Effect of patient position on the EEG bispectral index and entropy index under general anaesthesia

Perioperative depth monitoring techniques, such as electroencephalography bispectral index (BIS), entropy index, and auditory evoked potential, are commonly used to assess anesthesia depth. However, the influence of patient positioning changes, particularly in gynecological surgeries where a head-down position is often required, on the accuracy of these monitoring indices remains unexplored.

The aim of the our study was to observe the impact of patient position changes on the monitoring value of entropy and BIS to identify a more sensitive method of anaesthesia depth monitoring for gynaecological surgery patients.

We conducted a study involving 40 women undergoing general anesthesia, during which routine monitoring of vital signs, including electrocardiogram (ECG), heart rate (HR), noninvasive arterial blood pressure (NIBP), oxyhemoglobin saturation (SpO2), and end-expiratory carbon dioxide (PetCO2), was initiated. Entropy and BIS devices were affixed to the patients' foreheads after alcohol sterilization to record brain activity. Tracheal intubation was performed following anesthesia induction. Throughout anesthesia maintenance, the value of BIS and response entropy (RE) were monitored and maintained between 40 and 50 by adjusting the infusion rate of propofol and remifentanil with Target Controlled Infusion (TCI, Mintopharmacokinetics model). Dosing for infusion control utilized corrected weight (height-105). Data were recorded before and after position changes, including tilting the operating table to head-down positions of 15 and 25 degrees, returning to a supine position, and elevating the head to 15 and 25-degree angles. BIS and entropy values at different time points were compared between the groups.

Both BIS and entropy values increased from supine to head-down position and decreased from supine to head-up position, with entropy changes preceding those of BIS. Heart rate increased after head-up and decreased after head-down, while mean blood pressure (MBP) exhibited the opposite effect on heart rate. Significant correlations were found between heart rate and BIS (correlation coefficient: -0.43) and RE (correlation coefficient: -0.416), as well as between MBP and BIS (correlation coefficient: 0.346) and RE (correlation coefficient: 0.384).

Changes in patient position can significantly affect the value of RE and BIS, as changes in entropy occur earlier than changes in the BIS.