In this study, we demonstrated that (I) ORi and PaO2 exhibit a high level of correlation, (II) ORi may provide valuable insights for clinicians in evaluating hyperoxemia, and (III) under the guidance of ORi and SpO2 together, FiO2 titration can be performed, contributing to achieving optimum oxygenation by obtaining lower PaO2 values and reducing the incidence of hyperoxemia.

Unlike studies that solely investigated the correlation between ORi and PaO2 or the success of ORi in detecting hyperoxemia, our study also suggests that FiO2 titration can be effectively performed noninvasively with ORi.

Almost all clinicians use SpO2 as an indicator for adjusting FiO2, both during general anesthesia and in the intensive care unit. Currently, the clinical use of SpO2 as a noninvasive oxygenation monitor is indispensable and serves as a standard monitoring method for providing optimum oxygenation (Yoshida et al. 2020; Scheeren et al. 2018; Chen and Min 2020).

When SpO2, crucial for detecting hypoxemia, reaches 100%, PaO2 may be at elevated values, and its level becomes unpredictable (Yoshida et al. 2020; Courson et al. 2022). The disadvantages of arterial blood gas analysis, the gold standard in hyperoxemia detection and oxygen monitoring, such as invasiveness, extra cost, time delay, blood loss during repeated measures, complications related to puncture, and inability to provide continuous data, limit its use and may lead to overlooking hyperoxemia (Vos et al. 2019; Cousins and O’Donnell 2004).

The damage caused by hyperoxemia in the body is significant. Studies indicate that hyperoxemia has physiopathological harmful effects akin to hypoxia. Potential effects of hyperoxemia include ventilation/perfusion disequilibrium, hypercapnia, atelectasis, acute tracheobronchitis, diffuse alveolar damage, acute respiratory distress syndrome (ARDS), systemic vasoconstriction, cardiac output depression, and increased mortality (Horncastle 2019; Hafner et al. 2015). Furthermore, excessive medical gas usage rates have detrimental effects economically and environmentally (Gómez-Chaparro et al. 2018). Oxygen, while essential, has side effects when applied excessively, necessitating accurate monitoring for oxygen optimization.

Due to the limitations of SpO2 in detecting hyperoxemia, the use of ORi, which provides continuous and noninvasive measurements, may be more effective in preventing hyperoxemia. Using SpO2 and ORi together can complement each other and ensure effective oxygenation monitoring.

Reviewing studies on the relationship between ORi and PaO2, Applegate et al. reported a positive correlation between PaO2 and ORi (r2 = 0.536) when PaO2 was < 240 mmHg, while Yoshida et al. found a relatively high positive correlation between ORi and PaO2 (r2 = 0.706) (Yoshida et al. 2020; Applegate et al. 2016). Koishi et al. reported a positive correlation between PaO2 and ORi (r2 = 0.671), including some data with PaO2 ≥ 240 mmHg (Koishi et al. 2018). Similarly, Vos et al. reported a strong positive correlation between PaO2 and ORi in the ORi-sensitive range (PaO2: 100–200 mmHg), and that ORi had a good trend ability according to PaO2 changes in this range (Karalapillai et al. 2020). In our study, including all times and both groups, we found a high level of positive linear correlation of 73.8% between PaO2 and ORi (p < 0.001). In the ORi + SpO2 group, this relationship was highly positive and linear at a rate of 75.8% (p < 0.001).

Additionally, the regression equations obtained to estimate ORi and PaO2 variables for the 1st, 2nd, and 3rd hours were significant in our study.

These data suggest that ORi monitoring offers a reasonable estimate of PaO2 and can serve as a potential noninvasive tool in evaluating hyperoxemia in patients receiving oxygen. However, Jin Hee Ahn et al. (Ahn et al. 2022) mentioned that they did not find any linearity, including PaO2 < 240 mmHg values, in the PaO2 and ORi correlation analysis they conducted with 231 data sets. They suggested that differences in studies might result from using different versions of the rainbow sensors (updated versions Revision O/ and Revision L). Although a high level of PaO2-ORi correlation has been demonstrated in many studies, including ours, improvements to ORi with necessary updates might reduce variations.

These characteristics of ORi are crucial for preventing hyperoxemia due to unnecessary and excessive oxygen use and associated complications. Hyperoxemia is as harmful as hypoxia, with a U-shaped relationship between oxygenation and harm. Hence, using SpO2 and ORi together is essential to prevent complications and mortality (Vos et al. 2019; Jonge et al. 2008; Martin and Grocott 2013; Asfar et al. 2015).

