The ORi is a unitless index that reflects the relative changes in PaO₂ within a moderate hyperoxic range, with values ranging from 0.00–1.00. It is derived from multi-wavelength pulse co-oximetry (Rainbow SET, Masimo Inc.), which analyzes changes in light absorption caused by both arterial and venous pulsatile blood flow at the measurement site, typically the finger [2]. With supplemental oxygen administration, PaO₂ rises above 100 mmHg, while SpO₂ rapidly approaches a plateau near 100%. In contrast, venous oxygen saturation (SvO₂) continues to increase progressively and stabilizes at approximately 80% once the PaO₂ reaches approximately 200 mmHg [2]. The resulting changes in light absorption across this PaO₂ range form the basis for the ORi calculation, which incorporates principles from both the Fick and oxygen content equations [2]. Although the ORi does not directly measure PaO₂, existing data on intraoperative comparisons of PaO₂ and ORi values have demonstrated a correlation coefficient of 0.56 for PaO₂ < 240 mmHg [3].

One of the critical roles of the ORi is to provide advanced warning of developing hypoxemia in high-risk surgical patients. Fleming et al. [4] investigated the added warning time (defined as the difference between the onset of the ORi alarm and onset of SpO₂ desaturation) for patients classified as having an American Society of Anesthesiologists physical status of III or IV. Their findings demonstrated that the ORi provided an advanced warning time of over 45 s compared with conventional SpO₂ monitoring. This additional lead time may facilitate earlier modification of airway management strategies, prompt the initiation of interventions, and/or enable timely requests for assistance [4]. An approximate estimation of the time to rapid desaturation could also aid in guiding appropriate clinical decisions.

In addition to patient-related factors, surgical factors such as intermittent apnea or one-lung ventilation (OLV) can significantly contribute to the development of intraoperative hypoxemia. The ORi has been investigated as a tool capable of detecting intraoperative hypoxemia earlier than conventional pulse oximetry [4]. During lung resection surgery, preemptive OLV was initiated prior to pleural opening, and the predictive performance of a 5-min change in the ORi for intraoperative hypoxemia yielded an area under the curve (AUC) of 0.966 (95% CI [0.935–0.997]) with a threshold ORi value of 0.110 [5]. The ORi has also shown utility in pediatric patients undergoing pectus excavatum repair, a population particularly vulnerable to intraoperative hypoxemia [6]. In this study, the duration of desaturation was reduced by 2 min and the need for high inspired oxygen concentrations (fraction of inspired oxygen [FiO₂] > 0.6) was significantly lower in the ORi-guided group than in the group managed with conventional pulse oximetry [6]. These findings suggest that ORi monitoring may facilitate a more individualized intraoperative FiO₂ management strategy, helping avoid unnecessary oxygen exposure while enabling earlier detection and intervention in patients at high risk for hypoxemia.

High-flow nasal cannula oxygen (HFNO) has been demonstrated in several studies to be a viable alternative to conventional oxygenation in laryngeal surgeries performed under spontaneous breathing conditions, enhancing both airway safety and surgical conditions. During suspension laryngoscopy, supraglottic ventilation is performed with suitable suspension laryngoscopes for jet ventilation. However, data on the application of HFNO in pediatric laryngeal surgery remain limited. Li et al. [7] investigated the use of HFNO in pediatric laryngeal surgery using the ORi for guidance. Their findings suggest that adjusting the oxygen concentration based on the ORi enables the maintenance of intraoperative oxygenation while achieving postoperative PaO₂ levels closer to physiological norms. The optimal cutoff value for the ORi was determined to be 0.195 when the PaO₂ was 150 mmHg.

The risk of perioperative hypoxia is significantly increased in patients undergoing anesthesia and surgery for obesity. ORi monitoring has been evaluated as an early indicator of impending hypoxemia in morbidly obese patients compared with conventional pulse oximetry. In one study, following tracheal intubation, the breathing circuit was disconnected until the SpO₂ decreased to 94%. The added warning time (defined as the interval from the onset of the ORi alarm [or SpO₂ at 97%] to the point that SpO₂ reached 94%) was 46.5 s (range: 36.0–59.0 s) in obese patients compared to 87.0 s (range: 77.0–109.0 s) in patients with a body mass index in the normal range [8]. Another study using a similar methodology demonstrated that the ORi provided a significant added warning time of 17 s before the onset of desaturation compared to standard SpO₂ monitoring. These findings suggest that the ORi could serve as an effective early warning tool to prevent unexpected hypoxemia in morbidly obese patients through the detection of deteriorating oxygenation before a decline in SpO₂ is observed [9]. Further investigation is needed to better understand the correlation between the ORi and PaO₂, as well as to evaluate the utility of the ORi as a guide for preoxygenation, particularly in morbidly obese patients.

