Limitations of SpO2 / FiO2-ratio for classification and monitoring of acute respiratory distress syndrome—an observational cohort study

Background The ratio of pulse-oximetric peripheral oxygen saturation to fraction of inspired oxygen (SpO$_{2}$/FiO$_{2}$) has been proposed as additional hypoxemia criterion in a new global definition of acute respiratory distress syndrome (ARDS). This study aims to evaluate the clinical and theoretical limitations of the SpO$_{2}$/FiO$_{2}$-ratio when using it to classify patients with ARDS and to follow disease progression. Methods Observational cohort study of ARDS patients from three high-resolution Intensive Care Unit databases, including our own database ICU Cockpit, MIMIC-IV (Version 3.0) and SICdb (Version 1.0.6). Patients with ARDS were identified based on the Berlin criteria or ICD 9/10-codes. Time-matched datapoints of SpO$_{2}$, FiO$_{2}$ and partial pressure of oxygen in arterial blood (PaO$_{2}$) were created. Severity classification followed the thresholds for SpO$_{2}$/FiO$_{2}$ and PaO$_{2}$/FiO$_{2}$ of the newly proposed global definition. Results Overall, 708 ARDS patients were included in the analysis. ARDS severity was misclassified by SpO$_{2}$/FiO$_{2}$ in 33% of datapoints, out of which 84% were classified as more severe. This can be partially explained by imprecision of SpO$_{2}$ measurement and equation used to transform SpO$_{2}$/FiO$_{2}$ to PaO$_{2}$/FiO$_{2.}$ A high dependence of SpO$_{2}$/FiO$_{2}$-ratio on FiO$_{2}$ settings was found, leading to major treatment effect and limited capability for tracking change in ARDS severity, which was achieved in less than 20% of events. Conclusions The use of SpO$_{2}$/FiO$_{2}$ interchangeably with PaO$_{2}$/FiO$_{2}$ for severity classification and monitoring of ARDS is limited by its inadequate trending ability and high dependence on FiO$_{2}$ settings, which may influence treatment decisions and patient selection in clinical trials.