To evaluate the relationship between abnormal protein or metabolite expression levels in peripheral blood and postoperative delirium in elderly patients undergoing cardiac surgery.

Inclusion Criteria:

1. patients between 55 and 80 years old,
2. American Society of Anesthesiologists (ASA) physical status of II-III and New York Heart Association grade of II-III.
3. The planned cardiac surgery.
4. Hospitalization duration not less than 5 days.
5. The anesthesia time will be no less than 2 hours.All patients had a normal ability to hear, read and cooperate with neuropsychological tests.

Exclusion Criteria:

1. The pre-operative Montreal Cognitive Assessment (MoCA) score was less than 25,
2. Patients with dementia, history of neurological or psychiatric disease, hospital with anxiety depression scale (HADS) over eight points,
3. Currently use sedatives or antidepressants, Patients with substance dependence on drugs or alcohol.
4. Patients with liver and kidney dysfunction, patients with a history of cardiac surgery.
5. Admitted to the ICU for more than two days within the month preceding the current surgery,
6. Unable to cooperate with researcher's investigations.

To evaluate the relationship between abnormal protein or metabolite expression levels in peripheral blood and postoperative delirium in elderly patients undergoing cardiac surgery.