Non-invasive assessment of oxygenation status using the oxygen reserve index in dogs

The oxygen reserve index (ORi) is a real-time, continuous index measured with multi-wavelength pulse CO-oximetry technology. It estimates mild hyperoxemia in humans, which is defined as a partial pressure of oxygen (PaO₂) level between 100 and 200 mmHg. The objectives of this study were to assess the correlation between ORi and PaO₂, as well as to determine its ability in detecting mild hyperoxemia in dogs.

This prospective observational study enrolled 37 anaesthetised and mechanically ventilated dogs undergoing elective procedures. Simultaneous measurements of ORi and PaO₂ were collected, using a multi-wavelength pulse CO-oximeter with a probe placed on the dog's tongue, and a blood gas analyser, respectively. A mixed-effects model was used to calculate the correlation (r²) between simultaneous measurements of ORi and PaO₂. The trending ability of ORi to identify dependable and proportional changes of PaO₂ was determined. The diagnostic performances of ORi to detect PaO₂ ≥ 150 mmHg and ≥ 190 mmHg were estimated using the area under the receiver operating characteristic curve (AUROC). The effects of perfusion index (PI), haemoglobin (Hb), arterial blood pH and partial pressure of carbon dioxide (PaCO₂) on AUROC for PaO₂ ≥ 150 mmHg were evaluated.

A total of 101 paired measurements of ORi and PaO₂ were collected. PaO₂ values ranged from 74 to 258 mmHg. A strong positive correlation (r² = 0.52, p < 0.001) was found between ORi and PaO₂. The trending ability ORi was 90.7%, with 92% sensitivity and 89% specificity in detecting decreasing PaO₂. An ORi value ≥ 0.53 and ≥ 0.76 indicated a PaO₂ ≥ 150 and ≥ 190 mmHg, respectively, with ≥ 82% sensitivity, ≥ 77% specificity and AUROC ≥ 0.75. The AUROC of ORi was not affected by PI, Hb, pH and PaCO₂.

In anaesthetised dogs, ORi may detect mild hyperoxaemia, although it does not replace blood gas analysis for measuring the arterial partial pressure of oxygen. ORi monitoring could be used to non-invasively assess oxygenation in dogs receiving supplemental oxygen, limiting excessive hyperoxia.