P:F ratio


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

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.