1. What is the PaO₂/FiO₂ (PF) Ratio?

Definition: The PF ratio is a clinical measurement used to assess pulmonary gas exchange and diagnose ARDS (Acute Respiratory Distress Syndrome).

Purpose: It helps clinicians evaluate the severity of lung injury and guide treatment decisions in critical care.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( PF \) — Oxygenation index (mmHg)
• \( PaO_2 \) — Arterial oxygen partial pressure (mmHg)
• \( FiO_2 \) — Fraction of inspired oxygen (0.21-1.0)

ARDS Classification:

• Mild ARDS: 200-300 mmHg
• Moderate ARDS: 100-200 mmHg
• Severe ARDS: ≤100 mmHg

3. Importance of PF Ratio

Clinical Significance: The PF ratio is a key diagnostic criterion for ARDS and helps monitor disease progression and response to therapy.

4. Using the Calculator

Tips: Enter the patient's PaO₂ (from arterial blood gas) and FiO₂ (oxygen concentration being delivered). FiO₂ ranges from 0.21 (room air) to 1.0 (100% oxygen).

5. Frequently Asked Questions (FAQ)

Q1: What is a normal PF ratio?
A: Normal is >300 mmHg. Values ≤300 suggest impaired oxygenation.

Q2: When is the PF ratio most useful?
A: For diagnosing and classifying ARDS severity, especially in mechanically ventilated patients.

Q3: How does PEEP affect the PF ratio?
A: Higher PEEP may improve the PF ratio by recruiting alveoli, but the Berlin definition requires calculation at PEEP ≥5 cmH₂O.

Q4: Can I use SpO₂ instead of PaO₂?
A: For non-invasive estimation, you can use the SpO₂/FiO₂ ratio, but PaO₂ is preferred for accurate ARDS diagnosis.

Q5: What other factors affect PF ratio interpretation?
A: Altitude, hemoglobin levels, acid-base status, and cardiac output can all influence the ratio.