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

Abstract

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/FiO₂) at ARDS onset. Since the proposal did not mandate PaO₂/FiO₂ calculation under standardised ventilator settings (SVS), we hypothesised that a stratification based on baseline PaO₂/FiOv 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/FiO₂≤200) and severe (PaO₂/FiO₂≤100) ARDS and followed until hospital discharge.

Interventions: We examined physiological and ventilator parameters in association with the PaO₂/FiO₂ 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/FiO₂≤300) ARDS and non-ARDS (PaO₂/FiO₂>300).

Primary and secondary outcomes: Group severity and hospital mortality.

Results: At ARDS onset, 173 patients had a PaO₂/FiO₂≤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/FiO₂ category (p<0.000001). Our model of risk stratification outperformed the stratification using baseline PaO₂/FiO₂ and non-standardised PaO₂/FiO₂ 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.

Trial registration numbers: NCT00435110 and NCT00736892.

Keywords: RESPIRATORY MEDICINE (see Thoracic Medicine).

Figure 1

Flow diagram of the study. The percentages are a proportion of the total patients initially classified as moderate and severe, respectively. AECC, American-European Consensus Conference; ARDS, acute respiratory distress syndrome; PaO₂/FiO₂, ratio of arterial partial pressure of oxygen to fraction of inspired oxygen; PEEP, positive end-expiratory pressure; SVS, standardised ventilator setting.

Figure 2

Receiver operating curves for risk assessment using the ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂) ratio of each patient under standardised ventilator setting (SVS) at 24 h, and compared to risk stratification at ARDS onset (baseline) and at 24 h (T24) without standardising the measurement of PaO₂/FiO₂ values.