Exploring oxygen reserve index for timely detection of deoxygenation in canine patients recovering from anesthesia

Pulse oximetry (SpO₂) identifies a decrease in the partial pressure of oxygen (PaO₂) when it falls below 80 mmHg, while oxygen reserve index (ORi), a dimensionless index ranging from 0 to 1, detects PaO₂ changes between 100 and 200 mmHg. This study investigates the usefulness of ORi in detecting impending deoxygenation before traditional SpO₂. Fifty-one dogs undergoing anesthesia were mechanically ventilated maintaining a fraction of inspired oxygen of 0.50 and an ORi of 1. Animals were classified according to their body condition score (BCS) as normal-fit (BCS 4-5/9), overweight (BCS 6-7/9), or obese (BCS 8-9/9). At the end of the procedure, dogs were placed in sternal recumbency, and after 10 min disconnected from the ventilator and maintained in apnea. ORi added warning time was determined at various ORi values as the time difference in reaching SpO₂ of 95% from ORi of 0.9 and 0.5, compared to the SpO₂ warning time from SpO₂ of 98%. During apnea, ORi decreased before noticeable SpO₂ changes. An ORi of 0.9 anticipated an SpO₂ of 95% in normal-fit dogs by 87 (33-212) [median (range)] seconds or in those with a BCS ≥ 6/9 by 49 (7-161) seconds. Regardless of the BCS class, the median time from ORi of 0.5 to SpO₂ of 95% was 30-35 s. ORi declined from 0.9 to 0.0 in 68 compared to 33 s between normal-fit and obese dogs (p < 0.05). In dogs, ORi added warning time could facilitate timely intervention, particularly in obese patients.