Oxygen reserve index vs. peripheral oxygen saturation for the prediction of hypoxemia in morbidly obese patients: a prospective observational study

Background: Pulse oximetry is a standart of anesthesia for perioperative monitoring. Due to the principles of Hb oxygen dissociation curve, peripheral oxygen saturation has an approximate sensitivity and specificity of 90% for the detection of hypoxemia.

Objectives: The primary outcome of the study was to evaluate ORi^Ⓡ as an early parameter to determine hypoxia in morbidly obese patients. The secondary outcome was to compare the effectiveness of ORi^Ⓡ with SpO₂ in non-obese patients.

Design: Prospective, observational study.

Setting: Department of elective operating room at tertiary hospital.

Patients and methods: Observational study included written informed consent from 51 patients with 19 < BMI < 25 kg/m² and 51 patients with BMI > 40 kg/m² undergoing an elective surgery requiring tracheal intubation. In addition to standard monitors, an ORi sensor was placed and baseline values were recorded. The patients were preoxygenated until end tidal expiratory oxygen concentration is reached to 90%. After anesthesia induction and tracheal intubation, the breathing circuit was not connected tracheal tube until the SpO₂ decreased to 95%. Shapiro-Wilk, Pearson Chi-square, t-test, and Mann Whitney U test were used for the study.

Main outcome measures: Times of tolerable apnea, ORi^Ⓡ and SpO₂ values at the end of preoxygenation, beginning of intubation, beginning of the ORi alarm, when SpO₂ reached 95%, and when ORi reaches a plateau.

Sample size: 102 patients.

Results: The alert period: time to reach ORi^Ⓡ from 0.24 to a value of 95% SpO₂ was observed as 32 s in morbidly obese patients and 94 s in patients with a normal body mass index. The SpO₂ alert period was determined as time difference between 97% and 95% SpO₂. The data were recorded as 15 s and 36 s, respectively. It was observed that tolerable apnea, ORi^Ⓡ, SpO₂ and added alert times were longer in patients with normal BMI compared to morbidly obese patients.

Conclusions: As a result, ORi^Ⓡ can provide an early warning to prevent unexpected hypoxia before saturation begins to decrease in morbidly obese patients.

Limitations: Inability to perform arterial blood gas sampling in the time periods when we looked at the parameters to determine the relationship between ORi^Ⓡ and PaO₂.

Gov identifier: NCT05480748 registered 2022-07-29.