Assessment of PaO2/FiO2 for stratification of patients with moderate and severe acute respiratory distress syndrome

Objectives: A recent update of the definition of acute respiratory distress syndrome (ARDS) proposed an empirical classification based on ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO2/FiO(2)) at ARDS onset. Since the proposal did not mandate PaO2/FiO(2) calculation under standardised ventilator settings (SVS), we hypothesised that a stratification based on baseline PaO2/FiO(2) would not provide accurate assessment of lung injury severity. Design: A prospective, multicentre, observational study. Setting: A network of teaching hospitals. Participants: 478 patients with eligible criteria for moderate (100<PaO2/FiO(2)<= 200) and severe (PaO2/FiO(2)<= 100) ARDS and followed until hospital discharge. Interventions: We examined physiological and ventilator parameters in association with the PaO2/FiO(2) at ARDS onset, after 24 h of usual care and at 24 h under a SVS. At 24 h, patients were reclassified as severe, moderate, mild (200<PaO2/FiO(2)<= 300) ARDS and non-ARDS (PaO2/FiO(2)>300). Primary and secondary outcomes: Group severity and hospital mortality. Results: At ARDS onset, 173 patients had a PaO2/FiO(2)<= 100 but only 38.7% met criteria for severe ARDS at 24 h under SVS. When assessed under SVS, 61.3% of patients with severe ARDS were reclassified as moderate, mild and non-ARDS, while lung severity and hospital mortality changed markedly with every PaO2/FiO(2) category (p<0.000001). Our model of risk stratification outperformed the stratification using baseline PaO2/FiO(2) and non-standardised PaO2/FiO(2) at 24 h, when analysed by the predictive receiver operating characteristic (ROC) curve: area under the ROC curve for stratification at baseline was 0.583 (95% CI 0.525 to 0.636), 0.605 (95% CI 0.552 to 0.658) at 24 h without SVS and 0.693 (95% CI 0.645 to 0.742) at 24 h under SVS (p<0.000001). Conclusions: Our findings support the need for patient assessment under SVS at 24 h after ARDS onset to assess disease severity, and have implications for the diagnosis and management of ARDS patients.