Abstract
Background/Objective: Postoperative pain and delirium are frequent and clinically relevant complications in patients undergoing major cardiac or neurosurgical procedures. The interaction between these conditions remains insufficiently characterized, particularly across heterogeneous surgical populations. This study aimed to investigate the relationship between postoperative pain intensity and delirium severity within the first 48 h after surgery in cardiac and neurosurgical patients. Methods: This retrospective observational analysis included 408 individuals-202 following cardiac surgery and 206 after neurosurgical procedures. Pain intensity was measured using the Numerical Rating Scale (NRS), while delirium presence and severity were assessed using the CAM-ICU and CAM-ICU-7 instruments. Associations between NRS scores, delirium severity, demographic characteristics, and ICU length of stay were examined. Results: Cardiac surgery patients experienced higher pain levels on postoperative day 1 compared with neurosurgical patients; this difference was not observed on day 2. In the cardiac cohort, higher NRS scores were positively associated with greater delirium severity on both postoperative days. No such association was detected in the neurosurgical group. Pain scores also differed across procedure types within each specialty, and several demographic variables (age, sex, ICU stay duration) were linked with variations in pain intensity. On postoperative day 1, pain intensity showed a moderate association with delirium severity (Spearman ρ = 0.23; 95% CI 0.14-0.32). Patients who developed delirium had higher pain scores (r = 0.25). In ordinal logistic regression, greater pain on postoperative day 1 independently predicted higher delirium severity (OR 2.24; 95% CI 1.70-2.94). Conclusions: Significant associations between postoperative pain intensity and delirium severity were identified in cardiac surgery patients, whereas no similar pattern emerged among neurosurgical patients. Given the retrospective design and incomplete data on perioperative pharmacotherapy, the findings should be interpreted descriptively and do not support causal conclusions. These results underscore the importance of systematic monitoring of pain and cognitive function in high-risk postoperative populations and highlight the need for prospective studies to elucidate the complex interplay between pain, perioperative factors, and postoperative delirium.