BACKGROUND: 

The use of intraoperative pulse oximetry (Spo₂) enhances hypoxia detection and is associated with fewer perioperative hypoxic events. However, Spo₂ may be reported as 98% when arterial partial pressure of oxygen (Pao₂) is as low as 70 mm Hg. Therefore, Spo₂ may not provide advance warning of falling arterial oxygenation until Pao₂ approaches this level. Multiwave pulse co-oximetry can provide a calculated oxygen reserve index (ORI) that may add to information from pulse oximetry when Spo₂ is >98%. This study evaluates the ORI to Pao₂ 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 Pao₂ obtained from each arterial blood gas measurement and changes in ORI with changes in Pao₂ from sequential measurements. Linear mixed-effects regression models for repeated measures were then used to account for within-subject correlation across the repeatedly measured Pao₂ and ORI and for the unequal time intervals of Pao₂ determination over elapsed surgical time. Regression plots were inspected for ORI values corresponding to Pao₂ of 100 and 150 mm Hg. ORI and Pao₂ were compared using mixed-effects models with a subject-specific random intercept.

RESULTS: 

ORI values and Pao₂ measurements were obtained from intraoperative data collected from 106 patients. Regression analysis showed that the ORI to Pao₂ relationship was stronger for Pao₂ to 240 mm Hg (r² = 0.536) than for Pao₂ over 240 mm Hg (r² = 0.0016). Measured Pao₂ was ≥100 mm Hg for all ORI over 0.24. Measured Pao₂ 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 Pao₂ 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 Pao₂ 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 Pao₂ ≥100 mm Hg when Spo₂ is over 98%. Similarly, ORI > 0.55 appears to be a threshold to distinguish Pao₂ ≥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 Pao₂ approaching 100 mm Hg when Spo₂ is >98%. The clinical utility of interventions based on continuous ORI monitoring should be studied prospectively.