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

BMJ Open. 2015 Mar 27;5(3):e006812. doi: 10.1136/bmjopen-2014-006812.


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).

Publication types

  • Clinical Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Area Under Curve
  • Blood Gas Analysis
  • Female
  • Hospital Mortality*
  • Hospitals, Teaching
  • Humans
  • Inhalation
  • Lung / physiopathology*
  • Male
  • Middle Aged
  • Monitoring, Physiologic / methods*
  • Oxygen / physiology*
  • Partial Pressure
  • Positive-Pressure Respiration
  • Prospective Studies
  • ROC Curve
  • Respiration, Artificial / methods
  • Respiratory Distress Syndrome / diagnosis*
  • Respiratory Distress Syndrome / mortality
  • Respiratory Distress Syndrome / physiopathology
  • Severe Acute Respiratory Syndrome / diagnosis*
  • Severe Acute Respiratory Syndrome / mortality
  • Severe Acute Respiratory Syndrome / physiopathology
  • Severity of Illness Index*
  • Tidal Volume


  • Oxygen

Associated data