Chest
Volume 132, Issue 2, August 2007, Pages 410-417
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Original Research
Critical Care Medicine
Comparison of the Spo2/Fio2 Ratio and the Pao2/Fio2 Ratio in Patients With Acute Lung Injury or ARDS

https://doi.org/10.1378/chest.07-0617Get rights and content

Background

The diagnostic criteria for acute lung injury (ALI) and ARDS utilize the Pao2/fraction of inspired oxygen (Fio2) [P/F] ratio measured by arterial blood gas analysis to assess the degree of hypoxemia. We hypothesized that the pulse oximetric saturation (Spo2)/Fio2 (S/F) ratio can be substituted for the P/F ratio in assessing the oxygenation criterion of ALI.

Methods

Corresponding measurements of Spo2 (values ≤ 97%) and Pao2 from patients enrolled in the ARDS Network trial of a lower tidal volume ventilator strategy (n = 672) were compared to determine the relationship between S/F and P/F. S/F threshold values correlating with P/F ratios of 200 (ARDS) and 300 (ALI) were determined. Similar measurements from patients enrolled in the ARDS Network trial of lower vs higher positive end-expiratory pressure (n = 402) were utilized for validation.

Results

In the derivation data set (2,613 measurements), the relationship between S/F and P/F was described by the following equation: S/F = 64 + 0.84 × (P/F) [p < 0.0001; r = 0.89). An S/F ratio of 235 corresponded with a P/F ratio of 200, while an S/F ratio of 315 corresponded with a P/F ratio of 300. The validation database (2,031 measurements) produced a similar linear relationship. The S/F ratio threshold values of 235 and 315 resulted in 85% sensitivity with 85% specificity and 91% sensitivity with 56% specificity, respectively, for P/F ratios of 200 and 300.

Conclusion

S/F ratios correlate with P/F ratios. S/F ratios of 235 and 315 correlate with P/F ratios of 200 and 300, respectively, for diagnosing and following up patients with ALI and ARDS.

Section snippets

Derivation Data Set

Corresponding measurements of Spo2 and Pao2 from patients enrolled in the National Heart, Lung, and Blood Institute ARDS Network trial11 comparing tidal volumes of 6 mL/Kg predicted body weight (PBW) with those of 12 mL/kg were utilized to establish the relationship between S/F and P/F ratios. Each ARDS Network site received approval from local institutional review boards to conduct the studies. The inclusion and exclusion criteria for the ARDS Network tidal volume study11 have been reported

Results

Of the 861 patients enrolled in the study comparing tidal volumes of 6 and 12 mL/kg PBW, 189 were enrolled at sites located at > 1,000 m in altitude (Fig 1). The remaining 672 patients provided 3,384 Pao2 and Spo2 measurements at known Fio2 values. Spo2 exceeded 97% in 711 patients, leaving 2,673 data points for analysis in the derivation data set. Of the 549 patients who were enrolled in the trial comparing high and low PEEP, 146 were enrolled at centers that were at altitudes of > 1,000 m,

Discussion

We hypothesized that the continuously available S/F ratio can serve as a surrogate for P/F ratio in the diagnostic criteria for ALI/ARDS. Using data from patients with ALI and ARDS who were enrolled in two large clinical trials,11,14 we found that S/F ratio correlates well with a simultaneously obtained P/F ratio. The correlation improves slightly if PEEP is included in the regression model. S/F ratios of 235 and 315, were found to correspond to P/F ratios of 200 and 300, respectively, which

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    This research was funded by National Institutes of Health grants N01-HR-46054 (to Drs. Rice, Wheeler, and Bernard), N01-HR-46064 (to Mr. Hayden and Dr. Schoenfeld), HL07123 (to Dr. Rice), HL70521 (to Dr. Ware), and HL81332 (to Dr. Ware) from the National Heart, Lung, and Blood Institute.

    The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

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