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

Abstract

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 (r = 0.536) than for PaO2 over 240 mm Hg (r = 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.

Conflict of interest statement

Conflict of Interest: See Disclosures at the end of the article.

Figures

Figure 1.

Plot of oxygen reserve index (ORI) compared with arterial partial pressure of oxygen (Pao2) obtained from 106 patients undergoing surgery in whom measured Pao2 from 485 arterial blood gas analyses was between 62 and 534 mm Hg. Patients had >1 sensor applied, with analysis done using 1594 ORI values. A, Locally weighted regression analysis showed a nonlinear relationship overall with a more positive linear relationship for Pao2 up to 240 mm Hg than >240 mm Hg. B, Linear regression analysis of ORI and Pao2 up to 240 mm Hg showed a positive relationship (r2 = 0.536); dashed lines indicate 95% confidence interval of regression line.

Figure 2.

Plot of oxygen reserve index (ORI) to arterial partial pressure of oxygen (Pao2) obtained from 106 patients undergoing surgery suggested cutoff ORI values were present. A, When ORI was >0.24, all Pao2 were ≥100 mm Hg. B, When ORI was >0.55, 96.6% of Pao2 were ≥150 mm Hg.

Figure 3.

Plot of change in oxygen reserve index (ORI) to change in arterial partial pressure of oxygen (Pao2) for Pao2 up to 240 mm Hg obtained from 106 patients undergoing surgery showed a positive relationship; dashed lines indicate 95% confidence interval of regression line. Falling ORI had 77.7% sensitivity and 76.7% specificity for detecting decreasing Pao2.

Figure 4.

Example of continuous intraoperative oxygen reserve index trend (ORI; black line), continuous pulse oxygen saturation trend (Spo2; green line), and intermittent arterial partial pressure of oxygen determination (Pao2; red diamonds) obtained during surgery. ORI decreased during 30 minutes before a documented large decrease in Pao2.

Figure 5.

Example of continuous intraoperative oxygen reserve index trend (ORI; black line), continuous pulse oxygen saturation trend (Spo2; green line), and intermittent arterial partial pressure of oxygen determination (Pao2; red diamonds) obtained during surgery. ORI was zero at times when low Pao2 was measured and Spo2 was recorded at 96% and then increased over time as Pao2 and Spo2 increased.