The ratio of oxygen saturation by pulse oximetry (SpO2) to the fraction of inspired oxygen (FIO2) is readily available and noninvasive, while measuring PaO2 requires an arterial blood gas. Therefore, the use of the SpO2 to FIO2 is attractive for the diagnosis and monitoring of a patient with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).
Parameters:
(1) SpO2 by pulse oximetry
(2) fraction of inspired oxygen (FIO2)
At key decision levels the 2 ratios can be correlated:
SpO2 to FIO2 ratio
PaO2 to FIO2 ratio
235
200
315
300
According to Rice at al:
SpO2 to FIO2 ratio =
= (0.84 * (PaO2 to FIO2 ratio)) + 64
This can be rearranged as:
PaO2 to FIO2 ratio =
= ((SpO2 to FIO2 ratio) - 64) / 0.84 =
= (1.19 * (SpO2 to FIO2 ratio)) - 76.2
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