To Evaluate the Relationship between Oxygen Reserve Index (ORi™) with Arterial Partial Pressure of Oxygen (PaO2) and Arterial Oxygen Saturation (SaO2)

Abstract

Background: Oxygen Reserve Index (ORi™) (Masimo Corp., Irvine, CA, USA) is a novel non-invasive monitoring tool that
provides supplementary information on oxygenation, especially within the moderate hyperoxia range. Conventional methods
such as arterial blood gas (ABG) analysis, while accurate, are invasive, intermittent, and associated with certain risks. This study
investigates the relationship between ORI, arterial partial pressure of oxygen (PaO2), and arterial oxygen saturation (SaO2) to
evaluate its clinical utility in perioperative oxygen management.
Aims and Objectives: The primary aim was to evaluate the relationship between ORI and PaO2 in patients undergoing elective
neurosurgical procedures under general anesthesia. Secondary objectives included examining the correlation of ORI with SaO2
and exploring its potential in guiding oxygen therapy titration.
Materials and Methods: This prospective observational study was conducted over one year, enrolling 50 adult patients
undergoing elective neurosurgery. Baseline parameters, including heart rate, respiratory rate, SpO2, and end-tidal CO2, were
recorded preoperatively. ORI values were measured using a multiwavelength pulse co-oximeter (Radical-7, Masimo). Arterial
blood gas samples were obtained intraoperatively at 45-minute intervals and during recovery. Statistical analysis was performed
using descriptive statistics, Pearson’s correlation, and regression analysis, with significance set at p < 0.05.
Results: A significant positive correlation (R² = 0.473, p < 0.0001) was observed between ORI and PaO2 for values up to 150
mmHg. Beyond this range, the correlation weakened, suggesting ORI’s effectiveness is confined to the moderate hyperoxic range.
A stepwise regression model combining PaO2, SaO2, SpO2, and etCO2 showed a strong combined correlation with ORI (R² =
0.512, p < 0.0001). ORI trends were found to reliably reflect changes in PaO2, particularly within the targeted hyperoxic range.
Conclusion: The study demonstrates that ORI provides valuable non-invasive insights into oxygenation trends, particularly
in moderate hyperoxia, and holds potential for guiding oxygen therapy. However, its accuracy in extreme hyperoxic or hypoxic
states and its application in diverse clinical scenarios require further validation through larger, multicentric studies.