1. What is the Horowitz P/F Ratio?

The Horowitz P/F ratio (PaO₂/FiO₂ ratio) is a measure of oxygenation used to assess the severity of acute respiratory distress syndrome (ARDS) and other lung conditions. It compares arterial oxygen partial pressure (PaO₂) to the fraction of inspired oxygen (FiO₂).

2. How Does the Calculator Work?

The calculator uses the simple P/F ratio formula:

Where:

• \( PaO_2 \) — Partial pressure of arterial oxygen (mmHg)
• \( FiO_2 \) — Fraction of inspired oxygen (decimal from 0.21 to 1.0)

Explanation: The ratio helps standardize oxygenation assessment across different oxygen delivery methods and concentrations.

3. Importance of P/F Ratio

Details: The P/F ratio is crucial for diagnosing and classifying 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 (0.21 for room air to 1.0 for 100% oxygen). Both values must be valid (PaO₂ > 0, FiO₂ between 0.21-1.0).

5. Frequently Asked Questions (FAQ)

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

Q2: How does P/F ratio differ from SpO₂/FiO₂ ratio?
A: 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: Are there limitations to P/F ratio?
A: Yes, it doesn't account for PEEP levels, mean airway pressure, or other factors affecting oxygenation.

Q5: How does altitude affect P/F ratio?
A: At higher altitudes, normal P/F ratios will be lower due to decreased atmospheric pressure.