1. What is the PF Ratio?

Definition: The PF ratio (PaO₂/FiO₂ ratio) is a measure of oxygenation used to assess the severity of hypoxemic respiratory failure.

Purpose: It helps clinicians evaluate lung function and classify acute respiratory distress syndrome (ARDS) according to the Pinson & Tang criteria.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

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

Explanation: The arterial oxygen level is divided by the fraction of inspired oxygen to normalize oxygenation across different oxygen delivery methods.

3. Clinical Significance of PF Ratio

Details: The PF ratio helps classify respiratory failure:

• Normal: >400 mmHg
• Mild ARDS: 200-300 mmHg
• Moderate ARDS: 100-200 mmHg
• Severe ARDS: <100 mmHg

4. Using the Calculator

Tips: Enter the PaO₂ value from arterial blood gas (in mmHg) and the FiO₂ (0.21 for room air up to 1.0 for 100% oxygen).

5. Frequently Asked Questions (FAQ)

Q1: What is a normal PF ratio?
A: A normal PF ratio is typically >400 mmHg when breathing room air (FiO₂ 0.21).

Q2: When is the PF ratio most useful?
A: It's most valuable for assessing oxygenation in mechanically ventilated patients and diagnosing ARDS.

Q3: What's the difference between PF ratio and SpO₂/FiO₂ ratio?
A: The PF ratio uses direct PaO₂ measurements while SpO₂/FiO₂ uses pulse oximetry, which is less accurate but non-invasive.

Q4: How does altitude affect PF ratio?
A: At higher altitudes, normal PaO₂ decreases, so expected PF ratios will be lower.

Q5: What are limitations of the PF ratio?
A: It doesn't account for PEEP levels, patient effort, or other ventilatory parameters that affect oxygenation.