Oxygen Reserve Index in Predicting Hypoxemia in Morbidly Obese Patients

This single-center prospective, observational study was approved by Institutional Ethics Committee (Decision No:2022/514/222/9, Date:30/03/2022) and was performed in accordance with the Declaration of Helsinki. Written, informed consent was obtained from 51 participants with BMI>40 kg/m2(morbidly obesity) and 51 participants with19<BMI<25 kg/m2(normal BMI), 18-75 aged, American Society of Anesthesiologists (ASA) physical status I-III scheduled for an elective surgical procedure requiring general anesthesia with endotracheal intubation. Participants with significant history of cardiopulmonary disease, 25<BMI<40 kg m2, ASA>4 and<18 years of age, difficult intubation, pregnancy, hemoglobinopathies and preoperative hemoglobin of less than 10.0 mg/dL were defined as exclusion criteria.

Standard monitors were routinely established for each participants, 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 was downloaded from the Root monitor.

Following the placement of monitors, baseline values were recorded. Participants were then preoxygenated with spontaneous ventilation and 100% inspirative oxygen concentrate (FiO2) at a flow rate of 8 liter/minute via a tight-fitting face mask until EtO2 is reached to 90%. Anesthesia was induced with intravenous midazolam, fentanyl, propofol and rocuronium in 100% FiO2.The trachea was intubated after 3-4 minutes under direct visualization using a videolaryngoscope to confirm placement.The endotracheal tube was not connected to the breathing circuit, and the participants remained apneic. The World Health Organization defines intraoperative SpO2 ≥ 95% as normal in its training materials, and treatment steps are mentioned for SpO2 ≤ 94% [12]. However, since we included morbidly obese participants with limited functional residual capacities in our study and wanted to stay with in the safe range, we allowed SpO2 to decrease up to 95%. At the same time, the alarm point for SpO2 is applied as 95% in our clinic's protocol for morbidly obese patients.

ORi and SpO2 values were recorded continuously. Subsequently, the anesthesia circuit was connected and patients were ventilated with 100% Fraction of Inspired Oxygen ( FiO2), tidal volume targeted 7 ml/kg and 5 cmH2O of positive end-expiratory pressure until ORi plateaued.Thereafter, anesthesia continued per routine.

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 6) during ventilation with 100% FiO2 when the ORi reaches a plateau.