A PaO₂/FiO₂ ratio of ≤ 300 and ≤ 200 are used in the diagnosis of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) for patients in acute hypoxic respiratory failure. The requirement for arterial blood gas analysis for diagnosis of ALI/ARDS and concerns about anemia, excessive blood draws may contribute to the under diagnosis of ALI and ARDS.

We know that in healthy subjects PaO₂ correlates well with SpO₂ in the range of 80 to 100%. Could a SpO₂/FiO₂ ratio be used to noninvasively diagnose ALI/ARDS? Rice et al. with the NIH ARDS network recently looked into using the SpO₂/FiO₂ ratio as a surrogate for the PaO₂/FiO₂ ratio in the screening for ALI/ARDS patients.

This group looked at corresponding SpO₂ and PaO₂ from patients that were involved in the ARDSnet 6 ml/kg vs 12 ml/kg tidal volume trial. The relationship of SpO₂/FiO₂ ratio vs PaO₂/FiO₂ ratio was then validated using similar data from patients that were enrolled in the ARDSnet ALVEOLI (lower PEEP vs higher PEEP) study. They ended up having 2673 data points from the ARDSnet low tidal volume trial and 2031 data points from the ARDSnet PEEP trial. Measurements with SpO₂ values >97% were excluded from the data set because the oxyhemoglobin dissociation curve is flat above 97%.

SpO₂/FiO₂ and PaO₂/FiO₂ ratios showed a linear relationship that did not change over varying levels of FiO₂ or PEEP. A SpO₂/FiO₂ ratio of 235 correlated with a PaO₂/FiO₂ ratio of 200 and a SpO₂/FiO₂ ratio of 315 correlated with a PaO₂/FiO₂ ratio of 300. The SpO₂/FiO₂ ratio showed excellent sensitivity and good specificity in prediction the corresponding PaO₂/FiO₂ ratio in the validation data set. This study shows that the SpO₂/FiO₂ ratios of 235 and 315 are appropriate surrogates for the PaO₂/FiO₂ ratios of 200 and 300.

The use of a SpO₂/FiO₂ ratio will allow earlier recognition of patients who likely have ALI/ARDS without yet having undergone arterial blood gas analysis. This may facilitate earlier diagnosis, earlier treatment and early enrollment into clinical trials. Further studies will still need to be done to fully validate the SpO₂/FiO₂ and PaO₂/FiO₂ ratio relationship in populations of critically ill patients other than those with ALI/ARDS.

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Rice TW, Wheeler AP, Bernard GR, Hayden DL et al. Comparison of the SpO₂/FiO₂ ratio and the PaO₂/FiO₂ ratio in Patients With Acute Lung Injury or ARDS. Chest 2007; 132:410-417.