1. What is the P/F Ratio?

The PaO2/FiO2 (P/F) ratio is a clinical indicator of pulmonary gas exchange that helps assess the severity of hypoxemia and acute respiratory distress syndrome (ARDS). It compares arterial oxygen partial pressure (PaO2) to the fraction of inspired oxygen (FiO2).

2. How Does the Calculator Work?

The calculator uses the P/F ratio formula:

Where:

• \( PaO_2 \) — Arterial oxygen partial pressure (mmHg)
• \( FiO_2 \) — Fraction of inspired oxygen (0.21 for room air to 1.0 for 100% oxygen)

Explanation: The ratio normalizes the PaO2 for the amount of oxygen being delivered, allowing comparison across different oxygen delivery methods.

3. Clinical Significance

Details: The P/F ratio is used to classify ARDS severity:

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

4. Using the Calculator

Tips: Enter PaO2 in mmHg (from arterial blood gas) and FiO2 as a decimal (e.g., 0.5 for 50% oxygen). FiO2 should be between 0.21 (room air) and 1.0 (100% oxygen).

5. Frequently Asked Questions (FAQ)

Q1: What is a normal P/F ratio?
A: A normal P/F ratio is typically >300 mmHg when breathing room air (FiO2 = 0.21).

Q2: How does P/F ratio differ from SpO2/FiO2 ratio?
A: The SpO2/FiO2 ratio uses pulse oximetry instead of arterial blood gas, but is less accurate and not interchangeable with P/F ratio.

Q3: When should P/F ratio be calculated?
A: It should be calculated when assessing patients with acute hypoxemic respiratory failure or suspected ARDS.

Q4: What are limitations of the P/F ratio?
A: It doesn't account for positive end-expiratory pressure (PEEP) levels or other factors affecting oxygenation.

Q5: How does altitude affect P/F ratio?
A: At higher altitudes, normal PaO2 is lower, so P/F ratio thresholds for ARDS may need adjustment.