1. What is the Horowitz PF Ratio?

Definition: The PF ratio (PaO₂/FiO₂ ratio) is a measure of lung function that assesses oxygenation efficiency.

Purpose: It's primarily used in critical care to evaluate respiratory failure severity and classify ARDS (Acute Respiratory Distress Syndrome).

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( PF \) — Horowitz index (mmHg/unitless)
• \( PaO_2 \) — Arterial oxygen partial pressure (mmHg)
• \( FiO_2 \) — Fraction of inspired oxygen (0.21-1)

Explanation: The ratio compares oxygen level in arterial blood (PaO₂) to the oxygen concentration being inhaled (FiO₂).

3. Clinical Significance

Interpretation:

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

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What's a normal PF ratio?
A: A normal PF ratio is typically >400 mmHg on room air (FiO₂=0.21).

Q2: How is this different from SpO₂/FiO₂ ratio?
A: The SpO₂/FiO₂ ratio uses pulse oximetry instead of arterial blood gas, but is less accurate.

Q3: When is this ratio most useful?
A: Primarily for diagnosing and monitoring ARDS in mechanically ventilated patients.

Q4: Does altitude affect the PF ratio?
A: Yes, PaO₂ decreases with altitude, so interpretation should consider elevation.

Q5: What if my patient is on supplemental oxygen?
A: The calculator works for any FiO₂ - just input the actual oxygen concentration being delivered.