The Relationship Between Oxygen Reserve Index and Arterial Partial Pressure of Oxygen During Surgery Article Swipe

BACKGROUND: The use of intraoperative pulse oximetry (Sp o 2 ) enhances hypoxia detection and is associated with fewer perioperative hypoxic events. However, Sp o 2 may be reported as 98% when arterial partial pressure of oxygen (Pa o 2 ) is as low as 70 mm Hg. Therefore, Sp o 2 may not provide advance warning of falling arterial oxygenation until Pa o 2 approaches this level. Multiwave pulse co-oximetry can provide a calculated oxygen reserve index (ORI) that may add to information from pulse oximetry when Sp o 2 is >98%. This study evaluates the ORI to Pa o 2 relationship during surgery. METHODS: We studied patients undergoing scheduled surgery in which arterial catheterization and intraoperative arterial blood gas analysis were planned. Data from multiple pulse co-oximetry sensors on each patient were continuously collected and stored on a research computer. Regression analysis was used to compare ORI with Pa o 2 obtained from each arterial blood gas measurement and changes in ORI with changes in Pa o 2 from sequential measurements. Linear mixed-effects regression models for repeated measures were then used to account for within-subject correlation across the repeatedly measured Pa o 2 and ORI and for the unequal time intervals of Pa o 2 determination over elapsed surgical time. Regression plots were inspected for ORI values corresponding to Pa o 2 of 100 and 150 mm Hg. ORI and Pa o 2 were compared using mixed-effects models with a subject-specific random intercept. RESULTS: ORI values and Pa o 2 measurements were obtained from intraoperative data collected from 106 patients. Regression analysis showed that the ORI to Pa o 2 relationship was stronger for Pa o 2 to 240 mm Hg ( r 2 = 0.536) than for Pa o 2 over 240 mm Hg ( r 2 = 0.0016). Measured Pa o 2 was ≥100 mm Hg for all ORI over 0.24. Measured Pa o 2 was ≥150 mm Hg in 96.6% of samples when ORI was over 0.55. A random intercept variance component linear mixed-effects model for repeated measures indicated that Pa o 2 was significantly related to ORI (β[95% confidence interval] = 0.002 [0.0019–0.0022]; P < 0.0001). A similar analysis indicated a significant relationship between change in Pa o 2 and change in ORI (β [95% confidence interval] = 0.0044 [0.0040–0.0048]; P < 0.0001). CONCLUSIONS: These findings suggest that ORI >0.24 can distinguish Pa o 2 ≥100 mm Hg when Sp o 2 is over 98%. Similarly, ORI > 0.55 appears to be a threshold to distinguish Pa o 2 ≥150 mm Hg. The usefulness of these values should be evaluated prospectively. Decreases in ORI to near 0.24 may provide advance indication of falling Pa o 2 approaching 100 mm Hg when Sp o 2 is >98%. The clinical utility of interventions based on continuous ORI monitoring should be studied prospectively.