Effect of Hypothermia on BIS and PSI Correlation During Cardiopulmonary Bypass

Hypothermia during cardiopulmonary bypass (CPB) is known to influence cerebral metabolism and electroencephalographic activity. Processed electroencephalography-derived indices such as the Bispectral Index (BIS) and Patient State Index (PSI) are commonly used to monitor anesthetic depth; however, these indices are generated using different proprietary algorithms. While previous studies have demonstrated moderate to high correlation between BIS and PSI under normothermic conditions, it remains unclear whether this relationship is preserved during hypothermic CPB.

This prospective observational study aims to evaluate the effect of hypothermia on the relationship between BIS and PSI in adult patients undergoing elective on-pump coronary artery bypass grafting. Both indices will be monitored simultaneously throughout the intraoperative period using standard sensor placement.

Data will be collected at predefined time points representing normothermic, hypothermic, and rewarming phases of CPB. The primary analysis will focus on comparing the correlation between BIS and PSI at a normothermic reference point (36°C) and a hypothermic reference point (32°C).

To minimize confounding effects, anesthetic management and physiological parameters will be maintained within routine clinical target ranges, and relevant variables will be recorded throughout the procedure. Secondary analyses will explore temperature-dependent changes in each index and agreement between clinically relevant depth-of-anesthesia categories.

This study is designed to clarify whether hypothermia alters the relationship between commonly used processed EEG indices, which may have implications for intraoperative neuromonitoring and anesthetic management during cardiac surgery.

• Simultaneous BIS and PSI Monitoring (Other): This observational study does not involve any experimental intervention. All participants will receive standard anesthesia and surgical care according to institutional protocols. The study focuses on simultaneous intraoperative monitoring using Bispectral Index (BIS) and Patient State Index (PSI) devices. Sensors for both monitors will be applied prior to induction of anesthesia, and values will be recorded at predefined time points during normothermic, hypothermic, and rewarming phases of cardiopulmonary bypass. No changes will be made to routine clinical management, and all treatments will be determined by the attending anesthesiologist.

Primary outcomes

• Difference in Correlation Between Bispectral Index (BIS) and Patient State Index (PSI) Under Normothermic and Hypothermic Conditions [Intraoperative (during cardiopulmonary bypass at predefined time points)]: The primary outcome is the difference in Pearson correlation coefficients between BIS and PSI measured at predefined normothermic (36°C) and hypothermic (32°C) phases during cardiopulmonary bypass. Correlation coefficients will be calculated separately for each phase and compared using Fisher z-transformation to determine whether hypothermia alters the relationship between these indices.

Secondary outcomes

• Change in Bispectral Index (BIS) and Patient State Index (PSI) per Degree Celsius During Cooling and Rewarming [Intraoperative (during cooling and rewarming phases of cardiopulmonary bypass)]: This outcome evaluates temperature-dependent changes in BIS and PSI during cooling and rewarming phases of cardiopulmonary bypass. The rate of change in each index per degree Celsius (Δindex/°C) will be calculated using repeated measurements across predefined temperature points. Linear mixed-effects models will be used to estimate the relationship between core temperature and index values while accounting for within-subject variability.
• Effect of CPB Initiation on the Relationship Between Bispectral Index (BIS) and Patient State Index (PSI) [Intraoperative (comparison of pre-CPB and post-CPB initiation time points)]: This outcome assesses the impact of cardiopulmonary bypass initiation on the relationship between BIS and PSI. Pearson correlation coefficients will be calculated at the pre-CPB stable normothermic time point (T4) and after CPB initiation under normothermic conditions (T5). The change in correlation between these two time points will be evaluated to determine whether initiation of CPB alters the association between the indices.
• Agreement in Clinical Depth Categories Between Bispectral Index (BIS) and Patient State Index (PSI) [Intraoperative (during predefined normothermic and hypothermic phases of cardiopulmonary bypass)]: This outcome evaluates the agreement between BIS and PSI when classified into clinically relevant depth-of-anesthesia categories. BIS values will be categorized as >60 (light), 40-60 (adequate), and <40 (deep), while PSI values will be categorized as >50 (light), 25-50 (adequate), and <25 (deep). Agreement between the two indices will be assessed using Cohen's kappa coefficient. Comparisons will be performed separately for normothermic and hypothermic phases to determine whether temperature affects categorical agreement.

Sex: All, ages: 18 Years to 80 Years

Inclusion Criteria:

* Age ≥18 years
* Elective on-pump Coronary Artery Bypass Grafting (CABG) surgery
* Planned use of general anesthesia
* Written informed consent

Exclusion Criteria:

* Preexisting neurological disease
* Use of medications affecting Electroencephalography (e.g., antiepileptics)
* Severe hepatic dysfunction
* Emergency surgery
* Inadequate signal quality for anesthesia depth monitoring

• Meybohm P, Gruenewald M, Hocker J, Renner J, Graesner JT, Ilies C, Scholz J, Bein B. Correlation and agreement between the bispectral index vs. state entropy during hypothermic cardio-pulmonary bypass. Acta Anaesthesiol Scand. 2010 Feb;54(2):169-75. doi: 10.1111/j.1399-6576.2009.02138.x. Epub 2009 Oct 15. PMID 19839944
• Belletti A, Lee DK, Yanase F, Naorungroj T, Eastwood GM, Bellomo R, Weinberg L. Changes in SedLine-derived processed electroencephalographic parameters during hypothermia in patients undergoing cardiac surgery with cardiopulmonary bypass. Front Cardiovasc Med. 2023 Jul 4;10:1084426. doi: 10.3389/fcvm.2023.1084426. eCollection 2023. PMID 37469479

• AYÇA TAŞ TUNA, Professor, M.D., Principal investigator, Sakarya University Faculty of Medicine