Clinical Research Directory

Inclusion Criteria: * Age ≥ 65 years * American Society of Anesthesiologists (ASA) physical status I, II, III (according to the American Society of Anesthesiologists classification). * Scheduled for elective hip fracture surgery under spinal anaesthesia. Exclusion Criteria: * Patient refusal or inability to provide informed consent. * Contraindication to spinal/neuraxial anaesthesia (coagulopathy, local infection at injection site, patient refusal, severe spinal deformity). * Known hypersensitivity or contraindication to ondansetron (e.g., congenital long-QT syndrome). * Baseline cardiac arrhythmia, significant conduction abnormality, or QTc \> 450-470 ms on preoperative ECG. * Severe cardiovascular disease (decompensated heart failure, recent myocardial infarction, severe valvular disease), ASA IV-V. * Baseline bradycardia (heart rate \< 50 bpm) or use of a permanent pacemaker. * Concurrent use of medications that significantly affect heart rate or QT interval (e.g., class III antiarrhythmics), or of another 5-HT3 antagonist within 24 hours. * Emergency surgery, or planned combined spinal-general anaesthesia/conversion anticipated. * Pregnancy (not applicable in most cases given the population) and cognitive impairment precluding valid consent or symptom reporting.