Effect of Trendelenburg Position on Patient State Index

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)