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P:F ratio

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In pulmonary medicine, critical care and anesthesiology, the P:F ratio is the ratio of arterial oxygen concentration to the fraction of inspired oxygen. It reflects how well the lungs absorb oxygen from expired air. [1]

{P/F\ ratio} = \left (\frac{PaO_2}{Fi0_2}\right) \times 100

An example in a healthy person:

{476} = \left (\frac{100\ mm\ Hg}{21%}\right) \times 100

A higher ratio indicates better gas exchange:

  • Normal is 500
  • ARDS is < 200

Comparative studies suggest this measure correlates better with pulmonary shunts than does the A-a gradient.[2][3][4]

References

  1. ↑ Caroline Helwick, P/F Ratio May Be a Marker of Potential Intraoperative Ventilator-Induced Lung Injury, "Coverage of the American Society of Anesthesiologists (ASA) 2009 Annual Meeting", Medscape Medical News
  2. ↑ Covelli HD, Nessan VJ, Tuttle WK (1983). "Oxygen derived variables in acute respiratory failure". Crit. Care Med. 11 (8): 646–9. PMID 6409506. [e]
  3. ↑ El-Khatib MF, Jamaleddine GW (2004). "A new oxygenation index for reflecting intrapulmonary shunting in patients undergoing open-heart surgery". Chest 125 (2): 592–6. PMID 14769743. [e]
  4. ↑ Cane RD, Shapiro BA, Templin R, Walther K (1988). "Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients". Crit. Care Med. 16 (12): 1243–5. PMID 3191742. [e]
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