Respiratory Therapy Blog

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 underdiagnosis 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 datapoints from the ARDSnet low tidal volume trial and 2031 datapoints 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.