The ability of oxygen reserve index to detect mild hyperoxemia in mechanically ventilated dogs: a preliminary study

Introduction:Mild hyperoxemia in humans could be detected with the oxygen reserve index (ORi) for arterial partial pressure of oxygen (PaO2) between 100 and 200 mmHg. This study evaluated the correlation of ORi with PaO2 and its ability to detect mild hyperoxemia in dogs. Methods:This prospective observational study enrolled 23 adult anesthetized dogs undergoing elective procedures. The animals were ventilated under pressure-control ventilation with a peak inspiratory pressure of 10–12cm H2O and a respiratory rate adjusted to maintain normocapnia. Simultaneous measurements of ORi and PaO2 were collected, with a multi-wavelength pulse co-oximeter with a probe applied to the dog’s tongue, and a blood gas analyzer, respectively. Pearson correlation coefficient (r) was calculated between simultaneous measurements of ORi and PaO2. Youden index was used toidentify the ORi cutoff values predicting PaO2 hyperoxia thresholds ≥110, ≥120, ≥130, ≥140, and ≥150 mm Hg with the highest sensitivity and specificity. Diagnostic performances of ORi to detect the same hyperoxia thresholds were estimated using the area under the receiver operating characteristic curve (AUROC).The effects of perfusion index (PI),body temperature, arterial blood pH, partial pressure of carbon dioxide (pCO2), lactatemia, and hemoglobin on AUROC were evaluated. Results: A total of 56 paired measurements of ORi and PaO2 were collected. Mean PaO2 value was 148.8 ±32.48 mm Hg and mean hemoglobin was 12.58±1.84 g/dL. A moderate positive correlation (r=0.53,P<0.001) between ORi and PaO2 was found. The ORicutoff value with the highest sensitivity (91%) was 0.3, indicating PaO2 ≥120 mm Hg, with 70% specificity and AUROC=0.84 (95%CI=0.68–1.01). The ORi value cutoff with the highest specificity (86%) was 0.6, indicating PaO2≥140 mm Hg, with 60% sensitivity and AUROC=0.79 (95% CI=0.67–0.91). The AUROCs of ORito detect hyperoxia thresholds tended to increase with decreasing blood pH, PI, and lactatemia and with increasing pCO2 and temperature, while hemoglobin did not affect diagnostic performance of ORi. Conclusions: ORi may detect mild hyperoxemia with PaO2 between120 and 140 mm Hg and limit excessive hyperoxia in mechanically ventilated dogs, although it does not replace blood gas analysis for assessment of oxygenation.