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The following is a summary of “Clinical Performance of Spo₂ /Fio₂ and Pao₂ /Fio₂ Ratio in Mechanically Ventilated Acute Respiratory Distress Syndrome Patients: A Retrospective Study,” published in the March 2025 issue of Critical Care Medicine by Coppola et al. 

Researchers conducted a retrospective study to examine the correlation between acute respiratory distress syndrome (ARDS) severity classifications using the Spo₂ /Fio₂ ratio and the Pao₂ /Fio₂ ratio and to determine the relationship between the Spo₂ /Fio₂ ratio and venous admixture in patients on mechanical ventilation.   

They analyzed 258 individuals on mechanical ventilation with ARDS. The individuals were deeply sedated, paralyzed, and ventilated using volume-controlled ventilation with a tidal volume of 6–8 ml/kg of predicted body weight. Positive end-expiratory pressure (PEEP) was clinically determined by attending physicians. The respiratory rate was modified to maintain arterial carbon dioxide partial pressure between 40 and 50 mm of Hg, while PEEP and a fraction of inspired oxygen were set to achieve Spo₂ between 92% and 97%.   

The results showed that gas exchange, Spo₂, and respiratory mechanics were measured upon ICU admission and during the PEEP trial. Computed tomography data assessed lung compartments and recruitability. A nonlinear relationship was observed between the Spo₂ to Fio₂, and Pao₂ /Fio₂ ratio. Arterial pH and Paco₂ had no effect on this relationship despite potential confounders from pulse oximetry. The 2 ratios showed moderate agreement in classifying ARDS severity (intraclass correlation coefficient = 0.63). No differences were found between severity classifications based on these ratios regarding respiratory mechanics, gas exchange, lung imaging, ICU mortality, or within 2 levels of PEEP. A Spo₂ /Fio₂, ratio below 235 was detected in 89% of individuals with venous admixture exceeding 20%, similar to Pao₂, /Fio₂, ratio below 200.   

Investigators concluded that the SpO₂ /FiO₂ ratio effectively identified oxygenation impairment and ARDS severity like the PaO₂ /FiO₂ ratio, offering a quicker and more accessible method, though its applicability varied based on patient characteristics and required physician interpretation. 

Source: journals.lww.com/ccmjournal/fulltext/9900/clinical_performance_of_spo2_fio2_and_pao2_fio2.478.aspx