Pulmonology Book

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P/F Ratio

Aka: P/F Ratio, PF Ratio, Horowitz Index, Carrico Index
  1. See Also
    1. Arterial Blood Gas
    2. Acid-Base Homeostasis
    3. ABG Interpretation
    4. A-a Gradient
    5. Oxygen Saturation
  2. Indications
    1. Acute Respiratory Failure
      1. Allows for interpretation of PaO2 on Arterial Blood Gas when a patient is on Supplemental Oxygen
      2. PaO2 interpretation would otherwise require the patient to be on room air for ABG
      3. Used in SOFA Score and SMART-COP
  3. Precautions
    1. P/F Ratio may be unreliable when PaO2 >100 mmHg and FiO2 < 0.5
    2. P/F Ratio cannot distinguish between Acute Respiratory Failure and acute on chronic Respiratory Failure
  4. Labs: Calculation
    1. P/F Ratio = PaO2 / FIO2
      1. Where PaO2 = arterial PO2 (from Arterial Blood Gas)
      2. Where FIO2 = fraction of inspired oxygen (ranging from 0.3 for 30% to 1.0 for 100%)
  5. Interpretation: Rough Approximations on Room Air
    1. PF Ratio 400 to 500 mmHg: >90 mmHg on room air (normal)
    2. PF Ratio 300 mmHg: PaO2 <60 mmHg on room air (mild Respiratory Failure)
    3. PF Ratio 250 mmHg: PaO2 <50 mmHg on room air
    4. PF Ratio 200 mmHg: PaO2 <40 mmHg on room air
    5. PF Ratio 150 mmHg: PaO2 <30 mmHg on room air
    6. PF Ratio 100 mmHg: PaO2 <20 mmHg on room air
  6. Resources
    1. P/F Ratio (Nickson, LITFL)
      1. https://litfl.com/pao2-fio2-ratio/

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