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!
