This single-center prospective, observational study was approved by Institutional Ethics Committee (Decision No: 2022/514/222/9, Date:30/03/2022), was registered at ClinicalTrials.gov (NCT05480748) on 29th of July 2022, and was performed in accordance with the Declaration of Helsinki.

Written, informed consent was obtained from 51 patients with BMI > 40 kg/m2 (morbidly obesity) and 51 patients with 19 < BMI < 25 kg/m2 (non-obese), 18–80 years aged, American Society of Anesthesiologists (ASA) physical status I-III, scheduled for an elective surgical procedure requiring general anesthesia with tracheal intubation. Patients with significant history of cardiopulmonary disease, difficult intubation, pregnancy, hemoglobinopathies and preoperative hemoglobin of less than 10.0 mg/dL were defined as exclusion criteria. Our University Hospital’s surgical profile was consist of gynecologic procedures including hysterectomy, myomectomy and ovarian cystectomy with acute care surgery procedures including cholecystectomy, appendectomy and hernia repair. The data collection period was between August 1, 2022 and December 31, 2022.

Standard monitors were routinely established for each patient, including heart rate (HR), noninvasive blood pressure measurements. In addition, an ORiⓇ and SpO2 were measured simultaneously at 1-s interval with a pulse oximetry sensor (Rainbow sensor, R2-25) applied to the finger and connected to a Masimo Root with Radical-7 pulse oximeter (Masimo Corp.). Data for analysis were downloaded from the Root monitor. Rainbow® technology uses Pulse CO-Oximetry sensors connected to rainbow®-enabled devices. By this way the operators can monitor the oxygen content noninvasively. ORi is a part of this package. Light absorption is performed to increase the resolution of changes in oxygenation via a Pulse CO-Oximeter sensor. This utilizes multiple wavelengths of light. ORi is trended together with SpO2 as a real-time, continuous, unitless index between 0.00 and 1.00. This allows monitoring of patients’ oxygenation. Pulse oximeter uses spectrophotometry as its operating principle to determine oxyhemoglobin in peripheral arterial blood. The two wavelengths of light originating from oxygenated and deoxygenated hemoglobin in the blood are compared to each other.

Patients were admitted to the operating theatre without any premedication. A 20G cannula was used to establish intravenous access. Following the placement of monitors, baseline values were recorded. Patients were then preoxygenated with spontaneous ventilation and 100% FiO2 at a flow rate of 8 L/min via a tight-fitting face mask until end tidal expiratory oxygen concentration (etO2) reached to 90%. Anesthesia was induced by titrating intravenous propofol 2–3 mg/kg, fentanyl 1 mcg/kg, and rocuronium 0.6 mg/kg. The trachea was intubated after 3–4 min under direct visualization using a videolaryngoscope to confirm placement. The tracheal tube was not connected to the breathing circuit, and the patients remained apneic. The World Health Organization defines intraoperative SpO2 ≥ 95% as normal in its training materials, and treatment steps are mentioned for SpO2 ≤ 94% [18]. However, since we included morbidly obese patients with limited functional residual capacities in our study and wanted to stay within the safe range, we allowed SpO2 to decrease to 95%. At the same time, the alarm point for SpO2 was applied as 95% in our clinic’s protocol for morbidly obese patients. When SpO2 reached to 95%, an oral airway was placed. Afterwards the patients were ventilated by face mask with two hands technique. As a resque method, proper sizes of second generation supraglottic airway devices were kept ready.

ORiⓇ and SpO2 values were recorded continuously. Subsequently, the anesthesia circuit was connected and etCO2 was confirmed. Patients were ventilated with 100% FiO2, tidal volume targeted 6–8 mL/kg and 5 cmH2O of positive end-expiratory pressure until ORiⓇ plateaued. In order to keep etCO2 between 30 and 35 mmHg, the respiratory rate was adjusted. Thereafter, anesthesia maintenance was achieved by 2% inhaled sevoflurane with 1 MAC in 50% oxtgen and 50% air mixture. The steps are summarized in Timeline Diagram (Fig. 1).

The Timeline Diagram of the clinical trial

ORiⓇ and SpO2 data were compared at five specific time points: (1) baseline; (2) at the end of pre-oxygenation (when the EtO2 reaches to 90%); (3) at the beginning of intubation; (4) when SpO2 reaches 95%; and (5) when the ORi reaches a plateau with 100% FiO2.

We also recorded the tolerable apnea time defined as the time from the beginning of apnea until SpO2 reached 95% and ventilation was reinstated. The ORiⓇ alert period time was defined as the time between the onset of the ORiⓇ alarm (the time at which the ORiⓇ alarm would have started was also calculated using the manufacturer’s proprietary algorithm and the ORiⓇ alarm was set to ORi = 0.24 to stay within the safe range) and the SpO2 reaching 95%. We defined the SpO2 alert period as the time for SpO2 to decrease from 97 to 95%. The added warning time provided by ORiⓇ was defined as the difference between ORiⓇ warning time and SpO2 warning time.

SPSS version 25 statistical package program was used for statistical analysis. The data were summarized by using descriptive statistical methods (mean, median, frequency, percentile, minimum, maximum). Shapiro-Wilk test was used for normality tests of continuous variables. Pearson Chi-square Test of independence tests were performed for independence tests between two categorical variables. To investigate the differences between the two groups, t-test was used for continuous variables with normal distribution, and Mann Whitney U Test was used to compare data that did not. In addition, 95% confidence intervals were obtained with the Hodges Lehman median estimation in order to see the median changes.

The relationships between the classified variables forming the 2 × 2 crosstabs were investigated with Fisher’s Exact tests. The significance level was taken as 0.05 for all tests performed.

Tsymbal et al. [19] found the mean and standard deviation of obese group for Oxygen Reserve Index Values 0.41 and 0.09 respectively, and the mean and standard deviation of normal group 0.57 and 0.26. When this information was given into Gpower program, d parameter which is effect size was calculated as 0.822 seen in window at the below with alpha 0.05 and power of 0.95. Tsymbal et al. [19] also mentioned that they had used sampling information given by Szmuk P et al. [20], with power of 0.80 and alpha 0.05.

Gpower found that 42 patients for both groups, totally 84 patients with d effect size 0,822. Closely, we have calculated 80 patients by using Medcalc program. According to calculation of Gpower, non-centrality parameter of t test is 3.68 which was calculated by using sampling sizes of two groups and effect size. For this research it was planned to include 51 patients for each group in case of data collection errors, thus 102 patients were totally enrolled in the research. In summary, we have used sampling information from Tsymbal et al. [19] and calculated the parameter of effect size and used it with alpha 0.05 and power of 0.95 as seen in Gpower window. The calculation screen shot of Gpower program can be seen in Fig. 2.

The calculation of samole size using Gpower program