Dexmedetomidine-esketamine Combination and Chronic Postsurgical Pain After Spinal Surgery

Inclusion Criteria: 1. Aged \>= 40 years and \< 80 years; 2. Scheduled to undergo elective posterior cervical, thoracic or lumbar spine surgery under general anesthesia, with an expected operative duration \>= 2 hours; 3. Requirement for patient-controlled intravenous analgesia (PCIA) postoperatively. Exclusion Criteria: 1. Severe uncontrolled hypertension preoperatively (baseline ward blood pressure: SBP \> 180 mmHg or DBP \> 110 mmHg); 2. Severe bradycardia (heart rate \<= 50 bpm), sick sinus syndrome, atrioventricular block of grade II or higher without pacemaker implantation, or a history of myocardial infarction within one year, severe heart failure (NYHA class \>= III), or rapid ventricular arrhythmia; 3. Preoperative history of schizophrenia, epilepsy, Parkinson's disease, myasthenia gravis, or intracranial hypertension; 4. Preoperative history of hyperthyroidism or pheochromocytoma; 5. Inability to communicate preoperatively due to coma, severe dementia, or language impairment; 6. Severe cardiac insufficiency (preoperative LVEF \< 30% or NYHA class IV), severe hepatic dysfunction (Child-Pugh class C), severe renal dysfunction (preoperative dialysis), or ASA physical status \>= grade IV; 7. Other conditions deemed inappropriate for study participation by the investigator or attending physician.