Inclusion Criteria:  Patients aged between 18 and 80 years. Scheduled to undergo multi-segment (≥2) spine fusion surgery. Agreed to receive postoperative patient-controlled analgesia.  Exclusion Criteria:  Refused to participant in this trial. Poor blood pressure control in those with hypertension (BP >160/100 mmHg in the ward). Previous history of hyperthyroidism or pheochromocytoma. Previous history of schizophrenia, epilepsy or Parkinson disease. History of sick sinus syndrome, bradycardia (HR <50 beat per min), or atrioventricular block of grade II or higher without pacemaker. Severe heart dysfunction (New York Heart Association functional classification 4), hepatic insufficiency (Child-Pugh grade C), renal insufficiency (serum creatinine of 442 μmol/L or above, or requirement of renal replacement therapy), or ASA classification IV or above. Unable to complete preoperative assessment due to severe dementia or language barrier. Any other conditions that were considered unsuitable for the study participation.