Abstract

Background. Preoxygenation is very important to protect the patient from hypoxia before intubation. However, pulse oximetry has some limitations in detecting hypoxia.

Objectives. We aimed to compare the effectiveness of 2 preoxygenation techniques based on oxygen reserve index (ORI) levels.

Materials and methods. Twenty healthy male volunteers were included in the study. They inhaled 100% FiO₂ oxygen administered at 5 L/min as the 1^st technique (M1) with a ventilation mask as much as their tidal volumes for 3 min. The 2^nd technique (M2) applied 100% FiO₂ oxygen at 10 L/min flow using the same mask and 8 deep inspiratory volumes, which was aimed to be completed within 1 min. Maximum ORI levels, duration to reach that level, and time needed to reach the target ORI level (0.35) and return back to the “0” were measured.

Results. In the M1 group, ORI levels were significantly higher during and after 60 s, according to post hoc tests. In the M2 groups, ORI levels were significantly higher during and after the 4^th inspiration, according to post hoc tests. Oxygen reserve index values at the 60^th 2^nd (M1) and 8^th inspiration (M2) were compared as the 8^th inspiration corresponded to the 60^th second. The maximum ORI values were significantly lower in the M1 group compared to the M2 group (p < 0.001 and p = 0.006, respectively). Seven volunteers (36.8%) in the M1 group and 2 volunteers (10.5%) in the M2 group could not reach the target ORI (McNemar’s test, test statistic 3.2, degrees of freedom (df) = 1, p = 0.063). The time to reach the target ORI value and to reach maximum ORI values was significantly longer in the M1 group than in the M2 group (p = 0.008 and p < 0.001, respectively).

Conclusions. We observed that the 8-deep breath technique is more effective in preoxygenation compared to the 3-min tidal volume technique.

Key words: preoxygenation, oxygen reserve index (ORI), hypoxia prevention, 8-deep breath technique, 3-min tidal volume technique

Background

Administration of 100% oxygen before a “rapid-sequence” induction of anesthesia is recommended to prevent hypoxia during induction.¹

Historically, many preoxygenation techniques have been described.^{2, 3} Currently, there are 4 methods of preoxygenation used in routine anesthetic practice: (1) the deep breathing technique; (2) the rapid breathing using a fraction of inspired oxygen (FiO₂) of 1 (100%) for 2 to 5 min; (3) the 4 or 8-vital capacity methods; and (4) the Transnasal Humidified Rapid Insufflation Ventilator Exchange (THRIVE) technique.⁴

The use of intraoperative pulse oximetry (SpO₂) enhances the prevention of hypoxic events and is mandatory in anesthesia and critical care practices.^{5, 6}

However, because the relationship between the arterial partial pressure of oxygen (PaO₂) and arterial oxygen saturation (SaO₂) is not linear but rather sigmoid, SpO₂ may not provide an additional warning below the levels of 98% until PaO₂ decreases below 70 mm Hg.^{7, 8, 9}

On the other hand, increasing the PaO₂ above 90–100 mm Hg or more will no longer affect the SpO₂. As a consequence, when SpO₂ is ≥97%, the PaO₂ levels could be anywhere between 90 and 600 mm Hg.^{10, 11, 12}

These major limitations in clinical practice have forced researchers to develop new methods to measure tissue oxygenation levels. The oxygen reserve index (ORI™; Masimo Corp., Irvine, USA) is a new variable that represents oxygenation status, with a scale between 0.00 and 1.00, and enables noninvasive and continuous measurement of PaO₂ ranging from 100 to 200 mm Hg. Oxygen reserve index has significant potential for predicting both hypoxemia and hyperoxia.^{13, 14}

Objectives

Considering that SpO₂ monitoring is not sufficient to show hypoxic or hyperoxic events, we aimed to compare the effectiveness of 2 preoxygenation techniques using ORI measurements.

Materials and methods

Following the approval of the Institutional Ethics Committee of the University of Health Sciences Istanbul SUAM (dated 12/22/23, No. 398, and UMIN: 000051009/08/05/23) and the provision of informed consent by each participant, 20 healthy male volunteers with an American Society of Anesthesiology (ASA) of 1 status, ranging in age from 18 to 32 years, were recruited to the study. The study size was assessed using biostatistical methods based on relevant previous studies. All study participants were nonsmokers with no evidence of cardiovascular, respiratory or other systemic diseases. Additional exclusion criteria included hemodynamic disturbances in the operating room and pre-procedural anxiety. Patients with nonivasive blood pressure (NIBP) over 140/90 mm Hg and a preprocedural State Trait Anxiety Inventory (STAI) test level over 41 were also excluded. Each study participant was monitored continuously using electrocardiography (ECG), NIBP sensors and the Masimo Root Radical-7 (Masimo Corp., Irvine, USA) incorporating both ORI-PVI and SpO₂ sensors, which were the disposable adhesive type and were placed on the 4^th finger of the contralateral side of the NIBP monitoring and protected under a light-shielding cover. The ORI was based on Masimo Rainbow SET (Masimo Inc, Irvine, USA) technology, in which the pulsatile signals were extracted from 8 wavelengths ranging between 500 and 1,400 nm, enabling the detection of changes in PaO₂ after SaO₂ was maximally saturated according to changes in the peripheral venous oxygen saturation. Oxygen reserve index is a novel, multiwavelength pulse oximeter-based, nondimensional index that ranges from 0 to 1 as PaO₂ increases from about 80 to 200 mm Hg.

