Using Oxygen Reserve Index to Prevent Hyperoxia during General Anesthesia

Monitoring oxygen reserve index can detect hyperoxemia in patients under general anesthesia and possibly avoid unnecessary administration of high concentration oxygen, according to a study published in the Journal of Clinical Monitoring and Computing.

The study investigated the ability of investigated the ability of Masimo ORi (Oxygen Reserve Index) to serve as a noninvasive indicator of the arterial partial pressure of oxygen (PaO₂) during general anesthesia, to help avoid hyperoxia. ORi is an index of oxygenation in the moderate hyperoxic region (PaO₂ range of 100 to 200 mmHg). As an “index” with a scale between 0.0 and 1.0, ORi can be trended to notify clinicians of changes in a patient’s oxygen reserve.

Dr. Keisuke Yoshida and colleagues at the Fukushima Medical University School of Medicine in Japan sought to evaluate whether ORi could provide continuous, noninvasive insight into avoiding excessive hyperoxia by comparing the relationship between PaO₂ and ORi during various oxygen administration conditions.

They enrolled 20 patients scheduled for surgery requiring general anesthesia. ORi was measured using Masimo Root with the Radical-7 Pulse CO-Oximeter and rainbow sensors. PaO₂ was measured using the Siemens RapidLab 1265 blood gas analyzer.

For each patient, after inducing anesthesia, blood gas analysis to measure PaO₂ was performed four times, with ORi values recorded each time blood was drawn, providing 80 data sets. Initial analysis was performed with inspired oxygen concentration (FiO₂) set to 0.33, with the three subsequent analyses performed when ORi was around 0.5, 0.2, and 0, achieved by adjusting FiO₂.

Using linear regression analysis, the researchers found a “relatively strong” positive correlation between ORi and PaO₂ when PaO₂ was less than 240 mmHg. Using receiver operating characteristic (ROC) curve analysis, they calculated that the optimal cut-off ORi value to detect PaO₂ ? 150 mmHg was 0.21 (sensitivity 0.950, specificity 0.755). Using four-quadrant plot analysis, they found that ORi trended PaO₂ with a 100% concordance rate.

“ORi has unique characteristics reflecting the state of oxygenation in the hyperoxic range. The present study examined data under general anesthesia in the operating room, but ORi’s unique features can provide benefits not only in the operating room, but also in the ICU and other fields,” researchers concluded.

As study limitations, the researchers noted the small number of subjects in the study. In addition, they noted, “It is plausible that a given patient’s factors (eg age, physique, body temperature, finger perfusion, hemoglobin concentration) affect their ORi value. Therefore, ORi might not reflect all changes in PaO₂.”