Background and Goal of Study: The Oxygen Reserve Index (ORI) is not a measure of PaO2, but a non-dimensional index ranging between 1.0 (high reserve of oxygen) and 0.0 (no oxygen reserve). It has been proposed that the measurement of ORI could add information to SaO2 in order to anticipate a desaturation or to
assess a hyperoxic situation. The objective of this study is to assess the relationship between ORI and PaO2.

Materials and methods: This study included patients with an arterial line (cannulated as current practice). They were monitored with the co-oxymetry monitor Radical7 (Masimo, Irvine, CA). During the perioperative period we
only took the samples we needed to monitor from the gasometry results, comparing with the ORI values. To search the correlation between ORI and PaO2, a regression analysis was performed (r2) using Microsoft Excel 2011â
(Microsoft Corporation. No other statistical analysis was necessary.

Results and discussion: 28 ORI values and PaO2 measurements were obtained from intraoperative data collection in 13 patients (7M-6F, 2 ASA IV-8 ASA III-3 ASA II). All patients had SaO2 > 97%. Range of PaO2 values
was 144-259 mmHg; range of ORI values was 0,17-1,00. Regression study showed that there was no good global ORI-PaO2 ratio (r2=0,17637) (Fig 1). Although it was higher if we consider only PaO2 values < 200 mmHg (n=18) (r2=0,31967, PaO2 range 144-195, ORI range 0,17-0,89) (Fig 2), it does not reach a suitable correlation. Individual results did not show that high values of ORI (even > 0.75) suggested necessarily a hyperoxic state.

Conclusion(s): These results suggest that ORI could be useful to anticipate the patient desaturation. The correlation between ORI and PaO2 values is not suitable, although more studies are needed to fully assess its usefulness.
References: Anesth Analg 2016;123:626-33. Anesthesiology 2016;124:779-84.

Is there any relationship between oxygen reserve index (ORI) and arterial partial pressure of oxygen? A preliminary study in surgical patients