Patient-controlled Remimazolam Sedation Under Spinal Anesthesia

The goal of this clinical trial is to compare the efficacy of patient-controlled sedation (PCS) versus clinician-controlled sedation (CCS) using remimazolam in adult patients undergoing primary hip arthroplasty under spinal anesthesia. The main questions it aims to answer are: * Does patient-controlled sedation significantly reduce the total consumption of remimazolam compared to clinician-controlled sedation? * Are there differences in secondary outcomes, such as sedation depth , sedation-related adverse events, frequency of airway interventions, and patient/surgeon satisfaction? Researchers will compare the PCS group to the CCS group to see if the patient-controlled method leads to a reduction in total remimazolam consumption while maintaining effective sedation. Participants will: * Be randomly assigned to either the PCS group or the CCS group. * In the PCS group: Receive an initial 0.05 mg/kg remimazolam dose over 1 minute. If deeper sedation is desired, the patient can self-administer a 1 mg bolus of remimazolam via a button press, which has a 1-minute lockout interval. * In the CCS group: Receive an initial 0.05 mg/kg remimazolam dose over 1 minute, followed by a continuous infusion starting at 0.3 mg/kg/h. The clinician will assess the MOAA/S score every 10 minutes and adjust the infusion rate to maintain a target MOAA/S score of 3.

Inclusion Criteria: * Patients aged 19 to 70 years with an ASA (American Society of Anesthesiologists) physical status of I or II who are undergoing primary hip arthroplasty under spinal anesthesia. Exclusion Criteria: * Refusal to participate in the study * Patients with contraindications to spinal anesthesia * Patients who do not consent to intraoperative sedation * Patients with contraindications to remimazolam * Patients with a history of allergy or hypersensitivity to remimazolam * Patients with a baseline MOAA/S (Modified Observer's Assessment of Alertness/Sedation) score of 4 or lower * Pregnant women