Validation of SpO2/FiO2 as a Non-Invasive Surrogate of PaO2/FiO2 in Mechanically Ventilated COVID-19 Patients at High Altitude

Background: The ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO2/FiO2) is central to the classification of acute respiratory distress syndrome (ARDS). However, its assessment requires arterial blood gas analysis, which may be limited by availability, cost, and invasiveness. Consequently, the ratio of peripheral oxygen saturation to fraction of inspired oxygen (SpO2/FiO2) has been proposed as a non-invasive surrogate for estimating the degree of oxygenation impairment. Methods: A retrospective cross-sectional study was conducted in adult patients with COVID-19 admitted to the intensive care unit at an altitude of 2600 m above sea level (m.a.s.l.). Spearman correlation coefficients were calculated to assess the association between the SpO2/FiO2 and PaO2/FiO2 ratios and their corresponding imputation models. A generalized linear model was applied, and the diagnostic performance of the SpO2/FiO2 ratio and the imputation models for detecting severe and non-severe hypoxemia (PaO2/FiO2 cutoff value of 150) was evaluated using the area under the receiver operating characteristic curve (AUC). Results: A total of 473 patients receiving invasive mechanical ventilation were included, with a mean age of 62.4 years (SD 14.1), and a predominance of males (67.2%). An SpO2/FiO2 ratio cutoff value of ≥206 demonstrated excellent diagnostic performance, with an AUC of 0.983 (95% CI 0.97–0.99), high sensitivity (90.6%), high specificity (96.7%), and an overall correct classification rate of 93.9%. This performance remained consistent across multiple clinical scenarios. In patients with positive end-expiratory pressure > 10 cmH2O, the AUC was 0.982, with a specificity of 97.7%. In the presence of hyperbilirubinemia (total bilirubin ≥ 3 mg/dL), the AUC was 0.951. Among patients with hemoglobin levels 35 mmHg, an SpO2/FiO2 ratio ≥ 206 showed near-perfect specificity (99.4%) and a positive likelihood ratio of 120.9. Conclusions: The SpO2/FiO2 ratio is a reliable and non-invasive surrogate of the PaO2/FiO2 ratio in mechanically ventilated patients with COVID-19 living at high altitude, particularly for the identification of non-severe hypoxemia.