Prediction of PaO₂ from SpO₂ values in critically ill invasively ventilated patients: rationale and protocol for a patient–level analysis of ERICC, LUNG SAFE, PRoVENT and PRoVENT–iMiC (PRoPERLy II)

Introduction: In patients with acute respiratory distress syndrome (ARDS), mortality risk is typically assessed using the arterial partial pressure of oxygen (PaO₂) divided by the fraction of inspired oxygen (FiO₂), known as the PaO₂/FiO₂ ratio. Recently, the SpO₂/FiO₂ ratio, which uses peripheral oxygen saturation (SpO₂) instead of PaO₂, has been suggested as a reasonable alternative when there is little access to arterial blood gas analyses. Additionally, equations that predict PaO₂ from SpO₂ values could offer another viable method for assessment. Aim: To evaluate the accuracy of methods for predicting PaO₂ from SpO₂ values and compare risk stratification based on measured versus predicted PaO₂/FiO₂ ratios using a large database that harmonizes the individual data of patients included in four observational studies. Methods and analysis: The individual data from four observational studies (‘Epidemiology of Respiratory Insufficiency in Critical Care study’ [ERICC], ‘Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure’ [LUNG SAFE], ‘PRactice of VENTilation in critically ill patients without ARDS’ [PRoVENT], and ‘PRactice of VENTilation in critically ill patients in Middle–income Countries’ [PRoVENT–iMiC]) were harmonized and pooled into a database named ‘PRoPERLy II’. The primary endpoint of this planned analysis will be the accuracy of currently available methods to predict PaO₂ from SpO₂ values. The secondary endpoint will be the accuracy of classification based on SpO₂/FiO₂ ratio and the predicted PaO₂/FiO₂ ratio. Dissemination: Our planned analysis addresses a clinically important research question by comparing different equations for predicting PaO₂ from SpO₂ values.