Evaluation of oxygen reserve index as an early warning indicator of hypoxemia in anesthetized dogs

The oxygen reserve index (ORi) is a non-invasive parameter recently introduced in human medicine that detects impending hypoxemia earlier than peripheral oxygen saturation (SpO₂). This study evaluated the utility of ORi as an early warning indicator of hypoxemia in anesthetized dogs. Six beagle dogs were subjected to planned apnea under two oxygen conditions (FiO₂ 1.0 and 0.4). Changes in ORi, SpO₂, and arterial oxygen partial pressure (PaO₂) were monitored. Warning times (WT) were calculated as the time difference between each timepoint and SpO₂ reaching 90%. We defined Added warning times (AWT) as the time between an ORi warning point and the moment SpO₂ fell to 96%. When ORi decreased to half of its baseline value or reached 0.4, it provided early warnings of 43.2 ± 24.6 sec and 44.7 ± 37.0 sec, under FiO₂ 1.0, and 29.5 ± 21.9 sec and 37.0 ± 22.5 sec under FiO₂ 0.4, before SpO₂ began to decrease. A significant positive correlation was observed between ORi and PaO₂ when SpO₂ was ≥97% (r=0.74, P<0.0001). Receiver operating characteristic (ROC) analysis showed high diagnostic accuracy of ORi for predicting PaO₂ ≥150 mmHg (area under the curve [AUC]=0.92), with an optimal cutoff value of 0.48 (sensitivity 92%, specificity 78.8%). Unlike in humans, ORi continued to change at PaO₂ levels above 200 mmHg in dogs, suggesting species-specific differences. In conclusion, ORi can detect deterioration in oxygenation earlier than SpO₂ in anesthetized dogs and provides valuable non-invasive real-time monitoring of oxygen reserves during controlled apnea under anesthesia.