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 (SpO₂/FiO₂) 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 SpO₂/FiO₂-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 SpO₂, FiO₂ and partial pressure of oxygen in arterial blood (PaO₂) were created. Severity classification followed the thresholds for SpO₂/FiO₂ and PaO₂/FiO₂ of the newly proposed global definition.

Results

Overall, 708 ARDS patients were included in the analysis. ARDS severity was misclassified by SpO₂/FiO₂ in 33% of datapoints, out of which 84% were classified as more severe. This can be partially explained by imprecision of SpO₂ measurement and equation used to transform SpO₂/FiO₂ to PaO₂/FiO_2. A high dependence of SpO₂/FiO₂-ratio on FiO₂ 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 SpO₂/FiO₂ interchangeably with PaO₂/FiO₂ for severity classification and monitoring of ARDS is limited by its inadequate trending ability and high dependence on FiO₂ settings, which may influence treatment decisions and patient selection in clinical trials.

Key Points

1. Misclassification of ARDS Severity by SpO₂/FiO₂: The study found that SpO₂/FiO₂ misclassified ARDS severity in 33% of cases, with 84% of misclassifications resulting in overestimation of severity compared to the PaO₂/FiO₂ ratio.
2. Dependence on FiO₂ Settings: The SpO₂/FiO₂ ratio was highly dependent on FiO₂ settings, leading to inaccurate classification. The study observed the lowest accuracy (22%) when FiO₂ was set at 70%, suggesting that treatment-related FiO₂ changes could distort severity assessment.
3. Poor Trending Ability for Disease Progression: The SpO₂/FiO₂ ratio failed to reliably track changes in ARDS severity over time, correctly identifying severity progression in only 19.6% of cases. This limitation affects its utility for monitoring ARDS patients.
4. Comparison with PaO₂/FiO₂ Ratio: While SpO₂/FiO₂ correlated moderately with PaO₂/FiO₂ (r = 0.69), it demonstrated significant variability, making it unreliable for direct substitution.
5. Influence of Oxygen Saturation Variability: SpO₂ values were affected by measurement biases, including sensor variability and perfusion status. The study found a mean SpO₂ overestimation bias of +0.5% compared to SaO₂, with greater discrepancies at higher oxygen saturations.
6. Limitations in Conversion Equations: Various conversion formulas used to estimate PaO₂/FiO₂ from SpO₂/FiO₂ performed inconsistently. The equation from Rice et al. showed the best fit, but no single model provided a precise conversion.
7. Challenges in Resource-Limited Settings: While SpO₂/FiO₂ is proposed for ARDS diagnosis in settings without ABG availability, its inaccuracies may lead to over- or under-diagnosis, affecting treatment decisions.
8. Impact on Clinical Trials and Research: The study warns that using SpO₂/FiO₂ as an alternative to PaO₂/FiO₂ in clinical trials may introduce bias, impacting patient selection and outcome interpretation.
9. Recommendations for Improving Accuracy: To minimize misclassification, the authors suggest using SpO₂/FiO₂ only when SpO₂ is below 97% and optimizing FiO₂ settings to avoid artificial severity inflation.
10. Future Research and Alternative Methods: The study advocates for exploring machine learning models and non-invasive respiratory indices to refine ARDS classification in the absence of ABG data.

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