Delirium is an acutely occurred and fluctuating cerebral dysfunction characterized with inattention, altered consciousness, cognitive decline and/or abnormal perception. It is common in the elderly after cardiac surgery and is associated with worse outcomes. Causes leading to delirium are multifactorial but sleep disturbances remains an important one. In previous studies, sedative-dose dexmedetomidine improves sleep quality in ICU patients with mechanical ventilation; and low-dose dexmedetomidine improves sleep quality in postoperative patients without mechanical ventilation. In recent studies of elderly after noncardiac surgery, night-time infusion of low-dose dexmedetomidine reduces delirium and improves 2-year survival. The investigators hypothesize that, for elderly patients after cardiac surgery, night-time infusion of dexmedetomidine may also improve sleep quality, reduce delirium development and improve 2-year survival.

Delirium is an acutely occurred and fluctuating cerebral dysfunction characterized with inattention, altered consciousness, cognitive decline and/or abnormal perception. It is common in the elderly after cardiac surgery and is associated with worse outcomes.

The development of delirium is a consequence of multiple factors. For patients undergoing cardiac surgery, surgical stress and/or cardiopulmonary bypass can produce hyper-inflammatory and stress response, both of which are important factors leading to delirium. Meanwhile, patients in ICU after major surgery often develop sleep disturbances, which are also associated with increased risk of delirium.

Dexmedetomidine is a highly selective α2-adrenoceptor agonist with sedative, analgesic and anxiolytic properties. In previous studies of elderly patients admitted to ICU after non-cardiac surgery, continuous infusion of low-dose dexmedetomidine during nighttime improves sleep quality, reduces delirium, and improves 2-year survival.

The investigators hypothesize that, for patients admitted to ICU after cardiac surgery, nighttime infusion of low-dose dexmedetomidine can decrease the incidence of delirium and improve long-term survival. The purpose of this study is to investigate the effect of nighttime infusion of low-dose dexmedetomidine on postoperative sleep quality, delirium, and 2-year survival in elderly patients admitted to ICU after cardiac surgery.

Inclusion Criteria:

1. Age ≥60 years but <90 years;
2. Scheduled to undergo cardiac surgery with cardiopulmonary bypass under general anesthesia;
3. Expected to stay in the intensive care unit (ICU) for at least 1 night after surgery.

Exclusion Criteria:

Patients who meet any of the following criteria will be excluded.

1. Refuse to participate in the study;
2. Preoperative history of schizophrenia, epilepsy, Parkinsonism, or myasthenia gravis;
3. Preoperative obstructive sleep apnea (previously diagnosed as obstructive sleep apnea, or the snoring, tiredness, observed apnea, high blood pressure-body mass index, age, neck circumference and gender [STOP-Bang] questionnaires ≥3);
4. Inability to communicate during the preoperative period because of coma, profound dementia or language barrier;
5. Preoperative sick sinus syndrome, severe sinus bradycardia (< 50 beats per minute), or second-degree atrioventricular block or above without pacemaker;
6. Severe hepatic dysfunction (Child-Pugh class C);
7. Severe renal dysfunction (requirement of renal replacement therapy) before surgery;
8. Presence of delirium (diagnosed by the Confusion Assessment Method [CAM]/CAM for the Intensive Care Unit [CAM-ICU]);
9. Current treatment with dexmedetomidine or clonidine.