PF Ratio Explained in Respiratory Failure

Jul 15, 2024

PF Ratio Explained in Respiratory Failure

Introduction

  • The PF ratio stands for PaO2/FiO2 ratio.
    • PaO2: Partial pressure of arterial oxygen.
    • FiO2: Fraction of inspired oxygen or the percentage of oxygen delivered to the patient.
  • Used to assess the severity of hypoxemia and trend the progression of respiratory failure.

Importance of PF Ratio

  • Helps determine oxygen stability in patients.
  • Identifies patients who need intervention to avoid intubation.
  • Identifies patients who may need early intubation.

Calculation

  1. Formula: PaO2 / FiO2 (FiO2 must be in decimal form)
    • Example: Room air is 21% oxygen (0.21 FiO2).
    • Example Calculation: PaO2 of 100 / FiO2 of 0.21 = PF ratio of 476.
  2. Normal healthy PF ratio: Usually greater than 400.

PaO2 vs. SpO2

  • PaO2: Measures dissolved oxygen in the blood using an arterial blood gas test.
  • SpO2: Measures the percentage of hemoglobin bound by oxygen (pulse oximetry).
  • Relationship between PaO2 and SpO2 is represented by the oxyhemoglobin dissociation curve.
    • High PaO2 doesn’t necessarily mean higher SpO2 once at full saturation.

True Value of PF Ratio

  • Allows for assessing changes in hypoxemia without removing the patient from oxygen.

Example Calculations

  1. Patient 1: Normal Healthy
    • Room air, PaO2: 100, SpO2: 97-98%. PF ratio: 476.
  2. Patient 2: On 50% Oxygen (FiO2 = 0.5)
    • PaO2: 100, SpO2: 97%. PF ratio: 200.
    • Theoretical PaO2 on Room Air: 42 (SpO2 ~75%). Extremely hypoxic.
  3. Patient 3: On 30% Oxygen (FiO2 = 0.3)
    • PaO2: 100, SpO2: 97%. PF ratio: 333.
    • Theoretical PaO2 on Room Air: 70 (SpO2 ~93-94%). Okay status.
  4. Patient 4: On 100% Oxygen (FiO2 = 1.0)
    • PaO2: 100, SpO2: 97%. PF ratio: 100.
    • Theoretical PaO2 on Room Air: 21. Very critical condition.

Conclusion

  • The PF ratio provides a quantitative value for assessing the degree of respiratory failure.
  • The ability to trend PF ratio over time helps in monitoring patient status and making decisions regarding escalations or de-escalations of therapy.
  • It is also used to evaluate the degree of ARDS in patients.
  • Upcoming lessons will discuss interventions in respiratory failure further.

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