The goal of this observational study is to determine the correlation between cerebral oxygenation values measured by near-infrared spectroscopy (NIRS) and other routine monitoring parameters in patients placed in the semi-sitting position.

The primary questions investigated are :

Do NIRS values correlate with heart rate, invasive mean arterial pressure, and end-tidal carbon dioxide?

Do NIRS values reflect changes observed in arterial blood gas analysis?

Are cerebral perfusion-related parameters associated with patients' comorbidities such as diabetes, obesity, and hypertension?

Researchers will compare routine noninvasive monitoring (heart rate, blood pressure, peripheral oxygen saturation) with invasive arterial monitoring and NIRS to assess whether NIRS provides additional information for evaluating cerebral perfusion.

Participants will be monitored for:

Heart rate

Invasive mean arterial pressure

End-tidal carbon dioxide

Arterial blood gas changes

Cerebral oxygenation with NIRS

Patients' comorbidities (e.g., diabetes, obesity, hypertension) will also be recorded, and their association with perfusion-related parameters will be analyzed.

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.

Inclusion Criteria:

• Patients scheduled for orthopedic surgery in the semi-sitting position
• ASA physical status I-III
• Voluntary participation with informed consent

Exclusion Criteria:

• Patients younger than 18 years
• Patients who do not provide consent
• History of cerebral ischemia
• History of head trauma
• Previous neurosurgical intervention
• Documented carotid stenosis ≥90%
• Previous neck surgery

Adult patients (18 years and older, ASA I-III) undergoing orthopedic surgery in the semi-sitting position, monitored for cerebral oxygenation (NIRS), invasive arterial pressure, and routine intraoperative parameters.