Oxigen Reserve index as a non-invasive indicator of arterial partial pressure of oxygen in anaesthetised dogs

The aim of this study was to evaluate the potential use of the oxygen reserve index (ORi) as a non-invasive estimator of the partial pressure of oxygen (PaO2) in a hyperoxemic condition (100 -- 200 mmHg) in anaesthetised dogs. In this range of PaO2 traditional pulse-oximeters cannot be applied as the arterial haemoglobin is fully saturated and the measurement return by the instrument is > 98%. The only way to assess the oxygenation status is arterial blood gas analysis, which is invasive, intermittent and challenging. The primary aim of the study was to investigate the relationship between the ORi, a parameter measured non-invasively with a multi-wavelength pulse co-oximeter, and PaO2, measured with the arterial blood gas analysis. A second goal was to trace how did the ORi change at changing the inspired fraction of oxygen (FiO2) in order to estimate the error of single ORi measurement from the scattering of ORi values. Ultimately, we focused on finding cut-off values of ORi that indicate upper-limits for PaO2 that have clinical application. The study enrolled 16 dogs undergoing general anaesthesia for elective procedures requiring the insertion of an arterial catheter and mechanical ventilation. An adhesive sensor was wrapped around the tongue and connected to a pulse co-oximeter (Rad 97, Masimo Corp.) that calculated the ORi. For each dog at least 2 arterial blood samples were collected for analysis. A total of 35 paired measurements were collected. A moderate positive correlation was observed between ORi and PaO2 with the correlation coefficient of 0.68. Tracing the ORi probes we calculated the error of single ORi measurement that was 0.02. The Youden index was used to identify 0.55 as value of ORi able to detect a PaO2 > 150 mmHg with the highest sensitivity and specificity. We can conclude that ORi can be used as a non-invasive monitoring of blood oxygenation status in anaesthetized dogs since it’s variation reflects the variation of the PaO2, although it does not replace blood gas analysis for assessment of oxygenation.