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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*

Todd W. Rice, MD, MSc; Arthur P. Wheeler, MD, FCCP; Gordon R. Bernard, MD, FCCP; Douglas L. Hayden, MA; David A. Schoenfeld, PhD; Lorraine B. Ware, MD, FCCP; for the National Institutes of Health, National Heart, Lung, and Blood Institute ARDS Network
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*From the Division of Allergy, Pulmonary, and Critical Care Medicine (Drs. Rice, Wheeler, Bernard, and Ware), Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN; ARDS Network Clinical Coordinating Center (Mr. Hayden and Dr. Schoenfeld), Massachusetts General Hospital, Boston, MA.

Correspondence to: Todd W. Rice, MD, MSc, Division of Allergy, Pulmonary, and Critical Care Medicine, T-1218 MCN, Nashville, TN 37232-2650; e-mail: todd.rice@vanderbilt.edu


*From the Division of Allergy, Pulmonary, and Critical Care Medicine (Drs. Rice, Wheeler, Bernard, and Ware), Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN; ARDS Network Clinical Coordinating Center (Mr. Hayden and Dr. Schoenfeld), Massachusetts General Hospital, Boston, MA.


Chest. 2007;132(2):410-417. doi:10.1378/chest.07-0617
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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.

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