Oxygen reserve index to predict oxygen status in anaesthetized dogs

Oxygen reserve index (ORi) is a non-invasive continuous parameter ranging from 0 to 1 that positively correlates with arterial oxygen content (PaO2) between 100-200 mmHg in humans. This prospective study investigated the relationship of ORi with PaO2, and its use to predict oxygen content in dogs. In 21 anaesthetised mechanically ventilated healthy dogs undergoing elective procedures, PaO2 was measured by a blood gas analyser and at the time of blood collection the ORi recorded by Masimo multi-wavelength pulse CO-oximeter. Pearson coefficient (r) was used to assess the correlation between ORi and PaO2. To identify if factors known to affect the pulse oximeter reading, such as the weight, local perfusion, pH, PaCO2, and temperature, influenced ORi measurements, paired data were fitted in a linear model and the correlation between the model residuals and the confounders was calculated. Youden index was used to identify the ORi value that predicted PaO2 ≥ 150 mmHg with the highest sensitivity and specificity. A total of 51 paired measurements of ORi and PaO2 were collected, and a moderate positive correlation (r = 0.6) was found. Plethysmographic curve appeared to influence the accuracy of ORi rather than the perfusion index. Only body weight mildly affected ORi measurements (r = 0.48, p < 0.05). An ORi ≥ 0.48 indicated a PaO2 ≥ 150 mmHg with as sensitivity of 87%. The oxygen reserve index may be used to titrate oxygen administration in anaesthetised dogs, although it does not replace blood gas analysis for arterial oxygen content measurement.