1. What is the P/F Ratio?

Definition: The P/F ratio (PaO₂/FiO₂ ratio) is a measure of arterial oxygen partial pressure to fractional inspired oxygen concentration, used to assess lung function and oxygenation status.

Purpose: It helps clinicians evaluate respiratory function, particularly in critical care settings and for diagnosing acute respiratory distress syndrome (ARDS).

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( P/F \) — PaO₂ to FiO₂ ratio (mmHg)
• \( PaO₂ \) — Arterial oxygen partial pressure (mmHg)
• \( FiO₂ \) — Fraction of inspired oxygen (0.21-1.0)

Explanation: The ratio compares the oxygen level in arterial blood to the oxygen concentration being inhaled.

3. Clinical Significance of P/F Ratio

Details: The P/F ratio is a key indicator in respiratory assessment:

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

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: Why is the P/F ratio important in ARDS diagnosis?
A: It provides an oxygenation index that's independent of ventilator settings, making it useful for standardized assessment.

Q2: What's the difference between P/F ratio and SpO₂/FiO₂ ratio?
A: P/F uses direct PaO₂ measurement (more accurate), while SpO₂/FiO₂ uses pulse oximetry (less invasive but less precise).

Q3: When should I use this calculation?
A: When assessing patients with respiratory distress, monitoring ARDS progression, or evaluating response to oxygen therapy.

Q4: How does PEEP affect the P/F ratio?
A: Positive end-expiratory pressure (PEEP) can improve oxygenation and thus the P/F ratio, which should be considered when interpreting results.

Q5: What are limitations of the P/F ratio?
A: It doesn't account for factors like cardiac output, hemoglobin levels, or oxygen consumption that also affect oxygenation.