To evaluate whether prolonged exposure below a Patient State Index (PSI) threshold of 25—considered indicative of excessive anesthesia depth via processed electroencephalography—is associated with postoperative delirium (POD) in cardiac surgery.

A retrospective, single-center cohort study.

A university hospital in Sweden.

Adults (aged ≥ 18 years) who underwent cardiac surgery with cardiopulmonary bypass between January 2021 and February 2024 were included. Exclusion criteria included complex procedures and major perioperative complications.

None.

POD occurred in 18.1% of 733 patients. PSI values were recorded every 15 seconds perioperatively. POD was assessed using the Nursing Delirium Screening Scale and a validated chart-based instrument. Patients who experienced POD had significantly longer durations and greater cumulative areas with PSI < 25. In multivariable logistic regression, both PSI duration < 25 (adjusted odds ratio [OR], 1.04 per 10-minute increase; 95% confidence interval [CI], 1.01-1.06) and PSI area < 25 (adjusted OR, 1.01 per 10-unit increase; 95% CI, 1.00-1.01) were independently associated with POD. Risk stratification based on PSI threshold < 25 identified a low-risk group (POD, 12%) and a high-risk group (POD, >30%). Sensitivity analyses confirmed the robustness of PSI < 25 as a predictor, while thresholds of 20 and 30 showed weaker associations.

Exposure below the recommended anesthetic depth threshold (PSI of 25), measured by both duration and cumulative area, was a significant risk factor for POD after cardiac surgery. These associations persisted even after adjustment for multiple confounders. PSI-guided anesthesia may represent a modifiable intraoperative factor with the potential to reduce the occurrence of POD.