BravoCalc

PF Ratio Calculator

Calculate the PaO₂/FiO₂ ratio for respiratory assessment, ARDS diagnosis, and critical care monitoring. Essential tool for evaluating oxygenation efficiency and lung function.

PF Ratio Calculator
Calculate PaO₂/FiO₂ ratio for respiratory assessment and ARDS classification

Arterial Oxygen Pressure

Normal range: 80-100 mmHg on room air

Measurement Notes:

  • • Obtain from arterial blood gas (ABG)
  • • Wait 20-30 min after FiO₂ changes
  • • Ensure patient stability
  • • Correct for temperature if needed

Fraction of Inspired Oxygen

Room air = 21%

Common FiO₂ Values:

Room air: 21%
Nasal cannula: 24-40%
Simple mask: 35-50%
Non-rebreather: 60-90%

PF Ratio Reference Ranges

≥400
Normal
300-399
Mild Hypoxemia
200-299
Mild ARDS
100-199
Moderate ARDS
<100
Severe ARDS
What is the PF Ratio?
Understanding the PaO₂/FiO₂ ratio and its clinical significance

The PF ratio, also known as the PaO₂/FiO₂ ratio or P/F ratio, is a critical measurement used to assess the efficiency of oxygen exchange in the lungs. It represents the ratio of arterial oxygen partial pressure (PaO₂) to the fraction of inspired oxygen (FiO₂), providing a standardized way to evaluate oxygenation independent of the oxygen concentration being administered.

Formula Components

  • PaO₂: Partial pressure of oxygen in arterial blood (mmHg)
  • FiO₂: Fraction of inspired oxygen (0.21-1.0 or 21%-100%)
  • Normal Range: 400-500 mmHg in healthy individuals
  • Units: Always expressed in mmHg

Clinical Applications

  • • ARDS diagnosis and severity classification
  • • Ventilator weaning assessment
  • • Monitoring respiratory therapy effectiveness
  • • ICU patient oxygenation status

Calculation Formula

PF Ratio = PaO₂ (mmHg) ÷ FiO₂ (decimal)

Example: PaO₂ = 80 mmHg, FiO₂ = 0.4 (40%) → PF Ratio = 80 ÷ 0.4 = 200 mmHg

The PF ratio is particularly valuable because it normalizes oxygenation measurements across different oxygen delivery methods and concentrations, making it an essential tool for comparing respiratory function between patients and over time.

ARDS Classification by PF Ratio
Berlin Definition criteria for Acute Respiratory Distress Syndrome

Mild ARDS

PF Ratio: 200-300 mmHg

Clinical Characteristics:

  • • Moderate oxygenation impairment
  • • Often manageable with non-invasive ventilation
  • • Lower mortality risk (27%)
  • • Shorter ICU stay typically

Management Approach:

  • • PEEP optimization (5-10 cmH₂O)
  • • Conservative fluid management
  • • Prone positioning if indicated
  • • Close monitoring for progression

Moderate ARDS

PF Ratio: 100-200 mmHg

Clinical Characteristics:

  • • Significant oxygenation deficit
  • • Usually requires mechanical ventilation
  • • Moderate mortality risk (32%)
  • • Extended ICU management needed

Management Approach:

  • • Higher PEEP levels (10-15 cmH₂O)
  • • Lung protective ventilation
  • • Prone positioning strongly considered
  • • Neuromuscular blockade if needed

Severe ARDS

PF Ratio: <100 mmHg

Clinical Characteristics:

  • • Severe oxygenation failure
  • • Life-threatening condition
  • • High mortality risk (45%)
  • • Prolonged mechanical ventilation

Management Approach:

  • • Aggressive PEEP strategy (15+ cmH₂O)
  • • Prone positioning mandatory
  • • Consider ECMO if available
  • • Rescue therapies (inhaled NO, etc.)
Clinical Interpretation Guidelines
Understanding PF ratio values in clinical context

Normal to Mild Impairment

Normal
400-500 mmHg
Mild Hypoxemia
300-400 mmHg
Mild ARDS
200-300 mmHg

Patients in this range typically have good prognosis with appropriate management.

Factors Affecting PF Ratio

  • Altitude: Lower atmospheric pressure affects baseline values
  • Age: Elderly patients may have lower baseline ratios
  • Positioning: Prone vs. supine can significantly impact values
  • PEEP Level: Higher PEEP generally improves PF ratio

Moderate to Severe Impairment

Moderate ARDS
100-200 mmHg
Severe ARDS
<100 mmHg
Critical
<50 mmHg

Requires immediate intensive intervention and close monitoring.

Monitoring Frequency

Stable patients:Every 6-12 hours
Moderate ARDS:Every 4-6 hours
Severe ARDS:Every 2-4 hours
Critical changes:Continuous monitoring
Measurement Considerations
Important factors for accurate PF ratio calculation and interpretation

Arterial Blood Gas (ABG) Requirements

  • Timing: ABG should be drawn 20-30 minutes after FiO₂ changes
  • Stability: Patient should be hemodynamically stable
  • Temperature: Correct for patient's body temperature
  • Sample Quality: Ensure proper arterial sample without air bubbles

FiO₂ Documentation

  • Ventilator Settings: Record exact FiO₂ from ventilator display
  • Oxygen Delivery: Note method (nasal cannula, mask, etc.)
  • Flow Rates: Document oxygen flow rates for non-ventilated patients
  • Calibration: Ensure oxygen analyzer is properly calibrated

Common Pitfalls

  • Unit Confusion: Ensure FiO₂ is in decimal form (0.21-1.0)
  • Timing Errors: Don't calculate immediately after ventilator changes
  • Mixed Venous: Ensure arterial, not venous blood gas values
  • PEEP Changes: Account for PEEP effects on oxygenation

Quality Assurance

  • Double Check: Verify calculations with another clinician
  • Trending: Compare with previous values for consistency
  • Clinical Context: Correlate with patient's clinical status
  • Documentation: Record all relevant parameters and timing
Treatment Implications
How PF ratio values guide therapeutic interventions

Ventilator Management

PEEP Optimization

  • • PF > 300: PEEP 5-8 cmH₂O
  • • PF 200-300: PEEP 8-12 cmH₂O
  • • PF 100-200: PEEP 12-16 cmH₂O
  • • PF < 100: PEEP 16+ cmH₂O

FiO₂ Titration

  • • Target SpO₂ 88-95% in ARDS
  • • Minimize FiO₂ to reduce oxygen toxicity
  • • Consider recruitment maneuvers
  • • Monitor for barotrauma

Advanced Therapies

Prone Positioning

Consider when PF < 150 mmHg for >12 hours daily

Neuromuscular Blockade

For severe ARDS (PF < 120) in first 48 hours

ECMO Consideration

When PF < 50-80 mmHg despite optimal therapy

Weaning Considerations

Readiness Criteria

  • • PF ratio > 200 mmHg
  • • PEEP ≤ 8 cmH₂O
  • • FiO₂ ≤ 0.4
  • • Hemodynamic stability

Monitoring During Weaning

  • • Continuous SpO₂ monitoring
  • • Regular ABG assessment
  • • Watch for deterioration
  • • Patient comfort evaluation
Important Medical Disclaimers
Critical considerations for clinical use

Limitations

  • • Does not account for ventilation-perfusion mismatch
  • • May be affected by cardiac output changes
  • • Altitude and atmospheric pressure variations
  • • Patient positioning effects
  • • Measurement timing considerations

Emergency Situations

  • • Rapidly changing clinical status
  • • Hemodynamic instability
  • • Equipment malfunction concerns
  • • Sample collection difficulties
  • • Time-sensitive interventions needed