As a result of the reliable measurement of peripheral perfusion, the participants’ skin temperature was monitored using a thermocouple and maintained at 36.5°C throughout the experiment. During the experiment, oxygen gas flow was tightly controlled using a semi-open anesthesiology circuit and a proper face mask. The SpO₂ and ORI values were displayed and stored using Root with a Radical-7 device (Masimo Corp., Irvine, USA). A universal serial bus (USB) data output port on the Root monitor was connected to a computer (Ideapad 5 14ITL05; Lenovo, Beijing, China) running proprietary Pulse Oximetry Automatic Data Collection software (Masimo Instrument Configuration Tool (MICT) v. V1.2.4.5; Masimo Corp.) to create and store data files that were subsequently analyzed offline.

Each study participant underwent 2 different techniques of preoxygenation. Oxygen was administered by the following 2 methods using the following preoxygenation techniques in each study participant. The 1^st method (M1) oxygenated volunteers with 100% FiO₂ at a 5 L/min flow for 3 min while they were breathing normally, which means the inspired volume was as much as their usual tidal volumes (without any leakage, which was ensured by proper mask usage). In the meantime, we recorded SpO₂, ORI, total hemoglobin (SpHb), and Pleth variability index (PVI) values every 30 s and noted their maximum quantities. At the end of the 3 min, we took the masks off the volunteers and waited for ORI values to reach “0” again and noted its duration. Approximately 1 h after the administration of M1, the 2^nd method (M2) was employed to oxygenate the volunteers. This involved the administration of 100% FiO₂ and a 10 L/min flow, with the volunteers instructed to take 8 breaths in 1 min, with maximal effort, to reach their maximum forced inspired volumes. In the meantime, we recorded SpO₂, ORI, SpHb, and PVI values after each deep inspiration, and we also noted their maximum quantities, just like in the M1 group.

An ORI of 0.35 was used as the target to be reached, and the duration to achieve this level was recorded in each technique. In addition, after the oxygenation period, maximum ORI levels, time to reach maximum ORI levels, and time for the return of the ORI to its baseline value in room air after cessation of oxygen supplementation (3 min for the M1 technique; 1 min for the M2 technique) were recorded and compared. There was no potential source of bias.

Statistical analyses

Statistical evaluations were performed using the PICOS program on the E-PICOS website, and post hoc analyses using a Friedman’s test were performed with the R software (R Foundation for Statistical Computing, Vienna, Austria). Categorical values were given as percentages. Numerical values were given as median and min/max levels because of the small volunteer size of the group (n < 25). Categorical values, such as the ability to reach target ORI levels, were compared between the 2 methods using the McNemar’s test. A Wilcoxon signed-rank test was employed to assess the statistical significance of numerical variables between the 2 methods, given the limited number of study participants. Oxygen reserve index changes over time in the M1 and M2 groups were separately evaluated using the Friedman’s test, and post hoc analyses were made using the Nemenyi test. P-values <0.05 were accepted as statistically significant.

Results

Twenty volunteers aged 18–32 years with an ASA 1 status were recruited for the study. One volunteer was excluded because of high blood pressure levels altering high STAI 1 test results. Demographic data of the volunteers are provided in Table 1.

Oxygen reserve index changes over time in the M1 group and during inspirations 1–8 in the M2 group are shown in Table 2 and as boxplot graphics in Figure 1, Figure 2. The values of 6 measurements in the M1 group were compared using Friedman’s test (n = 19, degrees of freedom (df) = 5, test statistic = 42.322, p < 0.01). Oxygen reserve index levels were significantly higher during and after 60 s, according to post hoc tests (Table 3). In addition, values of 8 measurements in the M2 group were also compared with the Friedman’s test (n = 19, df = 7, test statistic = 104.759, p < 0.01). Oxygen reserve index levels were significantly higher during and after the 4^th inspiration, according to post hoc tests (Table 4).

