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.