Effects of Ondansetron Timing During Spinal Anesthesia for Urologic Surgery

This study will evaluate whether the timing of intravenous ondansetron administration affects cerebral oxygenation and hemodynamic stability in older adults undergoing endoscopic urologic surgery under spinal anesthesia. Participants aged 65 years or older will be randomly assigned to one of three groups: ondansetron administered before spinal anesthesia, ondansetron administered after spinal anesthesia, or placebo. Ondansetron will be administered intravenously at a dose of 8 mg. Cerebral oxygenation will be assessed using bilateral frontal near-infrared spectroscopy (NIRS). Regional cerebral oxygen saturation (rSO₂) will be recorded before spinal anesthesia and at 5-minute intervals during the first 60 minutes after spinal anesthesia. The study will also evaluate blood pressure, heart rate, vasopressor requirements, bradycardia, postoperative nausea and vomiting, post-spinal headache, and postoperative delirium. The aim is to determine whether the timing of ondansetron administration influences cerebral oxygenation and hemodynamic stability after spinal anesthesia.

Inclusion Criteria: * Male sex. * Age 65 years or older. * ASA physical status I-III. * Scheduled for elective transurethral resection of the prostate (TUR-P) or transurethral resection of bladder tumor (TUR-BT) under spinal anesthesia. * Ability to provide written informed consent. Exclusion Criteria: * Known hypersensitivity or contraindication to ondansetron. * Corrected QT interval (QTc) \>480 ms. * Significant cardiac conduction abnormality. * Concomitant use of medications known to prolong the QT interval. * Severe ischemic heart disease. * Severe chronic obstructive pulmonary disease. * Skin lesion, wound, or other condition preventing placement of frontal NIRS sensors. * Contraindication to spinal anesthesia.