = 85/1.0 = 85 mmHg (severe ARDS)
ARDS Classification:
• Mild: 200-300 mmHg
• Moderate: 100-200 mmHg
• Severe: <100 mmHg
*With PEEP ≥5 cm H₂O
• Moderate: 100-200 mmHg
• Severe: <100 mmHg
*With PEEP ≥5 cm H₂O
Limitations:
• Affected by altitude (lower PaO₂ at elevation)
• Doesn’t account for PEEP effects
• Less accurate at FiO₂ <0.5
• Influenced by cardiac output
• Varies with patient position (prone vs supine)
• Doesn’t account for PEEP effects
• Less accurate at FiO₂ <0.5
• Influenced by cardiac output
• Varies with patient position (prone vs supine)
Clinical Pearls:
• More reliable than PaO₂ alone
• Best measured after 30 mins stable FiO₂
• Serial measurements track progression
• Combines with chest imaging for ARDS diagnosis
• Used in ECMO candidacy criteria
• Best measured after 30 mins stable FiO₂
• Serial measurements track progression
• Combines with chest imaging for ARDS diagnosis
• Used in ECMO candidacy criteria
Quick Reference:
• Normal: >300 mmHg | Adequate oxygenation
• Mild ARDS: 200-300 mmHg | Consider NIV
• Moderate ARDS: 100-200 mmHg | Intubation likely needed
• Severe ARDS: <100 mmHg | Consider advanced therapies
• Critical value: <60 mmHg | ECMO evaluation
• Normal: >300 mmHg | Adequate oxygenation
• Mild ARDS: 200-300 mmHg | Consider NIV
• Moderate ARDS: 100-200 mmHg | Intubation likely needed
• Severe ARDS: <100 mmHg | Consider advanced therapies
• Critical value: <60 mmHg | ECMO evaluation
How to Use the P/F Ratio
- Get an ABG (for PaO₂).
- Note the FiO₂ (0.21 for room air, 1.0 for 100% oxygen).
- Divide PaO₂ by FiO₂ (e.g., PaO₂ 60 on 50% O₂ → 60/0.5 = 120, severe ARDS).
Limitations:
- Doesn’t account for PEEP in ventilated patients.
- Less precise with non-invasive oxygen (e.g., nasal cannula).
Why It Matters
The P/F ratio helps:
✔ Diagnose ARDS severity.
✔ Guide oxygen therapy & ventilator settings.
✔ Monitor lung function in critical care.
Pro Tip: Use an online P/F Ratio Calculator for quick results!
⚠️ Disclaimer:
The content on LabTestsGuide.com is for informational and educational purposes only. We do not guarantee the accuracy, completeness, or timeliness of the information provided. Always consult qualified healthcare professionals for medical advice, diagnosis, or treatment. LabTestsGuide.com is not liable for any decisions made based on the information on this site.
