PF Ratio Calculator

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p aO2 FIO2 Ratio  
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An image illustrating the step-by-step process of using a PF Ratio (PaO2/FiO2 Ratio) Calculator. The image shows a user entering their arterial oxygen pressure (PaO2) and fractional inspired oxygen concentration (FiO2) values into the calculator's input fields. The next step displays the user clicking on the 'Calculate' button and the calculator generating the PF Ratio value. This image visually guides the user on how to use the PF Ratio Calculator tool, which assists healthcare professionals in assessing lung function and oxygenation in critically ill patients.

Calculate the P/F ratio, a vital diagnostic score used to assess respiratory status quickly and accurately. The P/F ratio, derived from blood oxygen measures, quantifies ventilation efficiency and assists in clinical decision-making at the bedside or in mass casualty triage situations.

Defining the P/F Ratio

The PaO2/FiO2 ratio compares monitored patient oxygen saturation with the fractional-inspired oxygen concentration they receive from ventilation support. The ratio indicates how effectively the lungs are extracting airway oxygen to oxygenate circulation.

Higher readings signify better respiratory function and lower pulmonary impairment.

Calculating the P/F Ratio

Deriving the ratio requires:

1. Measure arterial O2 (PaO2)

2. Determine the concentration of supplied O2 (FiO2)

3. Divide PaO2 by FiO2

For example - PaO2 = 100 mmHg, FiO2 = 50% (0.5)

  • P/F Ratio = PaO2 / FiO2
    = 100 / 0.5 = 200

Easy as that!

Classifying Severity

Clinicians classify respiratory severity and prioritize interventions based on P/F stratification:

  • 400-500 = Mild impairment
  • 300-400 = Moderate impairment
  • 200-300 = Severe dysfunction
  • < 200 = Critical hypoxemia

Thus, the lower the ratio, the more supplemental O2 and ventilator support are indicated until values stabilize.

Moderate PF Ratio For ARDS

For patients with acute respiratory distress syndrome (ARDS), a moderate P/F ratio is considered:

200 - 300 mmHg

According to the Berlin Definition for classifying ARDS, this stratifies as:

  • Mild ARDS = P/F ratio 200 - 300 mmHg
  • Moderate ARDS = P/F ratio 100 - 200 mmHg
  • Severe ARDS = P/F ratio < 100 mmHg

So a P/F ratio falling between 200 - 300 mmHg correlates to a 'moderate' severity grading of lung function impairment for ARDS patients specifically.

Clinicians treat moderate ARDS cases aggressively with ventilation protocols and close monitoring, ready to escalate intervention if the P/F ratio drops below 200. However, ratios in this range indicate lung damage has not yet progressed to the dangerous 'severe' stage.

Tracking P/F this way determines ARDS severity to guide appropriate support measures.

Comparing Lung Function

The P/F ratios illuminate nuances across populations:

  • Healthy lung P/F ratios approach ~500
  • Asthmatics see temporary drops during attacks
  • COPD patients may have lower baselines
  • COVID-19 commonly causes sharp declines

The comparative analysis informs treatment modifications.

Interpreting Trends

Equally important as spot checks are analyzing directional trends:

  • Is the P/F ratio improving or worsening over hours?
  • Do increases aligned to interventions confirm efficacy?
  • At what follow-up thresholds is escalation protocol triggered?

Tracking data shapes the next actions.

Frequently Asked Questions (FAQs)

Q1: What is the PF ratio?

The PF ratio, or Pulmonary Function ratio, is a measurement used to assess lung function and respiratory health.

Q2: How is the PF ratio calculated?

The PF ratio is calculated by dividing the partial pressure of oxygen (PaO2) in arterial blood by the fraction of inspired oxygen (FiO2).

Q3: Why is the PF ratio important?

The PF ratio is important because it provides valuable information about how well the lungs are absorbing oxygen. 


In summary, the P/F ratio provides an indispensable snap diagnostic snapshot for quantifying respiratory function, gauging disease severity, and guiding oxygen therapy tactics at both individual and mass care levels when ventilation efficiency carries life-saving weight. 

Please note, that the calculator is for entertainment only. Consult a medical specialist for accurate assessments.

Read More: Horowitz Index for Lung Function (P/F Ratio)

                    How To Use Free Online PF Ratio Calculator