Spinal anesthesia is frequently used in urologic surgery in geriatric patients; however, it may cause significant hemodynamic changes such as hypotension and bradycardia. These changes can be more pronounced in elderly patients due to age-related physiological alterations. Ondansetron, a 5-HT3 receptor antagonist commonly used for the prevention of postoperative nausea and vomiting, has been suggested to attenuate spinal anesthesia-induced hemodynamic instability by modulating vagal reflexes.

This prospective observational study aimed to evaluate the effects of ondansetron on hemodynamic changes following spinal anesthesia in geriatric patients undergoing urologic surgery. Hemodynamic parameters were recorded during the intraoperative period, and the incidence of hypotension, bradycardia, and vasopressor requirements were assessed.

This study was designed as a prospective observational study conducted in geriatric patients undergoing urologic surgery under spinal anesthesia. Patient enrollment and data collection were performed prospectively between December 18, 2024, and April 18, 2025. The study protocol was approved by the TABED Ethics Committee (Approval No: 1-24-840, December 18, 2024).

Ondansetron was administered according to routine clinical practice. Hemodynamic parameters, including mean arterial pressure and heart rate, were recorded at predefined time points following spinal anesthesia. The incidence of spinal anesthesia-induced hypotension, bradycardia, and the need for vasopressor support were evaluated.

Although patient enrollment and data collection were conducted prospectively, the study was registered retrospectively at ClinicalTrials.gov due to delayed trial registration.

Inclusion Criteria:

• Patients aged 65 years and older Patients scheduled for elective urologic surgery under spinal anesthesia American Society of Anesthesiologists (ASA) physical status I-III Patients who received routine perioperative management, with or without ondansetron administration Patients who provided written informed consent

Exclusion Criteria:

• Refusal or contraindication to spinal anesthesia Known hypersensitivity or contraindication to ondansetron Preexisting severe cardiac conduction abnormalities (e.g., high-grade atrioventricular block, sick sinus syndrome) Baseline bradycardia (heart rate < 50 beats per minute) Chronic use of medications significantly affecting heart rate or blood pressure (e.g., beta-blockers, calcium channel blockers, antiarrhythmic drugs) Emergency surgery Incomplete perioperative hemodynamic data

The study population consists of geriatric patients aged 65 years and older undergoing elective urologic surgery under spinal anesthesia. Patients were managed according to routine clinical practice, with or without prophylactic ondansetron administration. Perioperative hemodynamic parameters, including blood pressure and heart rate, were prospectively recorded during the intraoperative period.