Evaluation of Cerebral Oxygenation During Orthopedic Surgeries Performed in the Beach Chair Position Under General Anesthesia

Semi-sitting (beach chair) position is commonly used during orthopedic surgeries such as shoulder arthroscopy, clavicle fracture fixation, and proximal humerus operations. However, this position is associated with a risk of cerebral hypoperfusion due to gravitational redistribution of blood volume, leading to decreased mean arterial pressure at the brain level. This can result in severe complications such as visual loss, ischemic stroke, and even brain death.

Routine noninvasive monitoring methods, including heart rate, blood pressure, and peripheral oxygen saturation, are standard during anesthesia. Because of the frequent occurrence of hypotension in this position, invasive arterial pressure monitoring is recommended to detect and manage hemodynamic fluctuations more precisely. Near-infrared spectroscopy (NIRS) is a noninvasive method that allows continuous measurement of cerebral oxygenation and can identify early reductions in cerebral perfusion that may not yet be reflected in traditional monitoring parameters.

The aim of this prospective observational cohort study is to determine the correlation between cerebral oxygenation measured by NIRS and other hemodynamic parameters (heart rate, invasive mean arterial pressure, end-tidal carbon dioxide) as well as arterial blood gas analysis during orthopedic surgery performed in the semi-sitting position. Additionally, the study will evaluate the relationship between cerebral perfusion parameters and patients' comorbidities (e.g., diabetes, hypertension, obesity, and other chronic conditions) to identify potential risk factors for cerebral desaturation in this surgical position.

The study will be conducted in a single tertiary academic center. A total of 30 adult patients (ASA I-III), aged 18 years or older, scheduled for elective orthopedic surgery in the semi-sitting position will be enrolled. Standard anesthesia care will be applied to all patients, and no additional intervention will be performed beyond routine monitoring. Data collection will include continuous NIRS measurements, invasive arterial pressure, heart rate, peripheral oxygen saturation, end-tidal CO₂, and arterial blood gas results.

Statistical analysis:

Descriptive statistics will be presented as means and standard deviations for continuous variables, and frequencies for categorical variables. Normality will be assessed using the Kolmogorov-Smirnov test. Correlations between NIRS values and other parameters (mean arterial pressure, ETCO₂, heart rate, blood gas values) will be analyzed using Pearson correlation coefficients for normally distributed variables. Group comparisons will be performed using paired t-tests for parametric data and Mann-Whitney U tests for nonparametric data. Bonferroni correction will be applied where multiple comparisons are made. A p-value of \<0.05 will be considered statistically significant.

Sample size was calculated using G\*Power (version 3.1.9.7) with a power of 80%, alpha of 0.05, and a medium effect size (f=0.25), resulting in a minimum of 28 patients required. Data will be analyzed using SPSS version 15.0.

This study aims to contribute to identifying optimal intraoperative monitoring strategies for maintaining cerebral perfusion during orthopedic procedures performed in the semi-sitting position, particularly in patients with multiple comorbidities.

• Association of Preoperatively Documented Hypertension with Intraoperative Low Cerebral Oxygenation (rSO₂) Measured by Near-Infrared Spectroscopy [Time frame: Preoperative period for comorbidity assessment and intraoperative period for cerebral oxygenation measurements (from pre-induction baseline to the end of surgery.]
• Association of Preoperatively Documented Diabetes Mellitus with Intraoperative Low Cerebral Oxygenation (rSO₂) Measured by Near-Infrared Spectroscopy [Time frame: Preoperative period for comorbidity assessment and intraoperative period for cerebral oxygenation measurements (from pre-induction baseline to the end of surgery.]
• Association of Preoperatively Documented Obesity with Intraoperative Low Cerebral Oxygenation (rSO₂) Measured by Near-Infrared Spectroscopy [Time frame: Preoperative period for comorbidity assessment and intraoperative period for cerebral oxygenation measurements (from pre-induction baseline to the end of surgery.]