Oxygen reserve index values in the 60^th second (values of M1ORI60 for the M1 and M2ORI 8^th for the M2 groups were compared because the 8^th inspiration corresponded to the 60^th second in M1 technique and maximum ORI values were significantly lower in the M1 than in the M2 group (p < 0.001 and p = 0.006, respectively, Table 5). Seven volunteers (36.8%) in the M1 group and 2 volunteers (10.5%) in the M2 group could not reach the target ORI (McNemar’s, test statistic 3.2, df = 1, p = 0.063). Times to reach the target values of ORI and to reach maximum ORI values were significantly longer in the M1 than in the M2 group (p = 0.008 and p < 0.001, respectively). Time to return to baseline ORI values were not significantly different between the 2 groups (p = 0.071, Table 5).

Discussion

Preoxygenation is a mandatory technique that extends the safe apnea time for endotracheal intubation, particularly in a “cannot intubate/cannot oxygenate” (CICO) scenario.¹⁵ The procedure is carried out by supplying 100% oxygen (FiO₂ of 1.0) before the induction of general anesthesia, and several methods of preoxygenation have been described in the literature. Two of the commonly known standardized approaches are 8 deep breaths in 1 min and tidal volume breathing for 3–5 min, both using 100% inspired oxygen, but there are studies questioning whether the techniques are effective or not.^{16, 17, 18, 19, 20}

Today, we know a lot about the oxygen cascade. The definition of hypoxia is typically based on SpO₂ measurements. However, it is recognized that SpO₂ does not provide sufficient information about hypoxia at the tissue and cellular levels. However, while we try to protect the patient from hypoxia, we have concerns about the hyperoxia and its side effects at the same time.^{21, 22, 23, 24, 25, 26, 27, 28} Because some studies have shown that SpO₂ and the partial oxygen pressure changes in arterial blood gas analysis do not match exactly, a new and more sensitive pulse oximeter-based indexing system (ORI) obtained with pulsatile multiple wavelength analysis was defined. Researchers have discussed that ORI may be more sensitive in the measurement of hypoxia, hyperoxia or both. For this purpose, we tried to evaluate 2 major preoxygenation techniques and their benefits on the ORI.

This study is distinct from previous investigations that have established safe apnea times and preoxygenation techniques based on SpO₂ levels. In our study, we observed the effects of different preoxygenation techniques on tissue oxygenation reserve according to the ORI. Furthermore, the M2 group exhibited a more pronounced increase in ORI values and a longer duration of elevated values than the M1 group, which can be a time-saving method in emergency intubations, rapid sequential induction, and in the case of a cesarean section, morbid obesity, etc.^{29, 30, 31, 32}

We found that the maximum ORI levels varied between volunteers, which shows that oxygen reserve capacity may be different individually, but whatever this level is, it can be reached more quickly with the M2 technique.

According to the literature, Applegate et al. found that an ORI > 0.24 can be considered a PaO₂ ≥ 100 mm Hg when SpO₂ levels are over 98%.³³ In a study by Szmuk et al.¹⁴ conducted with Masimo, an ORI value of 0.3 provided 85% sensitivity and 80% specificity for a PaO₂ < 150 mm Hg.¹⁴ However, an ORI > 0.55 appears to correlate with a PaO₂ ≥ 150 mm Hg level in light of this data. Our target ORI value was 0.35 because our aim was to put into practice the results of this study in difficult intubation situations.

The time to reach the target ORI was also shorter in the M2 group. Moreover, while 7 study participants in the M1 group could not reach their personal maximum ORI levels, there were only 2 cases that failed in the M2 group. We observed that the study participants adapted better to the M2 technique.

Although the presence of a safe apnea time has been shown to be important in procedures such as bronchoscopy, which is reported in case reports, especially in which apneic oxygenation is used, there are not enough data from clinical studies.^{34, 35} Instead of safe apnea time, we used reset duration of ORI. We saw that the ORI reset time was longer in the M2 technique.

In addition, while doing this study as we saw in Figure 2, the 3–4 deep breath technique which was defined in previous years failed, because the ORI values start to increase from the 4^th breath. Therefore, study participants need more time to raise their oxygen reserves to the target and max levels.

During preoxygenation, the saturation levels of the study participants were 100%; therefore, we reached the end of that monitoring parameter, indicating it was not a clinically effective tool in comparing the 2 preoxygenation techniques. This finding was similar to the results obtained by Koishi et al., which reported that SpO₂ began to decrease 72 s (mean) after ORI reached 0.00 and the SpO₂ was 99 (98–99%).³⁶

Limitations

The study group consisted of healthy volunteers. Therefore, it was not possible to examine the results of patients with different comorbidities. It would have been possible to perform simultaneous matching of the ORI measurements with PaO₂ levels in arterial blood gas analysis. Consequently, it would have been possible to directly measure the oxygen status of the study participants.

Conclusions

We compared 2 preoxygenation techniques using ORI in healthy volunteers. We observed that the M2 technique is more effective in preoxygenation compared to the 3-min tidal volume technique. The M2 technique is preferable, particularly in emergency or difficult intubation situations.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Consent for publication

Not applicable.