FIO2 and acute respiratory distress syndrome definition during lung protective ventilation

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 PaO,/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 (V-T = 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 (Q(VA)/Q(T)) 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. (Crit Care Med 2009; 37:202-207)