De Jonge et al., in their study examining the relationship between PaO2 and mortality, found the lowest mean mortality rate at a PaO2 of 113–150 mmHg (15–20 kPa) and indicated that the mortality rate increased when PaO2 < 68 mmHg (9 kPa) and > 225 mmHg (30 kPa) (Jonge et al. 2008). Rincon et al. (Rincon et al. 2014) reported that hyperoxemia (PaO2 > 300 mmHg/40 kPa) independently increases mortality in patients with traumatic brain injury (TBI) and advised against unnecessary oxygen administration.

Although the European Society of Intensive Care Medicine consensus specified that there is enough data to recommend that both hypoxemia and hyperoxemia should be avoided in TBI patients and agreed on a general normoxia recommendation with optimal PaO2 of 80–120 mmHg (10–16 kPa) in TBI patients with or without increased intracranial pressure, specific PaO2 targets need to be individualized (Courson et al. 2022; Robba et al. 2020).

Oxygen is a double-edged sword. While hyperoxemia has numerous harmful effects on the pulmonary, cardiac, metabolic, vascular, and cerebral systems and increases morbidity and mortality, studies have reported average PaO2 levels during general anesthesia to be 206 mmHg (Robba et al. 2020) and even exceeding 500 mmHg in some case groups (Yoshida et al. 2020; Ahn et al. 2022). Therefore, achieving optimum oxygenation within a narrow therapeutic range is vital (Courson et al. 2022).

Various classifications based on PaO2 values have been proposed for oxygen status. Although no definitive classification exists, it is generally defined as hypoxemia (PaO2 < 80 mmHg [< 10.7 kPa]), normoxia (81–100 mmHg [10.7–13.3 kPa]), moderate hyperoxemia (100–200 mmHg [13.3–26.7 kPa]), and severe hyperoxemia (PaO2 > 200 mmHg [> 26.7 kPa]) (Scheeren et al. 2018; Chen and Min 2020). In our study, using this classification, we investigated the effectiveness of ORi-guided FiO2 titration to achieve optimum oxygenation and avoid hyperoxemia. Normoxia (PaO2 80–100 mmHg [10.7–13.3 kPa]) was observed in more patients in the ORi + SpO2 group, and moderate hyperoxemia was not observed in any patient in the ORi + SpO2 group, with a significant statistical difference found between the groups. Moreover, the decreasing trend of PaO2 values over time in the ORi + SpO2 group and the statistically significant difference between PaO2 values in each time period demonstrate the effective application of FiO2 titration guided by ORi. Consistent with our study, Ahn et al. reported that adjusting FiO2 under the guidance of ORi and SpO2 resulted in a lower incidence of hyperoxemia with lower PaO2 levels (Ahn et al. 2022). In another study evaluating 50 patients undergoing breast surgery, lower oxygen requirements were obtained in the group using ORi to determine additional postoperative oxygen amounts. The beneficial effect of ORi on postoperative oxygen titration was demonstrated in this study (Martin et al. 2016; Kumagai et al. 2020). Another study investigating the effects of ORi-guided oxygen titration and hyperoxemia-mediated morbidity in one-lung ventilation reported lower mean FiO2 and PaO2 values with ORi monitoring. They concluded that ORi-guided oxygen titration can protect against hyperoxia, reduce hospital stay duration, and increase patient safety (Saraçoğlu et al. 2021). Consistent with our study, these studies indicated that ORi has a beneficial role in preventing hyperoxia and achieving optimum oxygenation through its guiding effect on FiO2 titration.

In our study, we believe that the lack of significant differences in detectable hyperoxaemia until the third hour may be due to the patients’ physiological reserve and stable oxygenation in the early phases of surgery. Additionally, the delay in detecting differences may be influenced by the combination of hourly monitoring and cumulative FiO2 titrations. As these adjustments accumulate, their impact on oxygenation becomes more pronounced, potentially explaining the differences observed later in the procedure. We believe this could be one of the reasons for the observed differences and underscores the importance of timing and frequency in FiO2 titration during intraoperative oxygen management.

Considering the increasing number of surgeries and intensive care patients worldwide, complications caused by hyperoxemia affect quality of life, increase morbidity and mortality, contribute to environmental harm due to unnecessary oxygen use, and result in workforce loss and substantial economic losses. ORi provides clinicians with a crucial tool to detect and prevent hyperoxemia noninvasively. The advantage of using ORi with SpO2 in preventing both hypoxemia and hyperoxemia is evident.