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Peer-reviewedCC BYJournal of intensive medicine

PaO2/FiO2 and SpO2/FiO2 ratios revisited: Useful, misleading, or both?

  • Prashant Nasa1
  • Denise Battaglini2
  • Deven Juneja3
  • Marcus J. Schultz4
  • Jozef Kesecioglu5
5 affiliations
  1. 1Department of Anaesthesia and Critical Care Medicine, The Royal Wolverhampton Trust, New Cross Hospital, Wolverhampton, United Kingdom
  2. 2Anesthesia and Intensive Care, IRCCS Ospedale Policlinico San Martino, Genoa, Italy
  3. 3Institute of Critical Care Medicine, Max Super Speciality Hospital, New Delhi, India
  4. 4Mahidol–Oxford Tropical Medicine Research Unit (MORU), Mahidol University, Bangkok, Thailand
  5. 5Department of Intensive Care Medicine, University Medical Center Utrecht, Utrecht University, Utrecht, the Netherlands
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Abstract

The ratio of arterial partial pressure of oxygen to the inspired oxygen fraction (PaO2/FiO2) serves as a cornerstone metric for the management of patients with acute respiratory distress syndrome (ARDS). This ratio is integral to the Berlin definition of ARDS and is used for severity classification, prognostication, and guiding therapeutic interventions. Furthermore, the PaO2/FiO2 ratio has been employed in landmark trials to ascertain patients’ eligibility for interventions such as prone positioning and extracorporeal membrane oxygenation. The modifications of the Berlin definition, the Kigali modification, and the 2023 New Global definition suggested using the ratio of peripheral oxygen saturation to FiO2 (SpO2/FiO2) as a surrogate for the PaO2/FiO2 ratio when arterial blood gas is inaccessible. Although these oxygenation ratios are valuable for timely recognition and categorizing ARDS, as well as informing clinical decision-making, several significant limitations limit their efficacy, such as dependence on ventilator settings and FiO2. The isolated use of these oxygenation ratios may not accurately monitor the evolution of the pathology and response to interventions. Additionally, the arbitrary cutoffs established for these oxygenation metrics frequently result in overlapping categories rather than distinct subgroups of ARDS. Finally, pulse oximetry measurements can be compromised by factors such as patient motion, perfusion variability, dyshemoglobinemia, and skin pigmentation-related bias, which may obscure hypoxemia. Considering these challenges, recent research has explored the trajectory of the PaO2/FiO2 ratio relative to static measurements and the reclassification of ARDS severity after 24–48 h for prognostication. Alternative metrics such as the oxygenation index, oxygenation saturation index, and (PaO2 ×10)/(FiO2 × PEEP) ratio (which incorporates positive end-expiratory pressure) have demonstrated greater ability to predict patients’ outcomes. This review summarizes the evidence base, clinical applications, and pitfalls of PaO2/FiO2 and SpO2/FiO2 ratios in ARDS and potential alternatives, particularly in the context of diagnosis, categorization, and management of ARDS.

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Cite

BibTeXRIS
BibTeX
@article{nasa2026pao2fio2,
  title = {{PaO2/FiO2 and SpO2/FiO2 ratios revisited: Useful, misleading, or both?}},
  author = {Prashant Nasa and Denise Battaglini and Deven Juneja and Marcus J. Schultz and Jozef Kesecioglu},
  journal = {Journal of intensive medicine},
  year = {2026},
  month = may,
  doi = {10.1016/j.jointm.2026.04.001},
  url = {https://europepmc.org/article/PMC/PMC13728404},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - PaO2/FiO2 and SpO2/FiO2 ratios revisited: Useful, misleading, or both?
AU  - Prashant Nasa
AU  - Denise Battaglini
AU  - Deven Juneja
AU  - Marcus J. Schultz
AU  - Jozef Kesecioglu
PY  - 2026
DA  - 2026/05/17
JO  - Journal of intensive medicine
DO  - 10.1016/j.jointm.2026.04.001
UR  - https://europepmc.org/article/PMC/PMC13728404
AB  - The ratio of arterial partial pressure of oxygen to the inspired oxygen fraction (PaO2/FiO2) serves as a cornerstone metric for the management of patients with acute respiratory distress syndrome (ARDS). This ratio is integral to the Berlin definition of ARDS and is used for severity classification, prognostication, and guiding therapeutic interventions. Furthermore, the PaO2/FiO2 ratio has been employed in landmark trials to ascertain patients’ eligibility for interventions such as prone positioning and extracorporeal membrane oxygenation. The modifications of the Berlin definition, the Kigali modification, and the 2023 New Global definition suggested using the ratio of peripheral oxygen saturation to FiO2 (SpO2/FiO2) as a surrogate for the PaO2/FiO2 ratio when arterial blood gas is inaccessible. Although these oxygenation ratios are valuable for timely recognition and categorizing ARDS, as well as informing clinical decision-making, several significant limitations limit their efficacy, such as dependence on ventilator settings and FiO2. The isolated use of these oxygenation ratios may not accurately monitor the evolution of the pathology and response to interventions. Additionally, the arbitrary cutoffs established for these oxygenation metrics frequently result in overlapping categories rather than distinct subgroups of ARDS. Finally, pulse oximetry measurements can be compromised by factors such as patient motion, perfusion variability, dyshemoglobinemia, and skin pigmentation-related bias, which may obscure hypoxemia. Considering these challenges, recent research has explored the trajectory of the PaO2/FiO2 ratio relative to static measurements and the reclassification of ARDS severity after 24–48 h for prognostication. Alternative metrics such as the oxygenation index, oxygenation saturation index, and (PaO2 ×10)/(FiO2 × PEEP) ratio (which incorporates positive end-expiratory pressure) have demonstrated greater ability to predict patients’ outcomes. This review summarizes the evidence base, clinical applications, and pitfalls of PaO2/FiO2 and SpO2/FiO2 ratios in ARDS and potential alternatives, particularly in the context of diagnosis, categorization, and management of ARDS.
SN  - 2097-0250
SN  - 2667-100X
ER  -