The goal of this observational study is to evaluate whether the Trendelenburg position affects electroencephalography-based depth of anesthesia monitoring and cerebral oxygenation during general anesthesia.

The main questions it aims to answer are:

• Does the Trendelenburg position cause a change in the Patient State Index (PSI), an Electroensephalography (EEG)-derived indicator of anesthetic depth?
• Are changes in PSI associated with changes in frontal cerebral oxygen saturation (rSO₂) and hemodynamic parameters?

The study will include adult female patients undergoing elective laparoscopic gynecologic surgery under general anesthesia.

Participants will receive standard anesthesia and routine intraoperative monitoring. In addition to standard monitoring, Patient State Index (PSI) and frontal cerebral oxygen saturation (rSO₂) will be recorded using non-invasive sensors. No additional intervention will be performed for research purposes.

Physiological parameters including PSI, cerebral oxygen saturation, mean arterial pressure, heart rate, oxygen saturation, and end-tidal carbon dioxide will be recorded at predefined intraoperative time points before and after the Trendelenburg position.

The study aims to determine whether position-related physiological changes influence EEG-based anesthesia depth indices and to improve the interpretation of intraoperative brain monitoring.

Electroencephalography (EEG)-based depth of anesthesia monitors are widely used to assess the hypnotic component of general anesthesia. The Patient State Index (PSI) is a processed EEG parameter derived from frontal EEG signals and provides a numerical estimate of anesthetic depth ranging from 0 to 100. Although these indices are primarily designed to reflect the pharmacologic effects of anesthetic agents, physiological factors that influence cerebral hemodynamics may also affect EEG-derived parameters.

The Trendelenburg position is frequently used during laparoscopic pelvic surgery to improve surgical exposure. This position can alter venous return, increase cerebral venous pressure, and influence intracranial dynamics. When combined with pneumoperitoneum, these physiological changes may modify cerebral blood flow and cerebral oxygenation. Such alterations may potentially influence EEG-based indices of anesthetic depth independently of anesthetic drug concentration.

Previous studies have demonstrated that body position can influence processed EEG indices such as the bispectral index (BIS), particularly in the beach-chair position. However, the response of the Patient State Index to the Trendelenburg position has not been clearly established.

This prospective observational study aims to evaluate whether the Trendelenburg position affects PSI values during general anesthesia and to investigate the relationship between PSI changes and cerebral oxygenation measured by near-infrared spectroscopy (NIRS).

Adult female patients undergoing elective laparoscopic hysterectomy under general anesthesia will be included. Standard intraoperative monitoring will be applied, including electrocardiography, non-invasive blood pressure, pulse oximetry, end-tidal carbon dioxide, and anesthetic gas monitoring. In addition, EEG-based depth of anesthesia monitoring will be performed using the PSI monitor (Masimo SedLine), and frontal cerebral oxygen saturation (rSO₂) will be measured using near-infrared spectroscopy sensors.

Physiological parameters including PSI, cerebral oxygen saturation, mean arterial pressure, heart rate, oxygen saturation, and end-tidal carbon dioxide will be recorded at predefined intraoperative time points: before induction of anesthesia, after tracheal intubation in the supine position, after pneumoperitoneum, and at several intervals following the Trendelenburg position.

No intervention will be performed for research purposes. Anesthetic management and surgical positioning will follow routine clinical practice. The study will analyze whether position-related physiological changes are associated with variations in PSI and whether these changes correlate with cerebral oxygenation or hemodynamic parameters.

Understanding the influence of patient positioning on EEG-based anesthesia depth indices may improve the interpretation of intraoperative brain monitoring and help prevent unnecessary adjustments of anesthetic drug dosing.

Inclusion Criteria:

• Female patients aged 18-65 years
• American Society of Anesthesiologists (ASA) physical status I-III
• Scheduled for elective laparoscopic hysterectomy under general anesthesia
• Ability to provide written informed consent

Exclusion Criteria:

• History of neurological disease (e.g., stroke, epilepsy)
• Known cerebrovascular disease, glaucoma, or retinal disease
• Severe cardiac or pulmonary disease
• Contraindication to near-infrared spectroscopy sensor placement
• Obesity (Body Mass Index > 35 kg/m²)

The study population consists of adult female patients undergoing elective laparoscopic hysterectomy under general anesthesia at a tertiary care university-affiliated hospital. Eligible patients will be recruited consecutively among those scheduled for surgery who meet the inclusion criteria and provide written informed consent.

Individual participant data will not be publicly shared because the dataset contains clinical perioperative monitoring data collected at a single center. De-identified data may be made available from the corresponding investigator upon reasonable request and subject to institutional and ethical approval.