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.