What is the PF Ratio?
The PF ratio, also known as the PaO2/FiO2 ratio or P/F ratio, is a critical measurement used in medicine to assess how well the lungs are functioning. It compares the amount of oxygen in the arterial blood (PaO2) to the concentration of oxygen being breathed in (FiO2). This ratio is particularly important for diagnosing and monitoring acute respiratory distress syndrome (ARDS) and other conditions affecting lung function.
A healthy individual breathing room air typically has a PF ratio between 400 and 500 mmHg. Lower values indicate impaired oxygen transfer from the lungs to the blood, which can signal serious respiratory conditions requiring immediate medical attention.
How to Calculate PF Ratio
The PF ratio is calculated using a simple division:
[\text{PF Ratio} = \frac{\text{PaO}_2}{\text{FiO}_2}]
Where:
- PaO2 (Partial Pressure of Arterial Oxygen) is the oxygen level in arterial blood, measured in mmHg from an arterial blood gas test
- FiO2 (Fraction of Inspired Oxygen) is the concentration of oxygen being breathed, expressed as a decimal (0.21 for room air, up to 1.0 for 100% oxygen)
The result is expressed in mmHg and indicates how efficiently the lungs are oxygenating the blood relative to the oxygen supply.
Calculation Example
Let's calculate the PF ratio for a patient on supplemental oxygen.
Patient values:
- PaO2: 80 mmHg (from arterial blood gas)
- FiO2: 0.40 (receiving 40% oxygen)
Using the formula:
[\text{PF Ratio} = \frac{80}{0.40} = 200 \text{ mmHg}]
A PF ratio of 200 mmHg indicates moderate impairment of lung function.
Clinical Interpretation
The PF ratio is used to classify the severity of ARDS according to the Berlin Definition:
- Normal: Greater than 400 mmHg
- Mild ARDS: 200-300 mmHg (with PEEP or CPAP at least 5 cmH2O)
- Moderate ARDS: 100-200 mmHg (with PEEP at least 5 cmH2O)
- Severe ARDS: Less than 100 mmHg (with PEEP at least 5 cmH2O)
Important Considerations
When interpreting PF ratios, keep these factors in mind:
- Altitude: High altitude affects normal PaO2 values and may require adjustment
- Patient positioning: Prone positioning can improve PF ratio in ARDS patients
- PEEP settings: Positive end-expiratory pressure affects oxygenation and should be documented
- Temperature: Fever can affect oxygen consumption and blood gas values
- Hemoglobin levels: Anemia can affect oxygen delivery despite normal PF ratios
The PF ratio is one component of a comprehensive respiratory assessment. Healthcare providers use it alongside other clinical findings, imaging, and laboratory values to guide treatment decisions for patients with respiratory compromise.