Introduction Postoperative delirium (POD) is a common complication following cardiac surgery and is closely associated with adverse clinical outcomes. The effect of perioperative dexmedetomidine on reducing POD remains controversial in the existing literature. In our previous meta-analysis, we obtained preliminary evidence suggesting that dexmedetomidine may reduce the incidence of POD by improving sleep quality, which may partly explain the heterogeneity reported in previous studies. Based on these findings, the present randomised controlled trial aims to test the hypothesis that preoperative intranasal administration of dexmedetomidine reduces the incidence of POD in patients undergoing cardiopulmonary bypass assisted cardiac surgery by enhancing preoperative sleep quality. Methods and analysis This trial is a single-centre, investigator-initiated, parallel, double-blind, randomised, placebo-controlled trial. Individuals aged 18 years or older who are scheduled for elective cardiopulmonary bypass—assisted cardiac surgery will be enrolled in the study. The planned sample size is 686. Participants will be randomly assigned to either the dexmedetomidine group receiving two doses of dexmedetomidine (1.5 µg/kg according to ideal body weight) administered between 21:00 and 21:30 on the night before surgery and 15 min before anaesthesia induction, or the placebo group, receiving an equivalent volume of normal saline at the same time points. The primary outcome is the incidence of delirium within 7 days after surgery. Secondary outcomes include the severity, subtypes and duration of delirium, length of postoperative hospital stay, in-hospital all-cause mortality, postoperative sleep assessed by the Numerical Rating Scale score, pain intensity, postoperative anxiety and depression scores. Mediation analyses will be conducted using the preoperative Sleep Quality Index to assess whether dexmedetomidine reduces POD by improving preoperative sleep quality. The Baron and Kenny causal steps framework in conjunction with bootstrap resampling will be employed to estimate the direct, indirect and total effects. Ethics and dissemination The study is approved by the Institutional Review Board of Xijing Hospital (KY20242259). Written informed consent will be obtained from all participants. The results will be submitted for publication in peer-reviewed journals. Trial registration number NCT06619912 .
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