Background and Objectives: Side effects of spinal anesthesia include arterial hypotension and bradycardia. Both of them may be induced by sympathetic nerve blockade as well as by the Bezold-Jarisch reflex, which may be mediated by peripheral serotonin receptors (5-HT₃ type). The aim of this study was to verify the hypothesis that blockade of type 3 serotonin receptors by intravenous ondansetron administration might reduce hypotension and bradycardia induced by spinal anesthesia.

Methods: Seventy-one patients participated in the study; 36 in the ondansetron group (receiving 8 mg intravenous ondansetron), and 35 in the placebo group (receiving isotonic NaCl solution). Measurements of heart rate and arterial blood pressure were taken every 5 minutes after spinal anesthesia was performed with 4 mL 0.5% hyperbaric bupivacaine solution.

Results: Decreases in mean, systolic, and diastolic arterial pressure as well as in heart rate, compared with baseline values were observed in both groups. Minimal systolic and mean blood pressure values obtained over a 20-minute observation period were significantly higher in the ondansetron group. There were no significant differences in diastolic blood pressure and heart rate values between the groups.

Conclusions: Ondansetron given intravenously attenuates the fall of systolic and mean blood pressure, but does not have an influence on diastolic blood pressure or heart rate.