Rapid sequence intubation (RSI) is the standard anesthetic procedure for tracheal intubation in patients at risk of gastric content aspiration. The goal of RSI is to minimize the interval between the drug-induced loss of protective airway reflexes and the successful insertion and inflation of a cuffed tracheal tube. Yoshida et al. [10] evaluated the usefulness of the ORi during RSI. In their study, a decline in the ORi was observed in 10 of the 17 patients (77%) undergoing preoxygenation, occurring a median of 32.5 s (interquartile range [Q1, Q3]: 18.1, 51.3) prior to a decrease in SpO₂ following the onset of apnea. This finding suggests that the ORi may provide early warning of impending desaturation, approximately 30 s before a detectable drop in SpO₂. Therefore, continuous ORi monitoring during RSI may help reduce the incidence of hypoxemia. This study also found that the ORi varies greatly among individuals, thus emphasizing the importance of monitoring ORi trends rather than specific values during RSI [10].

Intraoperative hyperoxemia is also commonly encountered in clinical practice and has been associated with complications such as hyperoxic acute lung injury and resorption atelectasis. Kallet and Matthay [11] showed that hyperoxia produces high levels of reactive oxygen species that overwhelm natural antioxidant defenses and destroy cellular structures. In one study, ORi- and pulse-oximetry-guided FiO₂ titration resulted in no cases of hyperoxemia, whereas moderate hyperoxemia was experienced in 31.6% of the patients in the control group [12]. Furthermore, another study demonstrated that ORi-guided oxygen therapy significantly reduced the incidence of postoperative delirium in elderly patients by effectively minimizing intraoperative hyperoxemia [13].

In addition to its role in intraoperative oxygen monitoring, the ORi has been investigated as a potential predictor of postoperative pulmonary outcomes. Lee et al. [14] conducted a prospective observational study to examine the association between the preoperative ORi and postoperative pulmonary complications. They found that a low ORi value measured 3 min after preoxygenation was significantly associated with an increased risk of postoperative pulmonary complications in surgical patients. These findings suggest that the ORi may serve as a useful tool for assessing lung function preoperatively and determining risk stratification for postoperative pulmonary complications.

Despite its clinical potential, the ORi has some limitations. As a unitless and relative indicator, it is not equivalent to PaO₂ and exhibits inter-individual variability owing to personalized scaling [2]. Moreover, ORi measurements can be affected by a patient’s peripheral perfusion status, such as in cases of shock or when high-dose vasopressors are administered [2].

As previously described, in recent clinical practice, the ORi has been increasingly used during anesthetic ventilator management to evaluate a patient’s oxygenation status. In the near future, ORi use may expand to patients in the intensive care unit (ICU) and general wards as well as to outpatients and patients with out-of-hospital cardiac arrest. Yoshida et al. [15] emphasize the usefulness of the ORi for patients in the ICU when suctioning through an endotracheal tube during ventilation. For respiratory infectious diseases, such as COVID-19, the ORi may be a valuable tool for reducing infection risk by decreasing the frequency of blood gas sampling and the need for arterial cannulation to protect healthcare workers. Malinverni et al. [16] investigated whether prehospital oxygen titration guided by the ORi improves normoxia in patients post-cardiac arrest. However, no significant difference was found with ORi-guided titration compared to standard SpO₂-based titration in terms of the normoxia index, PaO₂ at hospital admission, or neurological biomarkers. These ongoing attempts to employ the ORi in diverse medical environments suggest a growing interest in its broader clinical applicability.

In conclusion, recent investigations suggest that the ORi is a promising tool for monitoring oxygenation within the PaO₂ range of 100–200 mmHg and may serve as a risk or prognostic marker for postoperative pulmonary complications. Further studies are warranted to clarify the potential role of the ORi in perioperative management, particularly in patients receiving supplemental oxygen therapy.