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

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.