Objective: Pao2/Fio2 ratio (P/F) is the marker of hypoxemia used in the American-European Consensus Conference on lung injury. A high Fio2 level has been reported to variably alter Pao2/Fio2. We investigated the effect of high Fio2 levels on the course of P/F in lung protective mechanically ventilated patients with acute respiratory distress syndrome.
Design: Prospective, controlled, interventional study.
Setting: University teaching French medical intensive care unit.
Patients: Twenty-four patients with acute respiratory distress syndrome having P/F between 100 and 200 mm Hg at Fio2 0.5 received low-volume controlled ventilation (VT = 6 mL/kg predicted body weight) with a positive end-expiratory pressure at 2 cm H2O above the lower inflection point if present, or 10 cm H2O.
Intervention: The following Fio2 levels were applied randomly for 20 mins: 0.5, 0.6, 0.7, 0.8, 0.9, and 1.
Measurements and Results: Increasing Fio2 above 0.7 was associated with a significant increase in P/F (p < 0.001). The mean P/F change between Fio2 0.5 and 1 (Delta P/F) was 47% ± 35%. Sixteen patients (67%) had a P/F >200 at Fio2 1 whereas P/F was <200 at Fio2 0.5. Venous admixture (QVA/QT) decreased linearly for each Fio2 step (p < 0.001). The QVA/QT change between Fio2 0.5 and 1 was strongly correlated with Delta P/F (r = 0.84). Delta P/F was higher in patients with true shunt <30% (64% [54–93]) than in those with shunt >30% (20% [10–36]; p = 0.003).
Conclusion: The P/F ratio increased significantly with a Fio2 >0.7. P/F variation, induced by a switch from Fio2 0.5 to 1, was responsible for two thirds of patients changing from the acute respiratory distress syndrome to the acute lung injury stage of the American-European Consensus Conference definition. Fio2 should be carefully defined for the screening of lung-injured patients.
From the Service de Réanimation Médicale, Hôpital Sainte Marguerite, 270 Boulevard Sainte Marguerite, 13274 Marseille Cedex 9, France.
Supported, in part, by l'Association pour le Développement de la Recherche Médicale.
The authors have not disclosed any potential conflicts of interest.
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